Investigation of the Low-Frequency Breathing Motion in Two Turbulent Separation Bubbles

摘要

Unsteady measurements of fluctuating pressure and velocity are performed in a pressure-induced turbulent separation bubble (TSB). The streamwise distributions of mean pressure coefficient, fluctuating pressure coefficient, and forward-flow fraction are scaled with either the distance L_(50) between transitory detachment and transitory reattachment, or the distance L_(99) between incipient detachment and complete reattachment. The normalized distributions are compared to those obtained in a larger TSB investigated in a previous publication. It is shown that Lgg is better suited than L_(50) to scale the streamwise extent of the separated flows. The low-frequency breathing motion in both TSBs is identified through its pressure signature near incipient detachment and through the first POD mode of horizontal and vertical velocity fields. Contrary to what is observed for the streamwise extent of the TSBs, a Strouhal number based on L_(50) shows a better collapse of the power spectral densities near the dominant frequency of the breathing mode. This suggests that the size of the region of mean back-flow is an important parameter of the breathing motion.