Acoustic Mach number, jet Mach number or jet velocity: Choosing the optimal control property for jet noise experiments at different test rigs

Abstract

Wind tunnel experiments are great in order to study jet noise. Yet, the heat demand even for ISA cold engine thrust conditions is rather high. The additional heating and/or cooling of air in order to meet atmospheric norm conditions increases both energy demand, test time and test cost. Some energy lean experimental pressureized air test rigs make compromises on the temperature control system. Hence, norm conditions cannot be met and a way must be found how to optimally deal with the shortcomings of the test rig. This paper starts with an ideal test rig and the implications of testing especially subsonic jets under ISA norm conditions. Then, the shortcomings due to low-energy designs will be investigated by examining two factors: The jet temperature as well as the temperature of the acoustic chamber (below or above ISA norm temperature). The Ffowcs-Williams analogy for jet noise will be rewritten without any temperature terms. This will show that jet noise scales approximately with I ~ Mj^2 Uj^1 Mac^5. The derivation will be used in order to evaluate measurement strategies on two different pressureized air test rigs: (case 1) the remotely located compressor which is characterized by a constant total jet temperature, but - if unheated - is typically too cold for ISA norm conditions, and (case 2) a closely located compressor, where the compression heat is preserved, yet causes runaway temperatures within the jet and possibly within the acoustic test room. The aim of this paper is to show a solution to the discussion on which operational parameter is optimal for any test facility: the acoustic Mach number Mac, jet Mach number Mj or jet velocity Uj. A suitable test parameter produces a small error (e.g. less than 12% or 0.5dB) in comparison to ISA norm conditions over a wide range of any subsonic operation. Assuming that the test room is warmer than ISA, unheated (too cold) jet test rigs like AWB make small errors when using the jet velocity whereas slightly too hot jet test rigs like JExTRA are better of using jet Mach number or acoustic Mach number for the definition of their jet operations. The latter case is demonstrated using a data example from JExTRA.