enhancing aerodynamics performancs of naca 23012 by using

61 ا را– ا رإ دا ن در– ) ددر 1ا (1436 - رم2014د

Enhancing AErodynamics performancs of NACA 23012 Ahmed Alrayah & Obai Younis

Ahmed Alrayah ( )

Obai Younis ( )

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Enhancing Aerodynamics

performancs of NACA 23012 by

Using Backward Step



Abstract The main objective of this paper is to improve

the aerodynamics characteristics of National

Advisory Committee for Aeronautics (NACA

23012) by introducing backward facing steps.

Investigation of the influence of the step location

on pressure distributions and therefore, on lift

and drag, for a wider range of angle of attack

were considered in order to understand the

physical phenomena of flow around air foil, as

well as flow visualization. Upper, lower and

hybrid steps configurations were examined to

show their effect on lift and on lift-to-drag ratios.

The results suggest that incorporation of

backward-facing steps on the lower surface that

is located at the mid-chord and extend back to

the trailing edge with 50% depth on lower side of

the air foil thickness may lead to considerable

enhancements in lift coefficients. Drag generated

with introducing steps is very high especially

when hybrid steps are located.



1. Introduction The maximum lift of an air foil is associated

with the separation of the boundary layer on its

suction side. Thus the numerical prediction of

the maximum lift must deal with the pressure

distribution of an air foil section with partly

separated flow and with the interaction between

this pressure distribution and the boundary

layer [1]. Numerous experiments have revealed

the sequence of flow events depicted in "fig.1"

first; a vortex starts to develop near the air foil

trailing edge as the angle of attack is rapidly

increased past the stall angle. This vortex then is

converted downstream near the air foil surface

which causes an increase in lift due to the

suction (upper surface) induced by the vortex.

The magnitude of the lift increasing depends on

the strength of the vortex and its distance from

the surface. The stream wise movement of the

vortex depends on the air foil shape and the

pitch rate. As the vortex is connected past the

trailing edge, the lift briefly attains its maximum

value and then start to drop rapidly [2].



Fig.1. Sequence of flow separation.

Richard L.Kline and Floyd F.Fogleman

introduced a breakthrough concept in air foil

design with their stepped air foils developed

early. The Ultimate Paper Airplane [3] by Richard

L.Kline & F.Fogleman describes that the object of

this design was to develop an improved air foil

with enhanced lift, drag and stability

characteristics and adaptability over a wide

range of speeds. Shifting to the review of studies

conducted on stepped air foils, it was in 1994

that the benefits of KF air foils were scientifically

proven through experiments and flight testing by

Demeter G.Fertis [4]. Aerodynamic studies on

stepped air foils were conducted by Stephen

Witherspoon [5] and Fathi Finaish, for different

configurations defined by the step lengths,

depths, and the location of steps on air foil



chord. Experimental tests and numerical

investigation with steps on NACA 0012 and

23012 air foils showed that higher lift coefficients

were obtained with lower surface step located at

half-chord, extending till the trailing edge at all

angles of attack ranging from 0 to 10 deg.

Further, upper surface steps located at half-

chord and extending till 62.5% chord generated

higher L/D ratios when compared with

unmodified NACA 0012 air foil at incidence(s)

around 10deg.

