A typical value of free stream turbulence level that is acceptable in force measurement is  of the mean velocity though a lower value may be required in certain studies such as those relating to hydrodynamic stability.
A sketch of a low speed open circuit wind tunnel is shown in Figure 2.19. The important components of this wind tunnel are:
Figure 2.19: Schematic Drawing of a Wind Tunnel. |
HC = Honeycomb for straightening the incoming flow.
SC = Screens for filtering out the eddies that are drawn from the ambient and those generated at the lips of the honeycomb structure
CC = Contraction cone, where the favourable pressure gradient limits the growth of the wall boundary-layers and dampens free stream turbulence
V = Vibration isolation table
TC = Test cell where the desired experiment is carried out
D = Diffuser connecting the test cell to the suction side of a fan
F = Fan that creates flow in the tunnel along with a variable speed motor
CF = Concrete floor to absorb fan vibrations
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