Compressibility-Inducted Hysteresis in Upswept Afterbody Flows*

A freestream-aligned cylinder with an upswept base is a commonly used canonical surrogate to study military cargo aircraft fuselage wakes. Recent investigations with near-incompressible flow have shown the existence of two types of wakes depending on the upsweep angle: a “vortex-pair” (VP) dominated state and a “fully separated wake” (FSW), with drastic drag characteristics. This investigation aims to delineate the effects of compressibility on the wake of a cylindrical aft body with a 45° upsweep by performing well-resolved large eddy simulations at Mach numbers 0.1, 0.3, and 0.5. At Mach 0.1, the wake exhibits typical VP characteristics observed in experiments. Increasing the Mach number to 0.3 shifts the VP formation downstream due to a larger recirculation region near the start of the basal upsweep. Further increasing the Mach number transitions the flow to an FSW state, similar to observations with changes in upsweep angle at incompressible speeds. Additionally, the flow exhibits hysteresis with Mach number—the FSW state persists even when the Mach number is reduced back to 0.1. The FSW regime features larger but more uniform low-pressure regions on the base, resulting in lower drag than the VP regime.