Dynamics of pressure field on axisymmetric backward-facing step: subsonic flows

An experimental study aimed at investigating the unsteady pressure field over an axisymmetric backward-facing step (ABFS) was performed at subsonic Mach numbers of 0.6, 0.8, and 0.93. As a part of the experiments, surface flow visualization and unsteady pressure measurements were carried out. The mean reattachment point was identified, and consequently the mean reattachment length was obtained from the oil flow visualization. Pressure measurements indicate that the mean pressure reduces while the root mean square (rms) of the fluctuations increases with the increase in freestream Mach number. However, the coefficient of rms decreases. Conventional spectral analysis suggests that the peaks in the spectra of pressure fluctuations can be classified into three types, and the pressure field in general can be distinguished into two to three regions depending on the amplitude of these three types of peaks and the freestream Mach number. It has been shown that more than 80% of the total energy on the base is contributed by the first five modes, while on the rearbody, 80–86% of the total energy contribution comes from the first six modes. The dynamics of proper orthogonal decomposition (POD) modes indicate that the dominant mode/mechanism is the flapping motion of the shear layer combined with the expansion/contraction of the separated region. Spectral analysis of POD modes suggests the presence of three different fluid dynamic mechanisms and that the freestream Mach number strongly dictates the dominant mechanism that is driving the pressure field.