Study of OGMA´s Test Facility

Mariana Sequeira Glória Monteiro

Thesis to obtain the Master of Science Degree in

Mechanical Engineering

Supervisor: Prof. José Maria Campos da Silva André

Examination Committee

Chairperson: Prof. Carlos Frederico Neves Bettencourt da Silva

Supervisor: Prof. José Maria Campos da Silva André

Member of the Committee: Prof. Luis Rego da Cunha de Eça

Monteiro

November 2018

ii

Resumo

Depois de reparados e inspecionados, os motores dos aviões são ensaiados no banco

de ensaios, cuja principal função é reproduzir as condições de um motor a operar numa

atmosfera não confinada e sem vento.

As oficinas da OGMA, em Alverca, possuem um banco, inicialmente projetado para

ensaiar motores de pequenas aeronaves militares, que agora se pretende adaptar de modo a

alargar o âmbito de utilização a motores de aviões comerciais de maior potência e com “by-pass”,

que exigem caudais de ar superiores.

Este estudo começou por identificar os principais desafios de um banco de ensaios típico

e as características do escoamento neste banco, por meio de uma análise unidimensional com

modelos computacionais e analíticos. Por exemplo, com velocidades demasiado baixas à

entrada do banco de ensaios, podem formar-se separações instáveis e formação de vórtices a

montante do reator, que sejam depois ingeridos pela admissão. Quando isto ocorre o angulo de

ataque do escoamento com as pás do motor é alterado podendo resultar em perda de

sustentação do motor. Velocidades demasiado altas podem alterar o impulso medido no banco.

Seguidamente, estudaram-se medidas corretivas e pequenas modificações, procurando não

fazer alterações estruturais, susceptíveis de alargar a gama de motores que se podem ensaiar

no banco.

Aumentar o comprimento da bomba de injetor, instalar um difusor à saída e direcionar melhor o

escoamento de montante por meio de pás directrizes são algumas das soluções propostas.

iii

Palavras-chave: Banco de ensaios, Análise unidimensional, Aviões comerciais, medidas

corretivas

Abstract

After repaired and inspected, aircraft engines are tested on the test bench. This test

facility has as a function the reproduction of an environment similar to the conditions of an engine

operating in an unconstrained and windless atmosphere.

OGMA's facility in Alverca has a test cell designed to test engines of small military

aircrafts. It is intended to improve the performance of this test facility to extend the scope of use

for commercial aircraft engines with higher power output and with “bypass” which require higher

air flow rates.

This research began with the study of the main constraints of a typical test bench. The

characteristics of the flow were studied through a one-dimensional analysis with computational

and analytical models to obtain the overall behavior of the installation.

Corrective measures and small modifications were studied in order to avoid making

structural changes and as a mean to extend the range of possible engines able to be tested in

the test bench responding to the market requests.

Low air velocities at the engine inlet promotes the formation of vortices in the

surroundings of the engine´s inlet zone. The ingestion of vortices alters the angle of attack of the

flow with the blades of the engine, which can result in engine stall.

Increasing the length of the augmenter tube, the installation of ramps on the walls near

the engine bell mouth and the installation of guiding vanes to direct the flow are some of the

solutions presented to test engines that admit higher flow rates

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Keywords: Test Facility, One-dimensional analysis, Commercial aircrafts, corrective

measures

Contents List of Tables ............................................................................................................ vi

List of Figures ........................................................................................................... vi

Nomenclature ........................................................................................................... vii

Introduction ............................................................................................................... 1

Chapter 1 .................................................................................................................. 2

1.1 - Jet engines´ test cells ................................................................................ 2

1.2 – OGMA´s Test Cell ......................................................................................... 5

1.2.1 - Historical review ...................................................................................... 5

1.2.2 – OGMA´s test cell configuration ............................................................... 7

1.3 - Challenges of OGMA´s test cell ..................................................................... 9

1.3.1 - Flow separation in the edge near the roof ............................................... 9

1.3.2 - Separation along flat walls caused by the local distortion of the flow near

the engine’s inlet .............................................................................................. 11

Chapter 2 ................................................................................................................ 13

2.1 – Boundary layer separation over flat walls near the engine’s inlet ................ 13

2.2 - Thrust correlations ....................................................................................... 15

2.3 - Cell depression ............................................................................................ 17

2.4 - Augmenter Tube .......................................................................................... 19

2.4.1 - Theoretical Analysis of the Augmenter tube ....................................... 22

2.5 - Desired cell performance ......................................................................... 24

Chapter 3 ................................................................................................................ 25

3.1 - Engines ....................................................................................................... 26

3.2 - Unidimensional Model.................................................................................. 27

3.2.1 - Fluid resistance coefficients ................................................................. 30

3.2.1.1 - Inlet system .................................................................................... 31

3.2.1.2. - Inlet baffles .................................................................................... 31

3.2.1.3. - Sharp curve – roof´s corner ........................................................... 32

3.2.1.4- Grid of flow rectifier channels .............................................................. 33

3.2.1.5- Exhaust system ................................................................................... 33

3.3. - CFD simulation of the flow inside the augmenter tube ................................ 34

3.2.1 - RANS and Turbulence models .............................................................. 34

3.2.2 - Geometry .............................................................................................. 35

3.2.3 - Boundary conditions .............................................................................. 36

v

3.2.4 - Mesh ..................................................................................................... 38

3.2.5 - Mesh independence study ..................................................................... 39

3.3 – Results: Pressure recovered by the Augmenter tube .................................. 41

3.4 – Results ........................................................................................................ 43

3.5 - Results Discussion ...................................................................................... 46

Chapter 4 ................................................................................................................ 48

4.1 - Increase of the length of the augmenter tube ............................................... 49

4.1.2 - Results .................................................................................................. 51

4.2 - Inlet ramps ................................................................................................... 56

4.3. - Guiding Vanes ............................................................................................ 57

4.4. – Proposed solution ...................................................................................... 60

Conclusions............................................................................................................. 61

Future work ............................................................................................................. 62

Bibliography ............................................................................................................ 63

vi

List of Tables

Table 1- Characteristics of the engines analyzed in the study (data obtain from

manufacturers website) ............................................................................................ 26 Table 2 - OGMA´s tests results ................................................................................ 36 Table 3 - Boundary conditions: field functions ..................................................... 37 Table 4 -Mesh independence ................................................................................... 39 Table 5 - Pressure computed by the CFD simulations ........................................... 42 Table 6 - Values predicted by the unidimensional model ...................................... 44 Table 7 - Results for the unidimensional model vs OGMA´s study ....................... 46 Table 8 - Results of the unidimensional model ...................................................... 54 Table 9 - Results for the velocity at section 4 with the installation of ramps ....... 57 Table 10 - Results ..................................................................................................... 61

List of Figures

Figure 1 - Outdoor test cell (image from Rolls Royce) ............................................. 2 Figure 2-Typical enclosed test cell [1] ....................................................................... 3 Figure 3-OGMA´s test cell (Image from OGMA) ........................................................ 5 Figure 4-Test cell stations (image from OGMA) ....................................................... 7 Figure 5-OGMA´s test cell (image from OGMA´s) ..................................................... 8 Figure 6 - Flow separation at a corner [9] ................................................................. 9 Figure 7 -Flow near the engine inlet ........................................................................ 11 Figure 8- Formation of vortex as a function of the velocity ratio and CBR [2] ..... 14 Figure 9 - Different inlet configurations: a) horizontal inlet b) vertical inlet c)

vertical truncated inlet [2] ........................................................................................ 18 Figure 10 - Cell depression obtained for different inlet configurations [2] ........... 18 Figure 11 - Scheme of the augmenter tube studied by Brian[8] ............................ 20 Figure 12 – Mass Augmentation ratio as a function of the length of the tube [8] 21 Figure 13 - Scheme of the Augmenter tube ............................................................ 22 Figure 14- Engines analyzed in this study (image from manufacturers website) 26 Figure 15 - Test Cell sections .................................................................................. 27 Figure 16 - Inlet screen made of circular wire ......................................................... 31 Figure 17 - Test cell geometry ................................................................................. 35 Figure 18 - Error estimate (log-scale) versus the characteristic lenght. ............... 40 Figure 19 - Results obtained for the pressure inside the test cell simulating the

test of the engine V2527 with different meshes ...................................................... 40 Figure 20 - Results obtained for the pressure inside the test cell during the test

of engine V2527. ....................................................................................................... 41 Figure 21 - Results obtained for the Pressure inside the test cell during the test

of engine AE3007 ...................................................................................................... 41

