Copyright © A. Filippone (1999-2003). All Rights Reserved.

Wind Tunnels

In this Chapter

Wind tunnel testing is the technical support of any major development process involving aerodynamics. It is used for aircraft, helicopters, cars, trains, and laboratory research.

The wind tunnel provides the engineer with valuable data on scale models. The wind tunnel is the most lasting contribution of the Wright brothers to the science of aerodynamics.

Wind Tunnel Times

It is estimated that it took the Wright Brothers less than 20 hours of wind tunnel testing to produce their successful Flyer (although their empirical research was a life time achievement).

The Douglas DC-3, perhaps the most successful commercial aircraft ever built, required about 100 hours of wind tunnel testing. Wind tunnel time has been steadily increasing (and so have the costs) since: the Boeing 747 required over 1000 tunnel hours; the Space Shuttle nearly 10 years of time.

The figure below shows the demonstrated trend in the aircraft industry from the Wright Flyer I (1903) to the NASA Space Shuttle (data elaborated from Neumann, 1988). The year in the abscissa is the ending time.

Data Available on CD-ROM

The time spent in wind tunnel to develop a state-of-the- art racing car (ex. Formula 1) is also staggering. The time used by an important team is of the order of several hundred per season.

Fields of Operation

Some operations ordinarily performed in the wind tunnel are the following:

  • Drag/Lift measurements on aircraft, helicopters, missiles, racing cars.
  • Drag/Lift/Moment characteristics of airfoils and wings.
  • Static stability of aircraft and missiles
  • Dynamic stability derivatives of aircraft
  • Surface Pressure distributions on nearly all systems.
  • Flow visualizations (with smoke, oil, talcum).
  • Propeller performances (torque, thrust, power, efficiency, etc.).
  • Performances of air-breathing engines.
  • Wind effects on buildings, towers, bridges, automobiles.
  • Heat transfer properties of engines and aircraft.

Some of the operations listed above can be performed in a water tunnel.


Wind tunnels may be classified according to their basic architecture (open-circuit, closed-circuit), according to their speed (subsonic, transonic, supersonic, hypersonic), according to the air pressure (atmospheric, variable- density), or their size (ordinary ones or full-scale). There is a number of wind tunnels (metereologic tunnel, shock tunnel, plasma-jet tunnel, hot-shot tunnel) that fall in a special category of their own.


The quality of the tunnel can be described by the range of the Reynolds and Mach numbers that can be tested, along with the turbulence levels and the testing equipment. Quantities generally given are the maximum speed in the test section, the size of the test section and the power of the motor.

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Note on Basic Instrumentation

The progress in electronics has widened the market of instrumentation for wind tunnel measurements. These systems are not described here. For pressure measurements systems that convert pressures into electric signals of appropriate frequency (transducers, strain gages. etc.) are used.

Measurements of temperatures, temperature gradients and heat transfer are made with thermocouples, thermistors, resistance sensors. Turbulence levels are measured with with laser systems (LDA, Laser Doppler Anemometry), particle tracking systems (PIV, Particle Image Velocimmetry), hot wires, thermal anemometers.

Analysis of flow direction (streamlines) can be made with a very simple technique, consisting in placing tufts on the surface of the models. Also used are dyes and oils (for surface streamlines and turbulence) and smoke (for field streamlines). For shock wave visualizations, the Schlieren photography has been used for many years.

Other methods include the shadowgraph technique and optical interferometry. For the highest speeds absorption methods are used. The figure below shows a flow visualization for a wing with a Hoerner tip. The experiment was performed by the author at the low speed wind tunnel of the University of Illinois.

Flow viz on wing tip

Figure 1: flow visualization on a wing

Other Hardware

A fundamental piece of hardware is the computer, which is not used for measurements is used to collect, but it is needed to store and reduce large volumes of experimental data.

With appropriate software, it is possible to control a wide range of parameters in real-time, to visualize the progress of the test with the tunnel running, start and stop the process, etc. The most up-to-date wind tunnels have now sophisticated computer controls.

