Strouhal number universality in high-speed cylinder wake flows

TL;DR

This study experimentally investigates flow oscillations in high-speed cylinder wakes at Mach 6, revealing a universal Strouhal number behavior across different Mach and Reynolds numbers, similar to low-speed flows.

Contribution

It demonstrates the universality of the Strouhal number in high-speed flows, extending previous findings at Mach 4 to Mach 6 and confirming similar flow dynamics at high speeds.

Findings

Strouhal number exhibits universal behavior across Mach and Reynolds numbers.

Flow oscillations show a phase difference of π radians between symmetric halves.

High-speed flow dynamics resemble low-speed wake behavior.

Abstract

Flow oscillations in the near-wake region of a 2D circular cylinder are experimentally investigated at Mach 6 over the Reynolds number range $2.3\times10^5$ to $5\times10^5$. The oscillation frequency is obtained by spectral proper orthogonal decomposition of high-speed schlieren data. The Strouhal number based on the length of the near-wake shear layers is found to exhibit universal behavior. This corroborates experimental findings at Mach 4 from recent literature, and further, the universal behavior is also seen to hold with respect to Mach number. Time-resolved pressure measurements at the flow separation points on the cylinder aft surface show that coherent oscillatory activity occurs with a phase difference of $\pi$ radians between the two statistically-symmetric halves of the flow. This aspect of the flow dynamics at high speeds is in common with its low-speed counterpart, i.e. the canonical problem of cylinder wake in an incompressible flow.