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Figure 22 - Results obtained for the Pressure inside the test cell during the test

of engine V2530. ....................................................................................................... 42 Figure 23 - Results obtained for the Pressure inside the test cell during the test

of engine V2528. ....................................................................................................... 42 Figure 24 - Velocity profile at three sections of the augmenter tube during the

simulation of the test of engine V2527 .................................................................... 43 Figure 25 - Pressure at each section when engine V2527 is tested ...................... 45 Figure 26 - Geometry of the augmenter tube .......................................................... 50

Figure 27-Velocity field of the 5 meters augmenter tube…………………………….51 Figure 28-Pressure recovered along the 5 meters augmenter tube…………….…51 Figure 29-Velocity field inside the augmenter with 18 meters…..……………….…52 Figure 30-Velocity profile at three different sections of the augmenter tube....…52 Figure 31-Results for the pressure recovered by the 18 meter´s augmenter

tube…………………………………………………….……….…………………………...…53

Figure 32 - Pressure in several sections of the test cell (orange- length-5meter

blue – length-18meters)…………………………………………………………………….55

Figure 33 - Installation of ramps [11] ....................................................................... 56 Figure 34 - Guiding Vanes in a rectangular elbow ............................................... 577

Nomenclature

𝐴𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔 – Area of the inlet of the engine

𝐴𝑁𝑜𝑧𝑧𝑙𝑒 – Area of the engine´s exit nozzle

𝐴𝑠 – Area of the secondary jet of the engine

𝐴𝑝 – Area of the primary jet of the engine

CBR- cell bypass ratio

D – diameter of the augmenter tube

𝐹𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 – thrust measured during the test

L – length of the augmenter tube

𝑚𝑐𝑒𝑙𝑙̇ – mass flow rate at the inlet of the cell

𝑚𝑒𝑛𝑔𝑖𝑛𝑒̇ - mass flow rate that enters the engine

�̇�0 – mass flow rate of the primary jet

𝑚1̇ – mass flow rate of the pherispheral flow that enters the augmenter

𝑝 - pressure

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𝑃𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔 – Pressure at the inlet of the engine

𝑃𝑒𝑥𝑖𝑡,𝑒𝑛𝑔 – Pressure at the exit of the engine

v – velocity

𝑣𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔 – Velocity of the flow at the engine´s inlet

𝑣𝑠 − velocity of the secondary jet of the engine

𝑣𝑝 − velocity of the primary jet of the engine

ρ – density

𝜌𝑝 – density of the primary jet of the engine

𝜌𝑠 – density of the secondary jet of the engine

𝜌𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔 – density of the flow at the engine´s inlet

Θ – Primary temperature ratio(Tres/TA)

𝜁 -head loss coefficient

𝛾- ratio between the mass flow of the peripheral air and the mass flow of primary air that enters

the augmenter tube.

π - primary pressure ratio

Ψ-mass augmentation ratio

1

Introduction

The test of the engines before they are assembled again in the aircraft is necessary to

check the correct functioning of all sub-systems and measure the impulse and fuel

consumption. OGMA’s test cell facility was designed in the 60s to test military jet reactors; more

recently, due to the market growth of civilian aircrafts’ maintenance, OGMA started to use their

test facility to test bigger commercial engines. These engines have high dilution rates, requiring

a much higher air flow rate than military reactors. The increase of air flow rate gives rise to

several aerodynamic instabilities that significantly limit the range of engines that can be tested

in OGMA. The scope of the present report is to identify the issues that limit the operational

range of OGMA’s test cell and put forward mitigation measures.

Chapter 1 provides a brief explanation of the functioning of a typical enclosed test cell

facility and describes the layout of OGMA´s test facility as well as the typical challenges of this

facility.

Chapter 2 contains a brief literature review and explains the cell´s most important

parameters.

The computational model used to study the flow inside the augmenter tube and the

model used to predict the quality of the flow inside the chamber are explained in chapter 3.

The proposed improvements of the test facility, to allow a broader range of engines to

be tested, are described and evaluated in chapter 4.

The objectives of the current study are to evaluate the behavior of the test cell when

different engines are tested, study the performance of the augmenter tube and to propose

improvements to the test cell in order to widespread the range of engine that can be tested.

2

Chapter 1

1.1 - Jet engines´ test cells

The primary function of an engine test cell is to provide a controlled environment for

engine testing. Engines can be tested for different purposes. Engine manufacturers might be

interested in tests during the development phase, for research or improving the design;

maintenance workshops test all engines as a routine procedure, to verify repairs and insure the

adequate functioning of all sub-systems. Each type of testing requires specific characteristics of

the test cell. One important feature for routine testing and troubleshooting after maintenance is

the ability to install the engine, connect the instrumentations, run the test and obtain reliable data

in the shortest possible time.

Test cells can be divided in two groups: Outdoor Jet engine test facilities and enclosed

ones. The outdoor test facilities are open-air stands that support the engine and measure its

thrust. In the absence of wind and in standard atmospheric conditions, these facilities provide the

reference measurements. However, open air testing produces a lot of noise, requires a large

space and the tests are strongly dependent on weather and wind conditions.

Figure 1 - Outdoor test cell (image

from Rolls Royce)

3

For these reasons, the test bench of common maintenance shops are enclosed between

walls, so that engines can be tested regardless of bad weather conditions or noise restrictions.

However, new problems arise, when the engine is enclosed. The exhaust stack, usually equipped

with acoustic baffles, introduces a head loss, that can lead to re-ingestion of the exhaust gases.

Even if there is no re-ingestion of exhaust gases, the air suction at the engine’s inlet may produce

flow separation at the walls of the test cells. These separations may be entrained by the main

flow and enter the engine or perturb the exhaust. In OGMA’s test cell there is a further concern

regarding extensive flow separation in the roof’s edge, upstream of the engine.

To balance the head loss in the exhaust stack, test cells usually use the engine’s jet in a

jet-pump tube, commonly known as the “augmenter tube”.

Figure 2 shows a typical enclosed test cell, composed by the intake section, the engine

testing chamber, the augmenter tube and the exhaust section. The facility also requires a control

room and a fuel supply system. The configuration represented in this figure has a vertical air inlet

and a vertical air outlet. This arrangement is popular since it is less sensitive to wind disturbances

and less dangerous at ground level, around the facility.

Figure 2-Typical enclosed test cell [1]

4

The engine is mounted in a thrust stand in the test chamber. Air enters the facility through

the intake section, passes through the engine or around it, is sucked by the augmenter tube and

exits through the exhaust stack. Inside the test chamber, part of the air is drawn into the engine

entering through a bell mouth duct to prevent the air stream from approaching sideways the fan

blades. The remaining air (we will call it the peripheral air) flows around the engine. Both streams

converge into the augmenter and are exhausted together into the atmosphere. The momentum

of the exhaust jet of the engine drives the augmenter tube, that keeps the testing chamber below

atmospheric pressure and pumps the peripheral airflow. The amount of air that bypasses the

engine is controlled by the pressure gain of the augmenter tube and, therefore, depends on its

dimensions. If the airflow through the test cell is too low, recirculation of the exhaust gases may

occur and possible re-ingestion of combustion products by the engine, that may cause engine

surge. A low air flow rate may also reduce the air velocities near the walls of the test cell, leading

to separation and possible vortex detachments that may be ingested by the engine or enter the

augmenter tube. In both cases, surge instability is prone to occur. A too high airflow through the

test cell affects the correction factor, resulting on less reliable correlations. Furthermore, it may

produce an excessive load on the walls and thrust stand components. In summary, it is convenient

to avoid both extremes and the dimensioning of the augmenter tube it is of the utmost importance.

5

Figure 3-OGMA´s test cell (Image from OGMA)

1.2 – OGMA´s Test Cell

OGMA´s test cell is located at the engine test building in the northwestern section of the

OGMA’s campus in Alverca, Portugal.

1.2.1 - Historical review

OGMA´s test cell was built to test military reactors for the Portuguese air force fighters.

The exhaust gases of these engines reach temperatures of about 1800 ºC during the full-

afterburning regime and temperatures between 500 ºC and 700 ºC during maximum take-off

regime. This means that the main design concern was to cool down the exhaust gases before

delivering them to the atmosphere. On the other hand, the overall air flow rate was small.

The temperature of the exhaust gases of modern commercial engines, which have a

higher by-pass ratio, is much lower. The engine’s by-pass ratio is defined as the ratio between

the mass flow rate of the bypass stream and the mass flow rate entering the core of the engine.

Due to dilution, the temperature is low, while the air flow rate is much greater.

6

.

OGMAS’s test cell was designed by Lufthansa in 1981, for six different types of engines:

- Rolls-Royce Orpheus 803D-11, General Electric Engine J79, General Electric Engine J85

and General Electric Engine J69: jet engines that admit a maximum of 70 kg/s of air during

take-off.

- Pratt&Whitney JT8D and Pratt&Whitney TF30 P408: Low-bypass (0.96:1 and 0.878:1

respectively) turbofan engines that admit an air mass flow of about 140 kg/s during take-off

at sea level.