Wind Tunnel Structure

Main components of a tunnel are: Entrance cone, test section, regain passage, propeller/motor, return passage. Flow straighteners, corner vanes, honeycomb layers for reduced turbulence, air exchangers and diffusers are other common features.

Measurement equipment and testing procedures are topics on their own. They include instrumentation for the measurement of pressure, temperature, forces, moments, turbulence intensity, etc.

The Test Section

The test section is where the model is placed and held with appropriate struts. The section is generally rectangular (sometimes the test section is an open jet). The longitudinal dimension is about twice the maximum dimension of the section.

Scale Effects

Very seldom the models tested in a wind tunnel are in true size. Most likely they are scaled. Models on scale are hard to build and generally very expensive (just think of the surface roughness, tolerances, small details, etc.) To simulate the real conditions the aerodynamicist must keep the dimensionless parameters constant.

For example, a model 1:4 must be tested at four times the real speed. Hence the smaller the model, the higher the speed in the test section, the other parameters being constant (this limitation can be removed in a pressurized wind tunnel.)

Model Size

The aerodynamicist must find a compromise between model’s size and wind tunnel size. The decision is generally dictated by cost considerations. When the real Reynolds and Mach numbers cannot be reproduced, the experimental data are affected by the so-called scale effect.

Extrapolation to real scale depends on the type of experiment performed and the range of Reynolds and Mach numbers tested. Sometimes the scale effects are negligible, sometimes (like transonic flows, low speeds) they are not.

Interference Problems

Interference in the wind tunnel section due to the blockage of the flow by the test model is a problem that must be addressed with proper correction of the data. Another type of interference can occur at transonic and supersonic speeds, due to the reflection of the shock waves from the wind tunnel walls.

Schlieren Image

Figure 2: Shock reflection in supersonic wind tunnel

A typical example is shown in the figure above, that is a Schlieren image taken at UMIST. It is a double wedge airfoil (not a UFO) at M = 1.8 and incidence -12 degs. Please note the bow wave at the front end of the wedge.


Flow blockage occurs in wind tunnels of limited size during the testing of relatively large models. The blockage is defined as the ratio of the frontal area of the model to the area of the test section.

Blockage ratios of less than 10 % are needed, but sometimes far larger ratios are used. For aeronautical testing the blockage must be less than 5 %. In automotive industry this value can be achieved by very few wind tunnels.

The presence of the model in the test section blocks the incoming flow and has the effect of increasing the pressure on the tunnel walls. For this reason sometimes open section wind tunnels or tunnel with slotted walls are used. Correction for blockage has been an active subject of research.

Wind Tunnel Corrections

The test conditions are never the same as the operational conditions. Among the most well known effects there are the scale effects, the flow blockage, due to the presence of the model in the test section and wall boundary layers.

Other effects are dependent on the type of experiments performed, for ex. angle of attack corrections for a wing, due to induced downwash. Wind tunnel correction requires special analysis and processing techniques.

Summary of Wind Tunnels (available on CD-ROM)

  • Low-Speed Wind Tunnels
  • Transonic Wind Tunnels
  • Supersonic Wind Tunnels
  • Hypersonic Wind Tunnels
  • Tunnels with Moving Ground
  • Pressurized Wind Tunnels
  • Meteorological Wind Tunnels

Selected References

  • Pope A and Rae WH. Low-Speed Wind Tunnel Testing, John Wiley, New York, 1984.

  • Pope A and Goin K. High Speed Wind Tunnel Testing, Krieger Publishing Company, 1965

  • Goethert BH. Transonic Wind Tunnel Testing. Pergamon Press, London, 1961.

  • Hilton WF. High-Speed Wind Tunnels, High Speed Aerodynamics, Longman Green, London, 1952 (chapter 14).

  • Baker RC. Flow Measurement Handbook, Cambridge Univ. Press, 2000.

  • AGARD, Wind Tunnel Wall Corrections, AGARD AG-336, 1998.

  • AGARD, Advanced Aerodynamic Measurement Technology, AGARD CP-601, May 1997.

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Copyright © A. Filippone (1999-2003). All Rights Reserved.