Acoustic Mach number, jet Mach number or jet speed – what is the optimal control property for jet noise experiments at AWB

Abstract

Some experimental facilities are planned energy-lean, but this comes typically with slightly imperfect test conditions. Especially temperature control systems are often slow, demand energy and increase operational cost. Contrary to this, low-cost experimental pressurized air test rigs, such as the one at AWB (Acoustic Wind tunnel Braunschweig), respond to control commands much faster, but do not meet norm conditions. Therefore, the question needs to be answered which flow parameter should be used for operation control, the acoustic Mach number Mac, jet Mach number Mj or jet velocity Uj. This study starts with the definition of ideal ISA norm conditions and a re-writing of the FfowcsWilliams jet noise analogy without any temperature terms. Using a rough assumption for the mixed (shear layer) density, the jet noise scales according to I ~ Mj2 Uj Mac5. The derivation will be used in order to judge the three control options at a test rig with a constant total temperature for the jet. As a result, this study will recommend the testing of jet velocities for jet noise experiments at AWB.