In 1997, the cell was modified to test the Rolls-Royce 3007, a high-bypass turbofan engine

that admits an air mass flow of about 140 kg/s during take-off at sea level conditions. This engine

is currently tested at OGMA´s test facility without problems.

With market growth, the interest in testing new engines increased. OGMA is now looking

forward to test the engines IAE V2527_A5, V2528_D5, V2530_A5: all high-bypass turbofan

engines with higher admission airflows and bigger dimensions than Rolls-Royce 3007.

7

1.2.2 – OGMA´s test cell configuration

OGMA´s test cell is a U-type test cell with two vertical stacks, the inlet and the exit one.

The admission tower has a cross sectional area of 36 m2. The air enters through lateral blinds

and screens with a porosity of 94%, located on the upper level of the tower admission. The body

of this tower is internally divided by 12 acoustic barriers with a height of 4.5 meters and a width

of 12 cm. These acoustic barriers absorb part of the noise generated by the engine.

1 – Exit of gases 4 – Test Cell room 7 – Air Intake Test

2 – Air mixing conduct 5 - Cradle

3 – Augmenter tube 6 – Engine reception room

The engine is mounted on a measuring thrust bed, centered vertically and horizontally

within the cell. The outlet bell-mouth is close to the exhaust section of the engine. After the bell-

mouth there is an augmenter tube, that acts as an air pump to drive the fresh air through the

whole facility using the high momentum of the exhaust gases of the engine. After the augmenter

Figure 4-Test cell stations (image from OGMA)

8

Figure 5-OGMA´s test cell (image from OGMA´s)

tube, the airflow makes a 90 degree turn to the top of the cell where it mixes with the ambient.

This exhaust stack is important to dilute the combustion gases before delivering them to the

atmosphere.

As illustrated in figure 5, the test cell room is equipped with one hydraulic platform to

provide easy access to the engine and one cradle with two cranes where the engine adapter is

installed and locked.

For safety reasons, this room has thick concrete walls, a door for safe exit and fire

protections.

9

1.3 - Challenges of OGMA´s test cell

Designed a few decades ago to test military jets, OGMA´s test bench reveals several

aerodynamic instabilities when bigger and more powerful engines are tested, having higher

dilution rates and requiring higher air flow rates.

The most relevant instabilities arise from flow separation at the roof´s corner and other

boundary layer separations associated with the positive pressure gradient of the peripheral air,

around the engine. The separated vortices are, in some cases, ingested by the engine or may

enter, in other cases, in the augmenter tube, changing its pressure recovery and, therefore, the

whole flow rate through the test cell. In any case, the unsteadiness of the flow produces important

variable forces, affects the velocity direction at the engine’s inlet and perturbs the combustion

inside the engine. The ingestion of vortices causes engine surge problems due to momentarily

blocking of the intake of air or by altering the angle of attack of the flow with the blades of the

engine.

1.3.1 - Flow separation in the edge near the roof

After going down through the inlet tower, the flow goes around a sharp edge, at the

entrance of the test section. Figure 6, a historical drawing of this kind of edge flow, highlights the

main features. First of all, the flow separation near the roof wall, being undesirable in itself,

becomes dangerous when the separated flow does not re-attach and is carried out by the main

stream. Given the location of the engine, shortly downstream of the edge, the possibility that this

mass of separated flow be captured by the engine’s bellmouth is great. If it escapes the engine,

this mass can survive till the augmenter tube. Secondly, the main stream exhibits strong curvature

near the edge, this misalignment of the flow entering the engine’s bellmouth is also undesirable.

Figure 6 also represents, symbolically, a region of separated flow at the outer edge of the corner.

This separated flow is also unstable because it may also be carried out downstream.

Figure 6 - Flow separation at a corner [9]

10

All these effects become more problematic with bigger and more powerful engines. The

engine’s inlet gets closer to the test cell walls, to the roof’s edge and all this is enhanced by greater

fractions of air being ingested by the engine. Indeed, OGMA´s test cell proportions suggest that

the intake of more powerful engines is very close to the critical separation surfaces.

A first attempt used to try to increase the uniformity of the flow at the entrance of the

engine at OGMA´s facility was implemented by placing a grid of flow rectifier channels in the

section downstream the corner. However, as it is explained in chapter 2 these rectifiers have a

high head loss coefficient, causing very high cell depressions during the engines´ tests.

One efficient way of avoiding the separation at the corner is the installation of guiding

vanes. This improvement is analyzed in detail in section 4.3.

11

Figure 7 -Flow near the engine inlet

1.3.2 - Separation along flat walls caused by the local

distortion of the flow near the engine’s inlet

. Even if there was no edge near the roof, the engine’s inlet flow can cause boundary

layer separation at the walls, roof and ground.

The cross-sectional area of the stream tube that enters the engine shrinks quickly as the

flow accelerates near the bellmouth, as represented schematically in Figure 7. As a consequence,

the cross-sectional area of the peripheral flow (dashed lines) enlarges, yielding a local adverse

pressure gradient. This pressure gradient may induce a boundary layer separation over the walls,

roof and ground. Again, these separations tend to be unstable, especially when a great amount

of fluid is suddenly carried out with the main flow. The adverse pressure gradient is more severe

when a large fraction of the air flow enters the engine, because this means a greater reduction of

the stream tube that enters the engine and, by the same token, a greater enlargement of

peripheral stream tube and a greater pressure increase.

One way of preventing the boundary layer from separating is to increase the velocity of

the fluid that enters the facility. A higher velocity of the peripheral flow increases its momentum

and, more significantly, a higher velocity of the stream tube aspirated by the engine reduces the

variation of its cross section, and of the cross section of the peripheral flow. The air flow rate is

mainly governed by the pressure increase along the augmenter tube.

A low velocity of the peripheral flow can also lead to recirculation of the hot gases. The

recirculation of the exhaust gases generates temperature gradients at the engine inlet creating

temperature distortions that can affect the engine performance, and in more serious cases engine

surge.

However, a high velocity of the peripheral air may affect the correction factor, resulting on

less reliable thrust correlations and causing excessive loads on the walls and thrust stand.

12

So, a balanced value for the velocity of the peripheral air of the test cell is convenient to

run the tests in god conditions.

As it is explained in section 4.2 the installations of two ramps at the inlet of the engine or

the increase of the augmenter tube are some of the solutions presented to increase the velocity

of the peripheral air near the inlet of the engine that can be applied to avoid vortex formation

13

Chapter 2

Literature Review

2.1 – Boundary layer separation over flat walls near

the engine’s inlet

As the airflow approaches the bell mouth of the engine it accelerates. The cross-sectional

area of the airflow entering the engine is smaller than the cross-sectional area of the test cell, that

peripherical airflow that passes between the engine and the internal walls is quantified by a cell

bypass ratio (CBR).

𝐶𝐵𝑅 = 100% (

𝑚𝑐𝑒𝑙𝑙̇ − 𝑚𝑒𝑛𝑔𝑖𝑛𝑒̇

𝑚𝑒𝑛𝑔𝑖𝑛𝑒̇)

(1)

Where 𝑚𝑐𝑒𝑙𝑙 is the mass flow rate at the inlet of the test cell and 𝑚𝑒𝑛𝑔𝑖𝑛𝑒 is the mass flow

rate that passes through the engine (both fan and core).

It is known (explained in section 1.3) that under certain conditions inlet vortices can form

and be ingested by the engine. This can result in an engine surge event, stopping useful testing

from being conducted. The formation and generation of vortices during tests in enclosed test

facilities is not a new phenomenon and has been investigated quite extensive in the past.

Freuler and Dickman [2] used a 1/17 scale model of a full-scale jet engine test cell to

study the influence of the inlet shape, the dimensions of the test cell, the exhaust systems, the

length/diameter ratio of the augmenter and the geometry of the diffusor in the probability of vortex

formation inside the chamber.

14

Figure 8- Formation of vortex as a function of the velocity ratio and CBR [2]

As shown in Figure 8, the formation of vortices occurs at velocity ratios below 0.5. This

ratio is a function of velocity of the front cell airflow and the by-pass airflow velocity. For all three

cross sections studied, vortices were created at cell bypass ratios below 0.75.

Ho at al. [3] studied the effect of the upstream velocity gradient on the formation of

vortices. They concluded that a low velocity region near one of the horizontal surfaces would force

the vortex to detach from that surface. They showed that separation at the cell wall is likely to

occur for a deceleration velocity ratio on the order of 0.4 and 0.5. They also argue that to avoid

vortex formation a CBR of 90% was required.

It is commonly accepted in the industry that to avoid vortex formation and detachment a

test cell must have a cell by pass ratio of more than 80%. Typically, test facilities are now

designed to have CBRs between 100% and 200%.

