Using “NACA ducts” for Ventilation in an Aerodynamic Fairing

Prepared by:Cloe Bailey

Colorado State UniversityME-405 Senior Design

Thermo-Fluids Division Technical ContributionSubmitted: March 1, 2000

There are many factors to take into consideration when analyzing and designing a

competitive aerodynamic Human Powered Vehicle fairing (HPV). One of these factors

is the choice of a ventilation system for the vehicle. Ventilation plays an important role

in human performance. A person will perform best under comfortable environmental

conditions, especially when performing intensive physical activity such as bicycling.

These environmental conditions include hot and cold temperatures. Usually, when the

surrounding environment temperature is too cold or too hot, a person’s body also will

tend to be too hot or too cold, thus causing a person discomfort. In hot environments, a

body does not dissipate enough heat. When a body is performing heavy physical

activity, most of the body heat (approximately 60 percent) is dissipated in form of latent

heat.1 The heat may be transferred by radiation between the body and the surrounding

surfaces such as walls or windows, or in this case, by convection through the ventilation

system. Therefore, in order to alleviate such discomforts, an efficient ventilation system

must be designed.

The most common ventilation for an enclosed, aerodynamic fairing is the “NACA

duct.” This was researched, tested and designed by many engineers who were involved

in the National Advisory Committee for Aeronautics (NACA). Although the design of

the “NACA duct” was mainly used for the ventilation of aerodynamic airplanes, the same

theories, principles, and calculations can be applied to an aerodynamic HPV fairing.

This paper will focus on the various requirements of the “NACA duct” and will

present the design for an aerodynamic HPV tandem fairing.

In the research of the “NACA duct”, there were three main general design classes

of the possible plan forms for a ramp. These include ramps with parallel, diverging, or 1 “Thermodynamics” An Engineering Approach by Yunus A. Cengel and Michael A. Boles

convergent walls. The flow using the ramp with the parallel walls appear to constitute a

two-dimensional analysis. However, the flow using the ramp with the divergent walls

has two main differences from that of the parallel wall. Due to these differences, the use

of a two-dimensional analysis can be ruled-out. The first difference is that the flow near

the floor of the ramp is divergent at all of the mass-flow ratios, this enables the boundary

layer to be a two-dimensional flow. Second is that since the walls are no longer parallel,

the external stream must now flow over the top of the diverging walls into the inlet.

The “NACA duct” design is defined as an inlet having the general shape as shown

in Fig. 28. This shape has beneficial aerodynamic characteristics. Several parameters

affect the design of the “NACA duct”, including: entrance width-to-depth ratio, ramp-

wall divergence, and ramp angle. Two main factors that are essential for determining the

above parameters are pressure loss and boundary layer thickness.

Pressure

Measuring the pressure loss of the air-flow into the duct helps determine the size

and shape of the actual design of the “NACA duct”. One method this is to test a model

of the “NACA duct” in a wind tunnel (shown in figure 1). Total pressure tubes and the

static pressure tubes are placed inside the entrance of the duct. The pressure loss and the

recovery pressure are measured using the total pressure tubes. The contours or the

pressure loss at the entrance of the duct can be plotted and used to determine the average

pressure. In order to determine the steep-ness of the pressure, a pressure-distribution test

over the lip and the ramp of the entrance of the duct also had to be made. The following

equation can be used to calculate the estimate the total pressure loss at the entrance of the

duct:

Hqo

HA

qo

1 ( )VA

Vo

2

1 0.2 M2VA

Vo

2

1

2.5

where can be obtained from the data collected (see nomenclatures in the appendix).2

Since this equation was derived for an airplane, the Mach number (M) can be disregarded

when using this equation for a land vehicle, such as an HPV. It is also necessary for low-

speed moving objects, such as land vehicles, to calculate the pressure loss as the ratio of

the total-pressure loss to the free-stream impact pressure.

Boundary Layer Thickness

While testing the pressure of the duct inside the wind tunnel, testing of the

boundary layer thickness can also be measured, with the assumption that the fluid being

used is incompressible. The boundary layers that are of interest for the duct design are

the ones that are developed on the ramp floor and the ramp. However, since the tests

performed on the “NACA duct” model have shown that basically, the boundary layer on

the ramp wall does not have any initial thickness and only forms over a very small wetted

area, therefore its significance can be neglected.

In conjunction with the boundary layer thickness, other parameters must be

considered, such as the displacement and momentum thickness. However, for low-speed

vehicles, the momentum thickness can be disregarded. The following equation (see

appendix for nomenclature) can be used to calculate the free-stream pressure, which, in

turn, aids in the calculation of the boundary layer thickness:3

h

0

y HHo Po

d

2 “An Experimental Investigation of the NACA submerged-duct entrances” 3 “NACA Research Memorandum” No. 7I30

From the tests performed on the boundary layer thickness, the data showed that thicker

boundary layers considerably reduced the existing pressure recovery at the end of the

diffuser. After both the pressure and the boundary layer thickness have been determined,

the actual design of the “NACA duct” can be performed.

Ramp Design

According to the research done in NASA, the best ramp design is a diverging

wall, which diverts the boundary-layer air around the entrance of the duct. This allows

maximum pressure recovery and maximum velocity recovery. The values for these can

be determined by performing wind tunnel testing on a model of a “NACA duct”.

