A mathematical theory is developed enabling wind tunnels with porous walls to be designed to give zero tunnel blockage in subsonic compressible flow. The tunnel walls are taken to be porous over only a finite range
R
, and solid elsewhere, and a sealed jacket is placed over the porous section so that the pressure on the outside of the porous wall can be controlled. The porous wall is assumed to have the characteristic that the component of velocity normal to it is proportional to the pressure drop across it, the constant of proportionality, λ, being termed the ‘porosity’ of the wail. Infinite porosity and zero porosity correspond to free streamline and solid wall boundaries respectively, which are thus included in the theory as special cases. The problem solved in this paper is to determine the relation between λ,
R
, the tunnel height
H
, and the Mach number
M
, so that the ‘blockage’, or velocity increment at the model caused by the tunnel walls, vanishes. It is found that for a given value of the porosity the length of the porous wall,
R
, must be reduced with increasing Mach number to keep the blockage zero. Thus the tunnel needs to be fitted with adjustable sections of solid wall which can be moved across the porous surfaces to reduce their effective length (see figure 1). Both ‘solid ’ and ‘wake’ blockage are considered in the paper. The effects of wake blockage, which are particularly important at high subsonic speeds due to the rapid increase in drag, cannot be completely eliminated by varying
R
alone. This is because wake blockage, unlike solid blockage, causes a pressure gradient in the tunnel. This gradient and the blockage can be eliminated simultaneously only by introducing a further independent variable. A very convenient one for this purpose can be created by pumping air at a certain rate from the jacket and exhausting it outside the tunnel. The rate of removal of the air from the jacket can be adjusted to eliminate the induced pressure gradient completely.
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