The problem of vortex formation and ingestion can be eliminated if the velocity of the

peripheral flow increases. These higher velocities can be achieved by improving the performance

of the augmenter tube. As it is explained in detail in section 2.4, If the length of the augmenter

tube increases, the pressure in the surroundings of the engine inlet decreases, increasing the

velocity.

15

2.2 - Thrust correlations

Testing an engine in an outdoor test stand without wind must give the correct engine

thrust. However, that does not happen when testing an engine in an enclosed test cell due to the

peripheral flow that by-passes the engine. This flow creates aerodynamic effects that produce

forces that act on the engine. To be able determine the equivalent outdoor performance some

corrections must be done to the thrust measurements. These correction factors depend on the

configuration of the test cell, the position of the engine, the bell mouth and the airflow of the

engine.

Ideally these correlations are calculated comparing the engine performance in an outdoor

free field test facility with the results obtained in an indoor test facility. However, to perform outdoor

tests ideal weather conditions and no wind are required and high levels of pollution and noise are

generated, so alternatives have been developed.

The main aerodynamic effects that affect the thrust are the inlet momentum drag, the bell

mouth and thrust stand drag and the static pressure drag.

Inlet momentum drag- Inlet momentum drag is the most significant aerodynamic component of

the thrust measurement. As a result of drawing air into the test cell, a force is produced on the

engine. When an engine is tested in an outdoor stand (static engine testing), the magnitude of

this force may be substantial :1 to 10 percent of the measured thrust [4]. Since this force is a drag

term, it must be added to the measured thrust of the engine. The inlet momentum is a function of

the engine inlet airflow and the approach airflow velocity in front of the engine. The approach

engine velocity is affected by the geometry of the test cell and quantity of cell flow.

Bell mouth and Thrust stand drag- The forces applied upon the supports of the thrust cradle

and bell moth by the airflow that passes through the testbed enclosure. The force of the thrust

stand depends on the drag coefficient of the structures, the velocity of the peripheral flow and the

area of the thrust stand:

𝐹𝑡ℎ𝑟𝑢𝑠𝑡 𝑠𝑡𝑎𝑛𝑑

= 𝐶𝐷1

2𝜌𝑝𝑒𝑟𝑖𝑠𝑝𝑣𝑝𝑒𝑟𝑖𝑠𝑝

2𝐴𝑠𝑡𝑎𝑛𝑑

(2)

16

Static pressure drag - This force is caused by the acceleration of the peripheral flow that

bypasses the engine to enter the augmenter tube, this phenomena causes pressure gradients.

The sharp decrease of pressure between the inlet and exhaust of the engine cause horizontal

forces that act in opposite direction of the thrust. According to Advisory circular [4] this force is

extremely sensitive to the cell exhaust geometry and spacing between engine exhaust and the

cell exhaust.

The thrust measured may be calculated by applying the momentum equation for a control

volume corresponding to the engine:

∫ 𝜌𝑣𝑥

𝑆

(�⃑�. �⃑⃑�)𝑑𝑆 = ∫ −𝑝𝑛𝑥𝑑𝑆

𝑆

+ 𝐹𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑

𝐹𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 = 𝜌𝑝𝐴𝑝𝑣𝑝2 + 𝜌𝑠𝐴𝑠𝑣𝑠

2 − 𝜌𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔𝐴𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔𝑣𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔2

+ (𝑃𝑒𝑥𝑖𝑡,𝑒𝑛𝑔 − 𝑃𝑖𝑛𝑙𝑒𝑡,𝑒𝑛𝑔) ∗ 𝐴𝑁𝑜𝑧𝑧𝑙𝑒 − 𝐹𝑡ℎ𝑟𝑢𝑠𝑡 𝑠𝑡𝑎𝑛𝑑

(3)

Where the subscripts p and s refer to the primary and secondary jet of the engine respectively.

17

2.3 - Cell depression

Cell depression is defined as the difference between the ambient pressure and the static

pressure inside the test cell. Upstream of the engine, this depression is mainly due to the dynamic

pressure and the head losses caused by the acoustic panels, the bends and the screens located

at the inlet of the facility.

The dynamic pressure effect is unavoidable at the bellmouth, for a given geometry of the

bellmouth and a given flow rate through the engine, but the head losses can be minimized. A

higher air mass flow rate allows a higher fuel mass flow rate in the combustion chamber and

hence a higher power through output of the whole engine.

Based on experience of existing test cells, Jacques [6] recommends to limit the

depression to less than 150 mm H2O (1471 Pa).

A very low cell depression usually means a small air flow rate entering the facility. This

causes a more pronounced shrinkage of the streamtube entering the engine and a greater

enlargement of the peripheral flow. Boundary layer separations are prone to occur near the walls,

perturbing the flow, and in more severe cases, can lead to exhaust gases re-ingestion.

A higher cell depression than the recommended may reduce the air density and, in virtue

of the heavy head losses, can affect the stability of the air flow around the engine and cause

excessive unsteady loads on the test cell that can affect the accuracy of the measurements.

Cell depression may be expressed by a generalized Bernoulli equation (taking into

account head loss coefficients) applied to the airstream between the atmosphere (∞) and the

section upstream to the engine’s bellmouth, where the flow is still uniform (section 4, Figure 4):

𝑝∞ + 𝜌𝑔𝑧∞ +

1

2𝜌𝑣∞

2 = 𝑝4 + 𝜌𝑔𝑧4 +1

2𝜌𝑣4

2 + ∑ 𝐾1

2𝜌𝑣2

(4)

K denotes the overall head loss coefficient, made dimensionless by the reference dynamic

pressure.

In OGMA´s test facility the components that contribute more to the depression are the

inlet screens (that prevent birds and foreign objects to enter the test cell), the acoustic barriers

(aiming at reducing the noise to an acceptable limit), the sharp elbow between the vertical stack

and the roof of the testing room and the grid of flow rectifier channels (placed by OGMA as an

attempt to uniformize the flow at the engine´s inlet).

18

Freuler et al. [2] tested three model test cell inlet configurations: one horizontal inlet,

one vertical with turning vanes and one vertical truncated inlet as it is illustrated in figure 11.

Figure 9 - Different inlet configurations: a) horizontal inlet b) vertical inlet c) vertical truncated inlet [2]

Figure 10 - Cell depression obtained for different inlet configurations [2]

As a result, the horizontal inlet without flow screen or vanes obtained the lowest cell

depression but provokes the less uniform flow (highest flow distortion). In the ideal case a

compromise between cell depression and flow distortion is required.

OGMA´s test cell has a vertical entrance with no guiding vanes. As would be expected,

the values for the cell depression obtained during the tests on OGMA´s test cell are high. A

horizontal inlet or a vertical inlet with guiding vanes would lead to a lower cell depression than the

actual configuration.

19

The cell depression is one of the limiting factors for extending the range of possible

engines tested in OGMA´s test facility.

2.4 - Augmenter Tube

The augmenter tube pumps the flow from the test cell providing a pressure increase at

the expense of the high momentum of the exhaust jet of the engine. The mixing of this jet with

the entrained air reduces the momentum flow rate that produces the pressure increase as in a

normal ejector pump.

In an ideal ejector, the exhaust jet and the peripheral air that enters the augmenter tube

are almost completely mixed at the end. In the augmenter tubes of standard engine test cells, the

length is usually shorter, limiting the pressure increase to a convenient level.

The adequate performance of the augmenter tube has a great influence on the overall

quality of the test cell. The dimensions of this tube affect the pressure. A greater diffusion in the

augmenter tube results in higher air flow rates across the facility and a greater depression. As

explained in section 1.3.2 , the velocity of the peripheral flow is an important parameter to control

the boundary layer separation. The diameter of the tube, the length and its position relative to the

engine nozzle affect the depression created and therefore the amount of peripheral airflow.

The ratio between the diameter of the augmenter tube and the diameter of the engine´s

nozzle influences the quantity of peripheral airflow that enter the augmenter tube. One important

parameter to evaluate the efficiency of the tube is the entrainment ratio, defined as the ratio

between the mass flow of the peripheral air and the mass flow of primary air that enters the

augmenter tube.

γ =�̇�𝑝𝑒𝑟𝑖𝑠

�̇�𝑗𝑒𝑡

.

Choi and Soh [7] analyzed the flow field of a two-dimensional ejector system. According

to their experimental results, [Figure 11(a)], the ratio (AR) between the area of the augmenter

tube and the area of the nozzle of the engine is an important parameter to control the efficiency

of the pumping of the peripheral air. Choi and Soh [7] made computations for nozzle area ratios

(AR) of between 1 to 12 and for nozzle pressure ratios of 2.5, 3.5 and 4.5. It is possible to see

that the pumping characteristic is low at high area ratios. If AR is too high, the flow goes directly

20

through the augmenter tube without giving adequate pumping, if too low the jet of engine can

block the incoming flow. The pumping is measured in terms of entrainment mass of the peripheral

air per unit of area

Walsh and Fletcher [15] suggest that to avoid undesirable flow phenomena and to

minimize measures thrust deficiency the diameter of the augmenter tube should be of around

three times the engine nozzle diameter.