Geometric optimization of lifting surfaces

during flight has the potential to enhance aircraft

performance for a wide range of flight

manoeuvres. Shorter take-off and landing

distances, slower take-off and landing speeds,

lower fuel consumption, and longer range are

just few examples of performance characteristics

that can be enhanced by optimizing the geometry

of lifting surfaces during flight. Wide range of

methods have been employed using devices such

as flaps, slats, spoilers, and drooped leading

edges. These systems are designed to maximize

aerodynamic performance for limited flight

manoeuvres/conditions relying on traditional



aerodynamic concepts that utilize attached flows

[6]. The air foil should be able to function at high

angles of attack without stalling. If we couple

these desired parameters, a multitude of air foil

designs can be developed, each having its own

aerodynamic characteristics tailored to a specific

flight condition [7]. Flow over submerged bodies

such as an aircraft and or a submarine can be

worked upon to delay boundary layer transition,

postpone separation, increase lift, reduce skin-

friction and pressure drag, augment turbulence,

enhance heat transfer, or suppress noise. Future

applications in the field of aeronautics include

providing structurally efficient alternatives to

flaps or slats; cruise application on conventional

take-off and landing aircraft including boundary

layer control on thick span-loader wings; as well

as increased leading edge thrust, and enhanced

fuselage and upper surface lift with most of the

new developments to be made pertain to the

employment of various flow control techniques [8].

The geometry of a conventional NACA 23012

airfoil was modified by introducing backward

facing steps, which forms a relatively new family

of air foil designs, popularly known as “KF (Kline-



Fogleman) airfoils”. Stepped air foils use the

concept of trapped vortex and consequently there

is the trapped vortex flow control. Flow rotation

induced near the step face generates a primary

vortex as shown in "fig.2", a secondary vortex

forms in the opposite direction after the flow

reattaches itself to the boundary. Thus an air foil

with a step traps vortices in the cavity which

primarily are the noticeable as well as

distinguishing flow features as compared with

the flow over conventional air foils. The formation

of these vortices alter the flow field

characteristics thereby altering the lift, drag

characteristics depending on whether the step is

introduced on the upper or lower edge or both

upper and lower edge of air foil [9].

Fig.2. Typical flow field around an air foil with steps on upper and

lower edges.



Investigations of the influence of the step

location on pressure distributions and therefore,

on lift and drag, for a wider range of angle of

attack were considered in order to understand

the physical phenomena of flow around air foil,

as well as flow visualization. CFD simulations

package (Ansys, Fluent) is suitable to use for

computational requirement.

It’s suggested that the resulting flow field

around a stepped airfoil must improve lift and/or

reduced drag and produce better stall

characteristics, depending on the airfoil

configuration and where the step is located. We

aimed to identify the optimum setting of steps

and the pest position on airfoil and making the

effective use of vortices resulting from stepped

configurations at various flight conditions.

2. RESULTS AND DISCUSSIONS: Four cases has been tested numerically for

modified and conventional NACA 23012, the

steps were located at upper surface, lower

surface and “upper and lower surface at the

same time”. Conventional and modified airfoils

were then tested numerically at various angles of

attack (5°, 10°, 15°, 20°, 25°, and 30°).Reynolds



number of (Re = 61082.1 ), based on the chord of the

airfoil (1m) and free stream velocity of (27.4m/s),

is considered. A finite volume method based CFD

tool which solves the governing equations of

conservation of mass and momentum was the

flow solver used for the entire numerical

analysis.

Fig.3. Computational Domain

Inlet

Velocity =27.4 m/s

Pressure gauge =0

Airfoil=wall

Outlet Pressure gauge =0



The computational domain comprises of four

blocks for base airfoil configurations, while the

single upper step configurations had five mesh

blocks also the single lower step configurations

had a same mesh blocks, while the upper &

lower step configurations had seven mesh blocks

as shown in "figs.4, 5 and 6" respectively. The

flow has been modeled as steady, incompressible

in the flow domain comprising several blocks.

The fluid chosen is air at standard atmospheric

conditions which were set as the reference values

along with the free-stream velocity set

corresponding to the flow “Re” for each

simulation run for the preliminary cases of

study. The inlet has been set as “Velocity Inlet”

which includes all the front, top and bottom

edges of the flow domain.



� �Fig.4. Block structure and Grid distribution of the flow domain over

a base air foil.

Fig.5. Blocks and Grid distribution of domain over air foil with upper

surface step.



Fig.6. Blocks and Grid distribution of domain over air foil with upper

& lower surface step.

2.1 Grid-Independence Study Grid independence is carried out to ensure

that solution is independent of the grid size. The

mesh is refined until the solution does not

change which is represent the proper mesh.