Ramp Angle

For a diverging ramp wall, determining the ramp angle can be difficult, because

instead of a two-dimensional aspect (as in parallel walls), the diverging wall is assumed

to be three-dimensional. Because of this, the pressure loss in conjunction with the value

of the ramp angle is caused by the resulting geometrical change in the ramp plan form.

In a diverging wall ramp design, increasing the ramp angle also increases the angle

between the diverging walls, with two possible effects. The first one is an increased

angle between the diverging ramp walls which also increases possibility of separation.

The second effect is increased ramp angle which also increases the distance between the

ramp walls and the free-stream flow. This creates more difficulty for the airflow to

follow the shape of the diverging side-walls. These two effects, along with the increase

in the ramp angle lead to pressure losses in the corners of the entrance of the duct.

Entrance width-to-depth ratio

Obtaining the results from the ramp angle will help in determining the entrance

width-to-depth ratio. This ratio is the width of the ramp entrance to the width of the

submerged entrance. In many cases, using a given ramp angle is established by the

length that is available at the entrance of the duct. The required ramp length is much

larger for the deep and narrow entrance (for a diverging ramp), assuming that the duct

entrance area and the ramp angle are constant. However, since the ramp length is usually

restrained by a design limitation, it is better to determine the entrance width-to-depth

ratio by comparing the pressure recoveries at a constant ramp length. This can be

obtained by plotting the pressure-recovery data after the diffusion versus the ramp-

length.

Ramp-Floor Shape

There were two tests that were performed to determine the ramp-floor shape.

These tests were used to compare the pressure recoveries of both straight and curved

ramp floors. According to this test, the straight floor proved to be the better shape.

However, since the effect of the pressure recovery is very small, changes of the contour

of the floor shape may be required in order to attain a smooth junction between the ramp

floor and the fuselage skin.

Mass-Flow Ratio

As the mass-flow ratio decreases, more air that would have gone into the duct

entrance is spilled over the lip and the ramp walls, ahead of the entrance of the duct.

When increasing the divergence, the mass-flow ratio and the ramp divergence are

expected to decrease. These losses occur on the main surfaces of the duct, but in the

regions where the flow is directed away from the inlet of the duct, the mass-flow ratio

and the ramp divergence cannot enter the duct, and must therefore be considered as

external drag.

Application for “NACA ducts”

Although the research and testing performed on the “NACA ducts” were mainly

designed for air-crafts, the applications, theory and most of the calculations can also be

used for low-speed vehicles such an HPV tandem fairing. When designing a duct, the

submerged entrance cannot be applied as an inlet, although this has certain characteristics

that makes is particularly suited for a specific ducting design application. One of the

main appeals for using and designing a “NACA duct” is its aerodynamical cleanness.

Conclusion

Providing comfortable environmental conditions for both of the riders in an

aerodynamic fairing can be difficult. Many considerations must be made when choosing

and designing a ventilation system for an enclosed surface; some of these considerations

are mentioned above. The most common design for a ventilation system on a Human

Powered Vehicle is the “NACA duct”. This is very efficient, cost-effective, and can be

easily placed in the proper locations. As mentioned before, the tests and research

performed on the “NACA ducts” were designed for aircraft, the same applications can be

used in the design for a low-speed vehicle.

When performing the wind tunnel testing for a model design of a duct, the same

measurements for pressure losses at the beginning and the end of the duct entrance can be

made. However, since the design using the “NACA duct” is for a low-speed vehicle, the

momentum, Mach number, and high-velocity calculations need not be made because

these become very irrelevant. Another important parameter to consider for designing a

duct for a low-speed vehicle is the boundary layer at the location of the entrance of the

duct. Again, this can be determined by taking data from the wind tunnel testing and

determining the locations of the thick and thin boundary layers. Once these locations

have been found, placing the actual duct on the fairing will be more aerodynamically

sound.

APPENDIX

REFERENCES

ThermodynamicsAn Engineering Approach Third EditionYunus A. Cengel and Michael A. Boles1998 The McGraw-Hill Companies, Inc. Hightsown, NJ

“An Experimental Investigation of NACA Submerged-Duct Entrances”National Advisory Committee for Aeronautics, ACR No. 5120Charles W. Frick, Wallace F. Davis, Lauros M. Randall, and Emmet A. Mossman1945 NACA. Washington

Source Location: NACA library, Langley Memorial Aeronautical Laboratory

“Research Memorandum” - An Experimental Investigation of the Design Variables for NACA Submerged-Duct EntrancesNational Advisory Committee for Aeronautics, RM No. A7130Emmet A. Mossman and Lauros M. Randall1948 NACA. WashingtonSource Location: NACA library, Langley Memorial Aeronautical Laboratory

“Theoretical Investigation of Submerged Inlets at Low Speeds”National Advisory Committee for Aeronautics, Technical Note 2323Alvin H. Sacks and John R. Spreiter1951 NACA. WashingtonSource Location: Ames Aeronautical Laboratory. Moffett Field, Calif.

Nomenclature

H total pressure, (lb/ft2)P Pressure coefficient, (pl - po)/qo

V Velocity, (ft/sec)q dynamic pressure, (lb/ft2)H loss in total pressure, (lb/ft2) ducting efficiencyo free streamh height of total pressure

(Frick et al., 1945)

(Frick et al., 1945)

(Frick et al., 1945)

(Frick et al., 1945)