Unsteady pulsating flowfield over spiked axisymmetric forebody at hypersonic flows

TL;DR

This study investigates unsteady pulsating flow over a spiked hypersonic forebody through experiments and modal analysis, revealing vortex interactions, shock pulsations, and frequency characteristics invariant to Reynolds number variations.

Contribution

It provides new experimental insights into hypersonic pulsation phenomena over spiked bodies, including flow dynamics, pressure fluctuations, and frequency analysis at different Reynolds numbers.

Findings

Maximum 98.24% reduction in pressure fluctuation between high and low ReD cases.

Identification of two vortical zones influencing pulsation characteristics.

Spectral pressure decay follows inverse and -7/3 laws for different ReD cases.

Abstract

The paper gives experimental observations on the hypersonic flow past an axisymmetric flat-face cylinder with a protruding sharp-tip spike at a freestream Mach number of $M_\infty = 8.16$ at two different freestream Reynolds numbers based on the base body diameter ($Re_D = 0.76 \times 10^6$, and $3.05 \times 10^6$). Furthermore, modal analysis is done on schlieren images to understand the flow dynamics parallel with the unsteady pressure measurements. The protruding spike of length to base body diameter ratio of $[l/D]=1$ creates a familiar unsteady flowfield called 'pulsation.' Pressure loading and fluctuation intensity at two different $Re_D$ cases are calculated. A maximum drop of 98.24\% is observed in both parameters between the high and low ReD cases. Based on the analysis, a difference in the pulsation characteristics are noticed, which arise from two vortical zones, each from a system of two `$\lambda$' shocks formed during the `collapse' phase ahead of the base body. The interaction of shedding vortices from the $\lambda$-shocks' triple-points, along with the rotating stationary waves, contributes to the asymmetric high-pressure loading and the observation of shock pulsation on the flat-face cylinder. The vortical interactions form the second dominant spatial mode with a temporal mode carrying a dimensionless frequency ($f_2D/u_\infty \approx 0.34$) almost twice that of the fundamental frequency ($f_1D/u_\infty \approx 0.17$). The observed frequencies are invariant irrespective of the ReD cases. However, for the high-frequency range, the spectral pressure decay is observed to follow an inverse and -7/3 law for the low and high $Re_D$ cases, respectively.