Since the grid densities normal to parallel to the

flow direction and fineness of the mesh near the

air foil surface can affect the accuracy of the

solution, the effect of these parameters was

studied so that results did not deviate much as

the mesh size was increased. The initial results

generated especially in the case of lift

computations for the Reynolds number of (Re



=6

1082.1 ), were in descent agreement with the

experimental data [4].

Fig.7. compares experimental data [4] with the preliminary numerical

results for the cases studied.

Mesh resolution can be checked by plotting

the distribution of Y+ near the air foil wall, the

values of Y+ are dependent on the resolution of

the mesh and the Reynolds number of the flow,

and are defined only in wall-adjacent cells.

In this case, the Y+ values are slightly high

and maximum is 325 as shown in "fig.8". The lift

coefficient can be better predicted if the Y+ value

is kept in the range of 50-250 [22]. In order to

meet this condition, refine the cells adjacent to

wall using adaption as shown by "fig.9"



Fig.8. Wall Y+ Plot

Fig.9. Wall Y+ Plot after adaptation

Fig.10. Adapted mesh near the wall of airfoil.



2.2 Aerodynamics Characteristics:

2.2.1 Pressure distributions

Comparison has been taken for specific angle

of attack (15°) for base, upper surface step, lower

surface step and upper & lower surface step

,(hybrid step) air foils at the same time.

Fig.11. Pressure coefficient of base airfoil at 15° angle of attack.



Fig.12. Pressure coefficient of upper step airfoil at 15° angle of attack

Fig.13. Pressure coefficient of hybrid step air foil at 15° angle of attack.



From "figs. 11 and 12 " there is clear

deviation appears when the base airfoils is

compared with upper surface step airfoil, the

coefficient pressure start at leading edge point 0

and rapidly decrease toward the trailing edge.

The Cp had a maximum of 1.02 while minimum

value recorded as -5.90 as shown by "fig.10" this

is agreement with value observed in Cp in the

same figure. The upper surface step airfoil had a

maximum Cp not exceed 1.35 and a minimum

value of Cp is -2.56.

The step reduce the area which undergoes

air pressure, this indicate that the associated lift

of lower surface step air foil is higher than any

other type of air foils i.e. (base or stepped) for the

same angle of attack 15°. It can be concluded

that the minimum value observed have smaller

value over all values of pressure in other

configuration above which mean low pressure in

upper surface of air foil and better associated lift

due to “suction” on upper surface.



2.2.2 VELOCITY CONTOURS

In “figs.12 and 13" the velocity contours are

shown for all cases base and modified air foil, a

slight difference between base and upper surface

step air foil appear as shown by "fig.12" Velocity

contour had a range of values which vary around

air foil from 0 to 6.61 m/s for base air foil and

4.12 m/s for the upper step air foil. Contour

shape around base air foil indicates that there is

no vortex formed on upper side of air foil and

hence a slight separation point. The maximum

value of lift is observed at this angle of attack 15°

for both base and lower surface step air foil. The

upper surface step air foil it’s shown clearly there

are more vortices on upper side of air foil and

hence separation point position exceeds the mid

chord. In the side of step there is low in speed at

upper surface body as indicated by blue colour.

This redaction of flow causes formation of

vortices, increasing in drag and an increase in

probability of delaying the stall. Hybrid step on

figure (15) fixed the problem faced by upper step

this is appear clearly when comparison between

figures (14) and (15) take place.



Fig.14. Velocity contours of base and upper surface step airfoil for 15° angle

of attack.

Fig.15 Velocity contours of lower surface step and hybrid step airfoil for 15°

angle of attack.



Fig.16. CL for base and modified air foils vs. angle of attack “α”

Fig (16) represents lift coefficient “Cl” versus

angle of attack “α”, the highest value of “Cl”

achieved by lower surface step air foil while the

base had a second highest value of “Cl” achieved

at 15° angle of attack. Lower surface step air foil

show good stability in stall more than other

types, upper step have no effect on aerodynamics

characteristics of air foil. However the maximum

lift coefficient increases from about 1.3 for the

base air foil to 1.4186 for the hybrid step air foil.

