Structure of a turbulent separation bubble

Abstract

Flow in the separation bubble formed along the sides of a blunt flat plate with right-angled corners has been studied in terms of extensive single- and two-point measurements of velocity and surface-pressure fluctuations. The cross-correlations between the surface-pressure and velocity fluctuations are found to be useful for the study of large-scale vortex structure in the bubble. Large-scale vortices are shed downstream from the separation bubble with a frequency of about 0.6U∞/xR, where U∞ is the approaching velocity and xR is the time-mean length of the bubble. On top of this regular vortex shedding, there exists a large-scale unsteadiness in the bubble. Vortices which are much larger than the regular vortices are shed with frequencies less than about 0.2U∞/xR. The large-scale unsteadiness is accompanied by enlargement and shrinkage of the bubble and also by a flapping motion of the shear layer near the separation line. The intermittent nature of the flow in the bubble is clarified in some detail. The distributions of the cross-correlations between the pressure and velocity fluctuations demonstrate the vortex structure in the reattaching zone. The longitudinal distance between the vortices is estimated to be (0.7–0.8) xR and their convection velocity is about 0.5U∞ near the reattachment line. The cross-correlations also suggest the existence of a longitudinal counter-rotating system in the bubble. The distance between the axes of the rotation is of the order of 0.6xR. Variations of timescales, lengthscales and phase velocities of the vortices are presented and discussed.

Large Eddy Simulation of a Controlled Diffusion Compressor Cascade

“…Regions of higher-velocity fluid are transported towards the blade surface by these structures, and regions of reverse flow are similarly transported away from the wall (Figure 13d). These results are in good agreement with previous numerical and experimental studies of the separated flow on a flat-plate [14], [37], [39]. Figure 14 shows the approximate location where the flow reattaches at this given instant in time.…”

Numerical study of instabilities in separated–reattached flows

“…Figure 5 shows pressure spectra at several different locations in a separated boundary layer transition on a flat plate with a blunt leading edge [28] and a peak frequency band at about 0.8-0.9 U0/xR is clearly observable (U0 is the free stream velocity and xR is the mean bubble length). This peak frequency band was also observed in several experimental studies of separated-reattached flow over a plate with a sharp leading edge at high Reynolds number [29,30,31]. This peak frequency band was stated to be the characteristic frequency of the large vortices shedding from the free shear layer of the bubble.…”

A fluid-mechanics based classification scheme for surface transient storage in riverine environments: quantitatively separating surface from hyporheic transient storage

“…The second separation region at the obstacle head results in a more complex flow field than the upper boundary layer separation region. Reattached flow travels along the flow obstacle and separates at the obstacle head, where a mixing layer forms and sheds vortices downstream (Kiya and Sasaki, 1983). The flow field downstream of the obstacle head is dependent on the orientation of the obstacle's protrusion into the flow.…”