There is an optimum length for the augmenter tube, below that the two streamlines do

not completely mixed and above the optical length the pressure recovered starts to decrease due

to the wall friction losses.

According to Jacques [6] it is necessary a ratio between the length and the diameter of 8

to obtain the maximum airflow pumping efficiency. In OGMA´s test cell that ratio in 2.55. This low

ratio does not provide a sufficiently long flow passage for the total mix of the primary and

peripheral streams. OGMA´s test cell tube has a low increase of pressure and consequently a

low efficiency.

Brian Quinn [8] analyzed the quantity of peripheral flow entering the augmenter tube as

a function of the pressure and temperature of the primary jet and as a function of the length of the

tube. In his experiments he used an aluminum tube with an inlet area ratio (A1/A0) of 25.8.

To measure the pressure along the tube he placed 4 pressure taps spaced with 90

degrees around the mixing tube in sections separated by 0.25in. Brian [8] measured the rate at

which the peripherical flow enters the tube as the primary stagnation pressure was increased in

regular increments. In order to study the effect of the length, at the end of each measurements, a

small section of the tube was cut. This experiment allowed the study of the effect of the

temperature of the jet and the length of the tube on the pressure recovery.

a) Choi and Soh [7] experimental results b) Scheme of the augmenter tube-Brian[8]

Figure 11 – Augmenter´s performance experimental data

21

. Brian Quinn [8] defined the mass augmentation ratio as:

𝜓 = 𝜃−1/2

𝑚1̇

�̇�0

(5)

Where the subscript 0 corresponds to the jet air flow expelled from the reservoir and the

subscript 1 to the peripheral airflow that is pumped into the augmenter tube. 𝜃 is the primary

temperature ratio: ratio between the temperature of the primary fluid expelled from the reservoir

and the ambient temperature. 𝜋 refers to the primary pressure ratio (primary pressure over

ambient).

Figure 12 – Mass Augmentation ratio as a function of the length of the tube [8]

Figure 12 shows the results of his experiments. It is possible to conclude that increasing

the length up to 8 diameters the mass augmentation ratio also increases, since more time is

providing to the primary flow to transfer its energy to the entrained stream. After L/D=8 the

performance of the ejector decreases because of the effect of the wall friction. If considered that

22

the maximum performance is reached at L/D=8 ( 𝜓 ≈ 4.5, changing slightly with 𝜋) , considering

that OGMA´s augmenter has a value of L/D=2.5 (𝜓 ≈ 1.3, also changes slightly with 𝜋) it is

possible to conclude that the augmentation performance of OGMA´s tube is nearly 30% (𝜓 =

1.3

4,5= 0.28).

2.4.1 - Theoretical Analysis of the Augmenter tube

Figure 13 - Scheme of the Augmenter tube

To have a first approximation of the performance of an augmenter tube, a one-

dimensional analysis can be performed.

To study the pressure variation along the augmenter tube, the mass conservation

equation and the linear momentum equation along x are solved for the control volume between

A and B:

�̇�𝐴 = �̇�𝐵

�̇�1 + �̇�2 = �̇�𝐵

𝜌1𝐴1𝑣1 + 𝜌2𝐴2𝑣2 = 𝜌𝐵𝐴𝐵𝑣𝐵

(6)

Applying the momentum equation between A and B:

𝑑

𝑑𝑡∫ 𝜌𝑣𝑥𝑑𝑉 +

𝑉

∫ 𝜌𝑣𝑥

𝑆

(�⃑�. �⃑⃑�)𝑑𝑆 = ∫ −𝑝𝑛𝑥𝑑𝑆

𝑆

+ 𝑓𝑣𝑖𝑠𝑐𝑜𝑢𝑠

(7)

23

As the regime is steady, the viscous forces are negligible, and it is considered that the

velocity profile at the end of tube is uniform it is possible to write:

∫ 𝜌𝑣𝑥

𝑆

(�⃑�. �⃑⃑�)𝑑𝑆 = ∫ −𝑝𝑛𝑥𝑑𝑆

𝑆

(8)

∫ 𝜌𝑣𝑥

𝐴

(�⃑�. �⃑⃑�)𝑑𝐴 = 𝜌1𝑣12𝐴1 + 𝜌2𝑣2

2𝐴2

(9)

∫ 𝜌𝑣𝑥

𝐴

(�⃑�. �⃑⃑�)𝑑𝐴 = 𝜌𝐵𝑣𝐵2𝐴𝐵

(10)

Where the subscript 1 corresponds to the jet air flow from the engine and the subscript 2

to the peripheral airflow that is pumped to the augmenter tube, as it is illustrated in figure 13.

To simplify the problem, it is assumed that the pressure at the end of the augmenter is

the atmospheric and as the area of the augmenter tube is constant this leads to:

𝑝𝐵 − 𝑝𝐴 =

𝜌1𝑣12𝐴1 + 𝜌2𝑣2

2𝐴2 − 𝜌𝐵𝑣𝐵2𝐴𝐵

𝐴𝐴𝑢𝑔

(11)

Ideally, the two flow streams entering the augmenter would attain a velocity of 𝑣𝐵, if they

are totally mixed. However, the pressure gained by the augmenter tube is limited by its length.

As OGMA´s tube has only 5 meters, at the end of the tube the flow profile is not fully

uniformized.

24

2.5 - Desired cell performance

In order to have a correct operation of the test cell, some conditions must be satisfied. A

minimum ratio between the velocity of the flow that enters the engine and the velocity of the flow

that bypasses the engine is required as well as an acceptable cell depression and cell bypass

ratio.

It is possible to wider the range of engines tested in a given test facility by doing some

improvements (analyzed in section 4). In order to do so, several parameters must be within

specific ranges:

• Cell by-pass >0.8 [2],[3]

• Cell depression 0.5

[2],[3]

25

Chapter 3

This chapter presents a unidimensional model of the flow to predict the airflow rate and

test the improvements of the facility. This unidimensional model incorporates data from the

engines’ manufacturers and typical correlations from the literature. It will be validated with

measurements of real engines tested at OGMA. This check with experimental data gave us some

confidence about the ability of the model to predict the flow under modified conditions of the test

cell.

The backbone of the model is the balance of energy, including estimates of head losses,

and the mass balance. We found some information in the literature about the pressure recovery

in the augmenter tube, but not exactly for the same conditions. The pressure recovery depends

on the balance of momentum and this depends on the diffusion process. Given the scarcity of

dimensionless information, we considered that a set of CFD simulations would provide a good

estimate of the diffusion and of the pressure gain. Since our concern is the pressure increase, the

numerical model of the augmenter tube provides an adequate answer, with great flexibility

regarding the geometrical characteristics of the tube, even with coarse meshes. The numerical

model is especially useful because we are interested in a parametric study, to compare different

configurations.

26

Figure 14- Engines analyzed in this study (image from manufacturers website)

3.1 - Engines

This study is focused on the performance of the test facility during the test of four engines:

engine AE3007_8501, currently tested at OGMA without major problems, and engines IAE

V2527_A5, V2528_D5, V2530_A5: engines that OGMA is looking forward to test with the right

conditions.

All the data needed to perform this study was collected from the manufacturers’ handbooks,

except for some parameters that were calculated from measurements obtained during test runs.

Table 1 illustrates the specifications of both engines.

AE3007_8501 V2527_A5 V2528_D5 V2530_A5

Airflow 122 kg/s 367-380 kg/s 370-390 kg/s 370-400 kg/s

Length 2.93 m 3.2 m 3.2 m 3.2 m

Width 1.17 m - - -

Height 1.41 m 1.60 m 1.60 m 1.60 m

Area exhaust nozzle

0.503 m2 1.13 m2 1.13 m2 1.13 m2

Takeoff thrust 28.66-31.32 kN 110 kN 130kN 140kN

Weight 744 kg 2327 kg 2327 kg 2327 kg

Bypass ratio 5 5 5 5

Currently tested

Yes No No No

Table 1- Characteristics of the engines analyzed in the study (data obtain from manufacturers website)

The engine´s bypass ratio (not to be confused with the cell’s bypass ratio) is the ratio

between the mass flow rate that goes through the engine’s core and the mass flow rate that goes

only through the fan (bypass duct).

