Fig.J-17 An example of a classical wind tunnel for aeronautical model measurements. This type and size of wind tunnel may be found practically in all industrial countries around the world - although, especially in larger size and for higher velocities, they belong to the most expensive experimental facilities. Typical for aeronautical applications is this circulation or closed-circuit type (also called Prandtl or Gottingen type) layout, which makes possible achieving higher velocities with less demanding driving power requirements. Another advantage is an easier access to the investigated model since there is basically atmospheric pressure in the test section.





















Fig.J-18 The Eiffel or open-circuit type wind tunnel: air sucked into the nozzle from the atmosphere - and returned into the atmosphere from the collector. The construction is cheaper (no need to build the pressurised return duct). Common for non-aeronautical aerodynamics - testing aerodynamic properties of buildings or automobiles.


















Eiffel type wind tunnel is usually (with the exception of the largest tunnels) placed inside a protection hall to shelter it from meteorological factors (rain, snow, wind of variable directions which would cause changes of velocity profile in the test section). The original Prandtl's tunnel had constant cross section in the whole circuit and introduced the vanes in the sharp bends which decrease hydraulic losses. Eiffel introduced the collector, shaped as a diffuser, downstream of the test section. This decreases the kinetic energy of discharged air, which is dissipated by vortices in the protective hall. Even so the disspated power is large and this type is usually used for measurements at lower velocity range.
An essential factor for successful wind tunnel measurement is proper correction for the effect of the limited transverse dimensions of the test section. The better and reliable is the correction procedure, the larger may be the tested model (- this means lower velocity, lower energetic demands and better similarity). The latest development of recent decades are self-correcting wind tunnes with adjustable test section (e.g. with flexible test section walls adjusted by a large number of actuators) adaptively set-up according to measured wall pressure distributions.

Fig.J-19

Wind tunnel measurements are based upon the assumption that there is no difference between the stationary object/moving fluid and the moving object/stationary fluid cases. This, of course, is valid in most situations, but there are exceptions. The best known of them is the effect of road moving relative to an actual road vehicle.

A typical wind tunnel investigations
of separated flow regions in air flow
past a truck. Flow visualisation by
coloured "smoke". Note the use of the
fixed bottom plate
- Photo: Aerodynamic Laboratory,
Institute for Aerospace Research,
Ottawa, Canada


















Fish eye lens view of the
largest wind tunnel in the world,
- a typical example of the
Eiffel type arrangement.
Located at NASA's Ames Research
Center, Silicon Valley, Calif.
The test section dimensions are
80 by 120 ft.
Photo credit: NASA Ames RC.













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This is page Nr. J08 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
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