In this Chapter
- Wind Tunnel Times
- Fields of Operation
- Notes on Basic Instrumentation
- Wind Tunnel Structure
- Wind Tunnels (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
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.
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.
Some operations ordinarily performed in the wind tunnel are the following:
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.
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.
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.
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 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.
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.)
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 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.
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.
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.
Copyright © of this document is property of
Dr. A. Filippone. Text and figures cannot be reproduced without the copyright's holder express
consent, except for the print or download intended for the use of the individual user.
Wind Tunnel Times
Fields of Operation
Classification
Qualities
Related Material
[on CD-ROM]
Note on Basic Instrumentation
Other Hardware
Wind Tunnel Structure
The Test Section
Scale Effects
Model Size
Interference Problems
Blockage
Wind Tunnel Corrections
Summary of Wind Tunnels (available on CD-ROM)
Selected References