to test models of proposed aircraft.
In the tunnel, the engineer can carefully control the
flow conditions which affect
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
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
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
tunnel, or the open room of an
supersonic flows, the air density changes in the tunnel because
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.
- Beginner's Guide Home Page