(a) Engine V2527_A5 (b) Engine AE3007_8501

27

3.2 - Unidimensional Model

OGMA´s test cell was divided in several sections:

Figure 15 - Test Cell sections

Sections:

∞-1 – Screen section 2-3 – Flow rectifiers 4 – By-pass flow

1-2 – Acoustic treatment 3 – Engine intake 5 – Engine exit

6 – Augmenter´s inlet 7- Augmenter´s exit

28

Energy equation

For incompressible and frictionless flows in steady-state regime, the energy equation

states that the total energy is constant along a streamline – Bernoulli equation.

𝑝 +

1

2𝜌𝑈2 + 𝜌𝑔𝑧 = 𝑐𝑜𝑛𝑠𝑡 (12)

Where p represents the static pressure, 𝑝 + 𝜌𝑔𝑧 the piezometric pressure and 1

2𝜌𝑈2 the

dynamic pressure. Considering the local hydrostatic pressure as reference, the Bernoulli

equation is simply written as:

𝑝′ +

1

2𝜌𝑈2 = 𝑐𝑜𝑛𝑠𝑡 (13)

It is now possible to relate the variations of the static pressure with the velocity variations of the

fluid.

Continuity equation

The mass is conserved along the test cell. In a one-dimensional flow, mass conservation

equation is written as:

𝜌𝐴𝑣 = 𝑐𝑜𝑛𝑠𝑡

(14)

where v denotes the average velocity in a given cross-section of area A and density ρ. The flow

is considered incompressible in the whole test cell (except in the high-speed flow that enters the

engine and the hot gases of the engine’s primary jet).

𝐴𝑣 = 𝑐𝑜𝑛𝑠𝑡

(15)

29

This equation relates the velocity and cross-sectional area at each section of the test

cell. Section 3 is composed by two regions: A region where the flow accelerates towards the

bell mouth of the engine (the mass flow rate is �̇�𝑒𝑛𝑔 ) and another region where the flow

bypasses the engine (section 4; its mass flow rate being �̇�4). This by-pass flow is treated as

incompressible. It is possible to relate the velocities in section 3 and 4:

�̇�𝑒𝑛𝑔 + �̇�4 = �̇�3

(16)

Unidimensional Model

Bernoulli equation relates the velocity and pressure of different sections, accounting also

for local head losses, if the corresponding terms are evaluated with dimensionless coefficients

and supplemented to the equation. This generalized form of the Bernoulli equation, applied along

a streamline between the outer atmosphere (section ∞) and section 6 of the peripheral flow (at

the inlet section of the cylindrical stretch of the augmenter tube), going around the engine (see

Fig. 15):

As far as the flow can be considered incompressible, the geopotential term balances the

hydrostatic pressure difference. Therefore, both contributions were omitted in (17) and the

pressure (denoted p’, with a prime sign) stands for the excess pressure relative to the hydrostatic

pressure. Since the fluid is at rest in the atmosphere, 𝑣∞ = 0:

𝐾∞−3 is the overall dimensionless head loss coefficient of the intake components. It can

be estimated with reasonable accuracy from experimental data [22], as explained in next

section.

At the end of the augmenter tube, the fluid discharges to a large channel, with an opening

in the ceiling and the walls of the channel. This flow exhibits a small pressure increase due to

momentum diffusion of the exhaust jet into the surrounding entrained air. Nevertheless, the

dimensionless length of this stretch (length over diameter) is small and the pressure increase is

probably similar to the head loss of the turning of the vertical exhaust stack. Hence, the exit of the

augmenter tube is approximately at the atmospheric pressure (p7 = p∞).

𝑝∞

` +1

2𝜌𝑣∞

2 = 𝑝6` +

1

2𝜌𝑣6

2 + ∑ 𝐾∞−31

2𝜌𝑣2𝑖

(17)

𝑝∞

` = 𝑝6` +

1

2𝜌𝑣6

2 + ∑ 𝐾∞−31

2𝜌𝑣2𝑖

(18)

30

All the remaining data to carry this study (mass flow rate of the engine, dimensions of the

engine) was collected from the manufacturers’ handbooks, except for some parameters that were

calculated from measurements obtained during test runs.

The performance of the augmenter tube cannot be predicted by simple analytical models.

Experimental measurements, extrapolated with dimensional analysis play an important role but

they are not available for the specific configuration of the OGMA facility. On the other hand, CFD

simulations are very flexible and the pressure increase is not very sensitive to the details of the

numerical model (results in chapter 3). The depression, computed with a CFD model, was applied

to close equation 18 and compute overall flow rate. Given this flow rate, the mass balance

provides the averaged velocity at every section. The, specific Bernoulli equations between every

two sections allow the pressure distribution to be computed along the test cell.

This procedure gave, during the test of a given engine: the cell flow rate, the cell by-pass

ratio and the cell depression, that are the leading parameters of the test cell’s performance.

3.2.1 - Fluid resistance coefficients

To estimate the values for the fluid resistance coefficients of the inlet components,

experimental values presented by Idelchik [9] were used.

Data about fluid resistance coefficient was scattered among various textbooks of

hydraulics and aerodynamics and in scientific papers, Idelchik [9] found the need to collect and

organize this data. The head loss coefficients collected by Idelchik [9] are checked by laboratory

studies, obtained theoretically or by approximate calculations.

These coefficients depend on several parameters and are calculated in detail in the

following sections.

31

3.2.1.1 - Inlet system

As it is shown in the figure 3, the installation has screens on the top of the

admission tower. These screens prevent the entry of birds and objects that might be in the

surroundings. As an approximation, it is considered that the screen is made of circular metal wire.

According to Idelchik [9] the resistance coefficient of a screen is a function of the screen

porosity. Porosity is defined as the ratio between the volume of the void fraction and the total

volume. OGMA´s screen has a porosity of 94%. The Resistance coefficient also depends on the

Reynolds number and k0, a correction factor that takes into consideration the state of the screen.

OGMA´s screen was installed in 1981, so it is acceptable to consider that the screen is not

perfectly clean.

As all the tests are done with high inlet velocities, the regime is always turbulent and with

Reynolds number much higher than 400. So, the final value for the resistance coefficient, ζ =

0.075 , is taken from diagram 8-6 of [9]. As would be expected, the value of this coefficient is low

due to the high porosity of the screen.

3.2.1.2. - Inlet baffles

In the interior of the admission tower there are twelve parallel acoustic barriers with a

length of 6 meters and a width of 20 centimeters. In this area the flow is incompressible. In order

to calculate the fluid resistance coefficient of the baffles, it is considered that the baffles are in

good conditions, have sharp tips and the flow does not have more obstructions. As the baffles

have sharp tips, flow separation may occur at the end of the baffles but will not be considered.

Figure 16 - Inlet screen

made of circular wire

32

According to Idelchik [9] the fluid resistance coefficient depends on the shape of the

grating baffles, the angle of inclination of the baffles towards the stream, the dimensions of the

baffles and the ratio between the area of the obstruction and the clear area. This resistance

coefficient is taken from diagram 8-9 of [9]. As OGMA´s baffles have sharp tips the coefficient β2

is 1. Knowing the percentage of the inlet that is open (65%) and the measures of the baffles we

can take the value of ζ´ from diagram 8-4 of [9]. The head loss coefficient is:

ζ´= β2 ∗ ζ´ ∗ senθ = 1.6

(19)

3.2.1.3. - Sharp curve – roof´s corner

Coefficients associated with sharp elbows in rough walls are presented in diagram 6-7 of

Idelchik´s Handbook [9]. These coefficients depend on the angle of curvature (δ = 90º), the

relativity roughness of the wall, the dimensions of the tunnel and the Reynolds number. The

coefficient is calculated using equation 20.

𝜁 = 𝐾𝐷𝐾𝑅𝑒𝐶1𝐴𝜁𝑖

(20)

The Reynolds number is calculated using the inlet velocity which varies between 5.5 m/s

(engine currently tested at maximum power) and 21 m/s (more powerful engines that OGMA

wants to test with some alterations of the test cell). So, Reynolds number is always larger than

4.104.

KD and KRe both depend on the Reynolds number and on the relativity roughness of the

wall and are available on table 6-11 of [9]. It is considered that the tunnel is made of concrete with

a coarse surface and has a mean height of roughness peaks of 9 mm. A and ζi depend on the

angle of the curvature and have values of 1.2 and 0.99, respectively.

The final value of the fluid resistance coefficient is 1.782.

Besides the creation of non-uniform streamlines this curve also has a great contribute to

the cell depression that cannot be higher than 150 mm H2O (1471 Pa)

33

3.2.1.4- Grid of flow rectifier channels

In order to try to uniformize the flow upstream the engine bell mouth, a grid of flow

rectifiers was introduced after the sharp elbow. This grid was improvised with the cylindrical parts

of industrial drums, with about 1 meter of diameter and a slightly longer length. This measure

helps to increase the uniformity of the flow at the entrance of the engine but do not create a

velocity profile totally uniformized.

