Oscillations in cylinder wakes at Mach 4

Abstract

The wake behind a circular cylinder in Mach 4 flow is examined experimentally in the Reynolds number range 2 × 10 4 to 5 × 10 5 . Periodic oscillations of the sliplines in the wake are observed. The Strouhal number of the oscillations based on the diameter of the cylinder is found to increase monotonically from 0.30 to 0.50 with increasing Reynolds number. If the Strouhal number is formed using the length of the sliplines, however, it has a constant value of approximately 0.48 for all Reynolds numbers studied. This scaling indicates that the oscillations in supersonic flow are likely driven by acoustic signals propagating back and forth through the subsonic region between the separation points on the cylinder and the neck where the sliplines converge, unlike in subsonic flow where oscillations are caused by vortices shed from the cylinder surface.

Cavitation dynamics and vortex shedding in the wake of a bluff body

“…(2007). In addition, such near-wake regions can also exhibit ‘necking’ which occurs due to the interaction between slip lines (Schmidt & Shepherd 2015). Downstream of the near-wake region, compressible wakes begin to resemble their incompressible counterparts depending upon a threshold value of local convective Mach number .…”

Measurements of Instability in Supersonic Flow with Injection by Time-Resolved Flow Visualization

“…Similar systems have been used to measure instability waves in hypersonic boundary layers 18,19 and wake instabilities in high speed flows. 20 In addition to the imaging system, the model is instrumented with five PCB 132A31 pressure transducers along the length of the cone downstream of the injector. These are sampled at 2 MHz on a National Instruments PCI-6133 data acquisition card.…”

Strouhal number universality in high-speed cylinder wake flows

“…Unsteady flow features in this canonical wake problem at high speeds have surprisingly received no attention in literature, with the recent work of Schmidt and Shepherd [12] seemingly being the only exception. Time-resolved shadowgraph visualization of cylinder flow at Mach 4 by Schmidt and Shepherd [12] revealed coherent oscillations in the near-wake region.…”

“…Unsteady flow features in this canonical wake problem at high speeds have surprisingly received no attention in literature, with the recent work of Schmidt and Shepherd [12] seemingly being the only exception. Time-resolved shadowgraph visualization of cylinder flow at Mach 4 by Schmidt and Shepherd [12] revealed coherent oscillations in the near-wake region. Strouhal number behaviour over a decade range in the Reynolds numbers from their experiments suggest the near-wake shear layer length (see figure 1) to be the governing length scale for the oscillations.…”

“…Based on the experimental observations a universal value of approximately 0.48 for the Strouhal number was reported. The invariance of Strouhal number with Reynolds number was hypothesized as being indicative of an acoustic resonance mechanism that sustains the oscillations [12]. The current work aims to build on some of these observations toward obtaining a more detailed * Email address: subrahmanyam@iisc.ac.in understanding of unsteady flow features in the near-wake region of a high-speed circular cylinder.…”