Hypersonic Turbulent Boundary-Layer Density Fluctuations: Direct Numerical Simulation, Focused Laser Differential Interferometry

In this article, we present a comparison of experimental measurements of density fluctuations over a hollow cylinder to direct numerical simulation (DNS) data at similar conditions and wall-normal positions. The experimental observations are made using a closely spaced multiple-beam-pair focused laser differential interferometry (FLDI) flow diagnostic, allowing data to be acquired above 1 MHz. The power spectral density (PSD), autocorrelation and cross-correlation, and coherence spectra computed from FLDI and DNS data are presented for comparison. The PSD spectra obtained from experimental results and numerical analysis show good agreement in the energy-containing and inertial subrange, with a minor deviation in the higher frequencies of the dissipation range. A comparison of the cross-correlation maintains the agreement in the energy-containing and inertial ranges, but shows slightly higher correlation values for DNS data until it asymptotically approaches zero. The coherence spectra consistently show that the DNS data remain better correlated at higher frequencies. However, both datasets show steep reductions in correlation with increasing probe separation distances, extending into the lower frequencies of the energy-containing and inertial ranges.