The flow upstream of the rectifying drums arrives at an angle and produces massive

separations inside the drums. A further difficulty of this analysis is the fact that the drums’ head

loss induces a by-pass flow around the array of drums, changing the flow rate distribution.

Therefore, it is difficult to estimate the head loss coefficient due to this device without experimental

values of this specific configuration or numerical simulations requiring very fine meshes. Our

strategy to evaluate the head-loss coefficient of this array of drums inside the test cell was based

on the experimental values of the cell depression obtained by OGMA during a number of engine

tests and the values of the cell depression estimated by our unidimensional model for the same

flow rates. The head loss coefficient that fits the unidimensional model is approximately 9.415. A

high value was expected since the flow impinges in the drummers’ edges with a high angle of

attack, producing extensive separations that block the inner cross-sectional area of the drums.

As a consequence, the fluid velocity increases inside the drums and gives rise to a large energy

dissipation.

3.2.1.5- Exhaust system

OGMA´s exhaust system is composed by an augmenter tube and an exhaust stack. This

system is of great importance because it controls the back pressure of the engine and the

secondary airflow that passes through the test cell.

The exhaust stack expels the diluted combustion gases into the atmosphere. This stack

must reduce the air velocity to minimize noise generation. According to Jacques [6], the maximum

exhaust stack flue velocity should be limited to 30-40m/s to avoid excessive noise generation. At

the same time, the airflow at the stack must have sufficiently high velocity to carry the exhaust

gases into the atmosphere.

34

3.3. - CFD simulation of the flow inside the

augmenter tube

To calculate the pressure gain in the augmenter tube during the test of each engine, a

set of CFD simulations were performed.

The values for the velocity at the inlet of the test cell, the engines´ exit velocity and mass

flow rate were set as boundary conditions to match the real values obtained by OGMA during the

test of these engines.

The main objective of the CFD simulations is to determine the pressure gain in the

augmenter tube during the tests. As an attempt to simplify the model and obtain better

convergence results, the inlet tower of the test cell was not considered. OGMA´s test cell has a

grid of flow rectifier channels in the section downstream the corner. The geometry analyzed with

CFD starts after these rectifiers and it is considered that at that section the flow has a uniform

velocity profile.

3.2.1 - RANS and Turbulence models

The Mathematical model used to solve this problem is the Reynolds-Average Navier-

Stokes Equations (RANS).

In this case turbulence was modelled with SST K-ω. This model was chosen because it

combines the best features of the K-ε turbulence model in the free stream region with the k- ω

near the walls.

According to the STAR CCM user guide [10], one reported advantage of the K-ω model

over the K-ε model is its improved performance for boundary layers under adverse pressure

gradient.

The region near the wall was modelled with wall functions.

35

Figure 17 - Test cell geometry

3.2.2 - Geometry

The geometry was imported from Solidworks as a Parasolid file. To reduce computational

expenses, it was only considered the fourth part of the geometry with two symmetry planes.

The geometry studied has a height and a width of 6 meters and a length of 12 meters.

The injector tube has a diameter of 1.96 meters and a length of 5 meters. Due to the different

dimensions of the engines, the engine´s geometry was slightly changed. Figure 17 illustrates the

geometry.

36

3.2.3 - Boundary conditions

Defining proper boundary conditions is an important factor of the model.

The parameters that were used to impose the boundary conditions were retrieved from data

of engines manufactures and from OGMA´s measurements taken during the test of the engines

and are showed in table 2.

Engine �̇�𝒆𝒏𝒈𝒊𝒏𝒆 Engine´s exit

velocity

𝑽𝟑

AE3007_8501 122 kg/s 298m/s 5.5 m/s

V2527_A5 367 kg/s 360m/s 12.8 m/s

V2528_D5 377.8 kg/s 452m/s 13.1 m/s

V2530_A5 385.5kg/s 433m/s 14.6 m/s

Table 2 - OGMA´s tests results

Figure 17 illustrates the boundary conditions applied in each boundary:

1 – Velocity inlet- The inlet stack of the test cell was not simulated. A uniform velocity equal to

the velocity measured in that section by OGMA during the test of each engine was defined for

this boundary.

2- Mass flow inlet- A mass flow inlet boundary represents an inlet for which the mass flow rate

is known. The value for the mass flow rate of each engine, was imposed in this boundary.

3 - Velocity Outlet – To simulate the escape velocity of the jet, a velocity outlet was imposed. To

approach the problem in the most realistic manner, a specific field function was created for each

engine simulated, as an attempt to emulate the gradient of velocities present in the two streams

of the jet, the primary and the secondary stream.

37

Field Function

𝑽𝑨𝑬𝟑𝟎𝟎𝟕 = 𝟑𝟔𝟎 − 𝟐𝟎𝟎 ∗ 𝒓

𝑽𝑽𝟐𝟓𝟐𝟕 = 𝟔𝟎𝟎 − 𝟓𝟎𝟎 ∗ 𝒓

𝑽𝑽𝟐𝟓𝟐𝟖 = 𝟔𝟑𝟎 − 𝟓𝟎𝟎 ∗ 𝒓

𝑽𝑽𝟐𝟓𝟑𝟎 = 𝟕𝟎𝟎 − 𝟓𝟎𝟎 ∗ 𝒓

Table 3 - Boundary conditions: field functions

Where r represents the radius of the of the exhaust nozzle of the engine, being 0 at the

engine´s longitudinal axis.

4 - Pressure Outlet-

At the exit of the augmenter tube, a pressure outlet was imposed. This condition requires the

specification of the static pressure at the boundary. As an approximation, it was defined that at

the end of the tube the pressure is equal to the atmospheric, setting the static pressure to 0.

Other Boundary conditions:

Symmetry plane - The shear stress is set to zero in the boundaries where this condition is

applied. These boundaries were set as a mean to create a realistic simulation using only one

quarter of the test facility.

Cell and engine walls - In the cell and engine walls, the boundary condition was defined as wall.

This condition sets zero velocity at the surface (No-slip condition).

38

3.2.4 - Mesh

In order to reduce the influence of false diffusion which is a large source of error in the

numerical results and with the objective to achieve results as accurate as possible a very refined

mesh should be created. However, taking into consideration the processor capacity a mesh with

a characteristic dimension of 1cm was used, yielding a domain with about 1 million cells.

It was chosen to discretize the domain with the trimmed cell mesher.

According to the STAR CCM user guide [10] the trimmed cell mesher provides a robust and

efficient method of producing a high-quality grid for both simple and complex mesh generation

problems. It combines several highly desirable meshing attributes in a single meshing scheme:

• predominantly hexahedral mesh with minimal cell skewness

• refinement that is based upon surface mesh size

• surface quality independence

39

3.2.5 - Mesh dependence study

To check the exponential dependence of the results on the mesh size, simulations were

performed with four different mesh sizes ranging from 0.5 to about 1 million cells.

The exponential fit of the results yield a projected pressure difference p∞

- p6

= 6212 Pa.

According to that fit, the error estimate of a N-cells mesh (the characteristic length of the cells

being proportional to “h” = N-1/3) is

Error estimate = A exp (-B N1/3)

With A = 2.936E-10 Pa and B = 0.2209 m.

Table 4 summarizes the results of the mesh sensitivity study. The first column contains

the number of cells of the mesh. The second column contains the pressure increase in the

augmenter tube, in Pascal. The maximum pressure difference of these meshes is 9%.

Mesh size p∞

- p6

“h” Error

estimate

906765 6227 Pa 0.01033 15 Pa

693389 6124 Pa 0.01129 95 Pa

533005 5727 Pa 0.01233 489 Pa

516995 5591 Pa 0.01246 586 Pa

Table 4 -Mesh dependence

The fourth column of Table 4 is an error estimate of the pressure recovery based on the

above exponential fit. The log-plot of this error estimate suggests that the characteristic

convergence rate is indeed exponential after a reasonable mesh-density is achieved.

40

Figure 18 - Error estimate (log-scale) versus the characteristic length.

y = -0,2209x + 24,10282,5

3

3,5

4

4,5

5

5,5

6

6,5

80 85 90 95 100

Loga

rith

m o

f er

ror

esti

mat

e

1/"h" = N^(1/3)

906765 Cells

516995 Cells

693389 Cells

533005 Cells

Figure 19 - Results obtained for the pressure inside the test cell simulating the test of the engine V2527 with

different meshes

41

Figure 21 - Results obtained for the Pressure inside the test cell during the test of engine AE3007

3.3 – Results: Pressure recovered by the Augmenter

tube

Figure 20 - Results obtained for the pressure inside the test cell during the test of engine V2527.

