Definition, Sources, and Lowest Possible Levels of Wind-Tunnel Turbulence,
DEUTSCHE FORSCHUNGS- UND VERSUCHSANSTALT FUER LUFT- UND RAUMFAHRT E V BERLIN (GERMANY F R)
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Wind-tunnel turbulence is defined to include the fluctuations of all flow quantities in an empty tunnel test section. It can be separated into three modes, vorticity, entropy, and pressure. The pressure mode is discussed in more detail. It is shown that contributions from the free shear layers of open test sections or the boundary layers of closed test sections are accompanied by much higher velocity fluctuations than plane sound waves. The lowest possible velocity fluctuation level in an open test section is determined by the pressure field generated by the free shear layer. The German-Dutch Wind Tunnel is shown to reach this level. It is assumed that the lowest possible turbulence level is a closed test section is determined by the pressure field that is generated by the boundary layers in the test section. A spectral energy distribution and a Reynolds number scaling law for this contribution are derived.