Compressibility Effects in Weakly Turbulent Hypersonic Flow Fields

An improved method for performing measurements of turbulent quantities and spectra in hypersonic wind tunnel flows, using a miniature pitot probe, is presented. The results of an extensive investigation of velocity fluctuation spectra, performed in the weakly compressible, decaying turbulent field established in the test section of a small scale hypersonic (Mach 6) wind-tunnel, at low enthalpy conditions, are also presented. A discussion of the fundamental assumptions is provided, together with experimental and theoretical considerations of applicability and limits of the proposed method. The measured one-dimensional spectra exhibits a non-Kolmogorov power-law behaviour over a significant wavenumber range, with a –11/3 exponent which is not predicted by available scaling relations for compressible turbulence. On the other hand, such power-law exponent is in agreement with EDQNM and LES results for the irrotational component of velocity fluctuations in weakly compressible flows [1,2]. Based on these results, a new dimensionally-based scaling relation is obtained for the compressible energy spectrum, which is in agreement with the numerically observed radiative behaviour for weakly compressible turbulence.