Figure 20 illustrates the pressure gain in the augmenter tube during the test of engine

V2527. As analyzed is section 2.4, the pressure recovered by the augmenter is limited by its

length and is an important parameter because it controls the velocity near the entrance of the

engine. Until a certain limit, the longer the tube, the higher the pressure recovery. During this test

the pressure recovered is about 6227Pa.

Figure 21, 22 and 23 show the pressure variation in the test cell during the test of the

engine AE3007, V2530 and V2528 respectively. As expected, as the flow accelerates towards

the bell mouth of the engine, the pressure decreases.

It is possible to see that the pressure recovered by this augmenter tube (between section

6 and 7 of figure 26) during these tests is about 1280Pa, 7643Pa and 9550Pa

42

Figure 22 - Results obtained for the Pressure inside the test cell during the test of engine V2530.

Figure 23 - Results obtained for the Pressure inside the test cell during the test of engine V2528.

Table 5 summarizes the pressure gain in the augmenter tube during the test of the four

different engines computed with CFD simulations.

Engine P6-P∞

AE3007_8501 1280Pa

V2527_A5 6227Pa

V2528_D5 7643Pa

V2530_A5 9550Pa

Table 5 - Pressure computed by the CFD simulations

43

To study the diffusion in the augmenter tube, the velocity profile at three sections of the

tube was obtained. Figure 24 shows the velocity profiles, the first corresponds to a profile at a

section on the entrance of the tube, the second profile is computed at a section in the middle of

the tube and the third corresponds to a velocity profile of a section in the end of the tube. It is

possible to see that at the third section the flow is not totally uniformized.

Figure 24 - Velocity profile at three sections of the augmenter tube during the simulation of the test of engine V2527

3.4 – Results

Knowing the pressure recovered by the augmenter tube and solving the equations of the

unidimensional model, it is now possible to compute the velocities and pressures at each section

of the test cell. Table 6 summarizes the values predicted by the unidimensional model during the

test of the different engines.

44

Engine �̇�𝒆𝒏𝒈𝒊𝒏𝒆 𝒑𝟔 − 𝒑∞ 𝒗𝟑

(m/s)

𝒗𝟒

(m/s)

𝒗𝟔

(m/s)

CBR Cell

depression

𝒗𝟒𝒗𝟑

AE3007 122 kg/s -1280 Pa 5.4 2.6 39.9 0.92 -465 Pa 0.48

V2527 367 kg/s -6227 Pa 12.3 3.8 88.9 0.45 -2506 Pa 0.31

V2528 378 kg/s -7643 Pa 12.6 4.2 96.9 0.49 -2643 Pa 0.33

V2530 285 kg/s -9550 Pa 13.2 4.81 112 0.57 -2950 Pa 0.36

Table 6 - Values predicted by the unidimensional model

It is possible to see that engines V2527, V2528 and V2530 do not comply with the

requirements of a correct operation in the test cell. As described in chapter 2, to prevent flow

separation from the walls, a cell bypass ratio higher than 0.8 and a ratio between the velocities

at section 4 and 3 higher than 0.4- 0.5 is needed to a correct functioning of the test. According

to this study neither the first nor the second condition are satisfied. During the test of the V

series engines, the peripheral air near the engine´s inlet decelerates too much leading to flow

separation from both walls, roof and ground. This separation can lead to vortex detachments

that may be ingested by the engine or enter the augmenter tube. The ingestion of vortices

causes engine surge problems due to a momentarily blocking of the air intake of the engine or

by virtue of excessive angles of attack at the blades of the engine. So, these three engines

cannot be tested in the best conditions at OGMA´s test facility.

To avoid flow separation at the walls of the test cell, near the inlet of the engine, the

velocity of the peripheral air needs to be increased. The alternatives will be discussed later. As

explained in section 2.4, the air flow rate depends on the pressure increase along the augmenter

tube. Therefore, increasing the length of the augmenter tube is an improvement suggested to

avoid flow separation from the walls. Another alternative is the installation of ramps at the ceiling

and roof near the engine´s inlet. These ramps would reduce the cross-sectional area of the

peripheral flow, increasing its velocity.

One can also remark that the cell depression when engines V2527, V2528 and V2530

are tested is higher than recommended. Following a streamline beginning at the outer still

atmosphere, the depression is due to the dynamic pressure and the head losses caused by the

acoustic panels, the bends and the screens located upstream of the engine.

The dynamic pressure effect is unavoidable. However, the head losses can be

minimized. The highest contribution for this depression is caused by the grid of hollow drums,

used as flow-rectifier channels. We estimate that these channels, used by OGMA, have a high

head loss coefficient of 9.415. To avoid flow separation at the roof’s corner and to reduce the cell

depression it is mandatory to install guiding vanes at the curve. This improvement will be analyzed

in section 4.3.

45

Figure 25 shows the pressure variation along the sections of the test cell predicted by

the unidimensional model when the V2527 engine is tested.

Figure 25 - Pressure at each section when engine V2527 is tested

During the test of engine AE3007, the cell bypass ratio is higher than 0.8 and a ratio

between the velocities at section 4 and 3 is higher than 0.4 – 0.5. The peripheral flow does not

deaccelerate too much, avoiding flow separation from the walls. This engine is currently tested at

OGMA´s test facility without problems.

46

3.5 - Results Discussion

To validate the model, a comparison of the values obtained for the velocities, the cell

bypass ratio and the cell depression with the values delivered by OGMA was performed. Table 7

provides a summary of the results obtained for engines AE3007_8501, V2527, V2528 and V2530:

Engine �̇�𝒆𝒏𝒈𝒊𝒏𝒆 Results

𝒗𝟑

(m/s)

CBR Cell depression Thrust

AE3007 122 kg/s 1-D 5.4 0.92 -465Pa 37575N

OGMA 5.5 1.04 -490Pa 37814N

V2527 367 kg/s 1-D 12.3 0.45 -2506Pa 117368N

OGMA 12.8 0.54 -2540Pa 117878N

V2528 377 kg/s 1-D 12.6 0.49 -2643Pa 116636N

OGMA 13.1 0.54 -2690Pa 115906N

V2530 385kg/s 1-D 13.2 0.57 -2950Pa 138681N

OGMA 14.6 0.6 -3014Pa 139674N

Table 7 - Results for the unidimensional model vs OGMA´s study

It is possible to see that for the cell velocity at section 3 during the test of engine AE3007,

the unidimensional analysis predicts a value of 5,4 m/s while OGMA´s study predicts a value of

5,5 m/s. The velocity predicted is 2% lower than the velocity predicted by OGMA´s study.

Table 7 shows that concerning the cell depression when engine AE3007 is tested, the

unidimensional model has a value 4% lower than the results from OGMA´s studies. As the head

loss coefficient was calibrated to be the same as the OGMA´s study and it depends on V3, this

difference is explained by the lower velocity calculated for section 3. The unidimensional analysis

predicts a CBR of 0,92 while OGMA´s testing results give a value of 1.04.

It is possible to conclude that the deviations of the results are lower than 10% than the

results obtained by OGMA for the same test.

Concerning the test of engine V2527, the velocity at section 3 calculated by the

unidimensional model is almost 4% lower than the obtained in OGMA´s study. Consequently, the

values for cell depression and cell bypass ratio are also slightly lower. The thrust predictions are

close to the values delivered by OGMA.

47

Concerning the last two engines, the velocity at section 3 calculated by the

unidimensional model is 4% lower for engine V2528 and 9% for engine V2530 than the obtained

in OGMA´s study. Consequently, the values for cell depression and cell bypass ratio are also

slightly lower.

This check with experimental data gave us some confidence about the ability of the model

to predict the present flow and substantiates the expectation that it can predict as well the flow

under the modified conditions of the test cell that will be analyzed in chapter 4.

48

Chapter 4

Test cell Optimization

Designed a few decades ago to test military jets, OGMA´s test bench exhibits aerodynamic

instabilities when bigger and more powerful engines are tested, having higher dilution rates and

requiring higher air flow rates. The most relevant instabilities arise from flow separation at the

roof´s corner and other boundary layer separations at the walls, around the engine, associated

with the local positive pressure gradient of the peripheral air.

The wider the range of engines tested in a particular test cell the greater the chances of

inadequate cell depression and cell bypass that may lead to vortex formation and ingestion. As it

was analyzed in section 3.4, engines V2527, V2528 and V2530 do not fulfil the adequate

conditions of a correct operation of the test cell. During the test of the V series engines, the

peripheral air near the engine´s inlet decelerates too much, leading to flow separation from both

walls, roof and ground.

Some improvements will be proposed in this chapter to increase the peripheral velocity

near the engine´s inlet, creating adequate conditions for the test of these engines. The effect of

these improvements will be assessed with the unidimensional model, validated in section 3.5.