While the maximum drag coefficient increases

from about 0.058 for the air foil with no step to

about 0.0954 for the hybrid step air foil for the

same angle of attack 15°.



Fig.17. CD for base and modified air foils vs. angle of attack “α”

Fig.18. lift to drag ratio

In many lift-generating devices the important

quantity is the ratio of the lift to drag developed,

L/D = CL/CD such information is often

presented in terms of CL/CD versus “α” as

"fig.18". Obviously "fig.18" indicates the amount

of lift generated by the airfoil compared to its

drag. The figure indicates that the ratio

decreased by increasing angle of attack; this is

explained by the large values of drag increasing

in aerodynamic forces when modifications take

place. Results include the lift, drag coefficient

and associated angle of attack “α”. The variation



of “α” gives more good prediction of the

aerodynamics characteristics of the airfoil also

gives more visualization of the flow filed. Stall

angle has been detected at 15° angle of attack

when base airfoil is tested for Reynolds number

of (Re =6

1082.1 ) and associated (Cl=1.3). It’s

necessary to compare base airfoil results with

modified airfoil to ensure that the hypotheses

introduced early are correct. However stall has

been observed for all cases tested; when the

lower surface step is compared with base airfoil

it’s found that there is increasing in lift and drag

coefficient for the same angle of attack 15°. The

lower surface step does not delay the stall

characteristic while the upper surface step has

stability of stall has been observed at α=20°, “Cl

=1.025” and “Cd=0.252”. it’s necessary to realize

that there are highly decreasing in “Cl”

magnitude if its compared with other type of

airfoil this is indicate that the upper surface step

have no significant effect on airfoil performance

characteristics because there is decreasing in

“Cl” magnitude and increasing in “Cd” more than

other types mentioned above.



1. CONCLUSION NACA 23012 air foil had been compared with

modified (backward stepped) types in order to

enhance the aerodynamic characteristics of

conventional NACA 23012 air foil and to examine

the advantages and disadvantages of the new

design concept. The results presented constitute

the next step towards understanding the effects

of a step-induced vortex on the aerodynamic

characteristics of a conventional NACA 23012 air

foil for a range of angles of attack. The research

has been performed on a relatively new family of

air foils, which in itself needs further studies.

The objective of study was to enhance the

aerodynamic performance of the base NACA

23012 air foil for a chosen range of step

configurations and angles of attack. Hence the

major outcome is the increscent in lift over

conventional NACA 23012 air foil except when

upper surface is located, while Drag-coefficient

data indicate that with the introduction of a step;

drag increased. This is consistent in all the

modified air foil cases studied. High drag

generation is in need of more studies in order to

reduce it. Hybrid step generate much drag



cannot be avoided and make problems with

pressure over upper and lower wing. The best

step configuration can be specified through these

numerical tests were lower surface step air foil.

REFERENCES 1. Schlichting, H. "Boundary Layer Theory"

McGraw Hill Vol.1, 1979.

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lift: Review of present CFD". Progress in

Aerospace Sciences 38, (pp. 145-180) 2002.

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4. http://web.mit.edu/2.972/www/reports/airfoil

/airfoil.html. The page accessed on 15/10/2013

at 9:00 pm.

5. Rangnadhan voona, “Enhancing the

Aerodynamic performance of stepped airfoils",

master thesis unpublished, 2012.

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computational-fluid-dynamics” first edition 1995

7. Fertis, Demeter G.,"New Airfoil Design Concept

with Improved Aerodynamic Characteristics",



Journal of Aerospace Engineering, Vol. 7, NO. 3,

pp. 328-339, July, 1994.

8. Versteeg, Malalasekera" An-introduction-to-

computational-fluid-dynamics” first edition 1995

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