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 Schematic drawing of two wind tunnels (one subsonic, one supersonic)
 and an explanation of the differences in the design

Aerodynamicists use wind tunnels to test models of proposed aircraft. In the tunnel, the engineer can carefully control the flow conditions which affect forces on the aircraft. By making careful measurements of the forces on the model, the engineer can predict the forces on the full scale aircraft. And by using special diagnostic techniques, the engineer can better understand and improve the performance of the aircraft.

Wind tunnels are designed for a specific purpose and speed range and there is a wide variety of wind tunnel types and model instrumentation. The model to be tested in the wind tunnel is placed in the test section of the tunnel. The speed in the test section is determined by the design of the tunnel. The choice of speed range affects the design of the wind tunnel due to compressibility effects.

For subsonic flows, the air density remains nearly constant and decreasing the cross-sectional area causes the flow to increase velocity and decrease pressure. Similarly, increasing the area causes the velocity to decrease and the pressure to increase. We want the highest possible velocity in the test section. For a subsonic wind tunnel, the test section is placed at the end of the contraction section and upstream of the diffuser. From a knowledge of the conservation of mass for subsonic flows, we can design the test section to produce a desired velocity or Mach number since the velocity is a function of the cross-sectional area. On the figure, we note the changes in Mach number, velocity and pressure through a subsonic wind tunnel design. The plenum is the settling chamber on a closed return tunnel, or the open room of an open return design.

For supersonic flows, the air density changes in the tunnel because of compressibility. In fact, the density changes faster than the velocity by a factor of the square of the Mach number. In a supersonic flow, decreasing the cross-sectional area causes the flow to decrease in velocity and increase pressure. Similarly, increasing the area causes the velocity to increase and the pressure to decrease. This change in properties is exactly the opposite of the change that occurs subsonically. In addition, compressible flows experience mass flow choking. As a subsonc flow is contracted, the velocity and Mach number increase. When the velocity reaches the speed of sound (M = 1), the flow chokes and the Mach number can not be increased beyond M = 1. We want the highest possible velocity in the test section of the wind tunnel. For a supersonic wind tunnel, we contract the flow until it chokes in the throat of a nozzle. We then diffuse the flow which increases the speed supersonically. The test section of the supersonic tunnel is placed at the end of the diffuser. From a consideration of conservation of mass for a compressible flow, we can design the test section to produce a desired velocity or Mach based on the area in the test section. On the figure we note the changes in Mach number, velocity and pressure through a supersonic wind tunnel design.

Notice that in both supersonic and subsonic designs, the velocity is increased and the pressure is decresed relative to the station upstream of the test section. In a subsonic tunnel the area is contracting into the test section; in a supersonic tunnel the area is increasing. The physical reason for this seeming contradiction is given on the wind tunnel theory web page of the Beginner's Guide to Compressible Aerodynamics.



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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 05 2015

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