Structure of large-scale vortices and unsteady reverse flow in the reattaching zone of a turbulent separation bubble

Abstract

This paper describes experiments concerning the structure of large-scale vortices and the unsteady reverse-flow properties in the reattaching zone of a nominally two-dimensional separation bubble formed at the leading edge of a blunt flat plate with right-angled corners. The experiment was performed in a wind tunnel with a constant Reynolds number 2.6 × 104 (based on the main-flow velocity and the thickness of the plate). Split-film probes, being sensitive to instantaneous reversals of flow direction, were extensively employed. An important feature of this study is a judicious use of surface-pressure fluctuations as a conditioning signal to educe the structure of the large-scale vortices.Distributions of fluctuating-velocity vectors and contour lines of high-frequency turbulent energy in a few space–time domains are presented and discussed. The most economical interpretation of these space-time distributions is that the large-scale vortices in the reattaching zone are hairpin vortices whose configuration is sketched in the text. The unsteady flow in the reattaching zone is mainly governed by two agents; the motion of the large-scale vortices and the low-frequency unsteadiness. The unsteady flow is clarified in terms of the motion (in a space–time domain) of zeros of the longitudinal velocity close to the surface of the plate; the effects of the two agents on this motion are presented separately. On the basis of these results, a mathematical model of the unsteady flow in the reattaching zone is suggested and found to yield good comparison with measured reverse-flow intermittency and frequency of local-flow reversals. It appears that the separation bubble experiences shrinkage and enlargement in connection with the low-frequency unsteadiness and that the speed of shrinkage is much greater than that of enlargement. The strength of the large-scale vortices in the reattaching zone seems to be dependent on the phase of the low-frequency unsteadiness.

The stability of laminar symmetric separated wakes

“…However, this frequency is about the same as the frequency found by Kiya & Sasaki (1985), Cherry et al (1984), Castro & Haque (1987) and others, and associated with the large-scale shedding around reattachment. It is substantially higher, by a factor of some 5-10, than the very low-frequency content found by all these authors at locations in the upstream half of their bubbles.…”

“…These are often associated with shear layer 'flapping' (e.g. by Cherry et al 1984;Kiya & Sasaki 1985), with an associated quasi-periodic extension and collapse of the bubble, and typically have time scales some 5-10 times greater than those associated with the passage of large vortex structures within the shear layer around reattachment. The latter always occurs in a fully turbulent bubble, of course, but the presence of the former seems to depend on the nature of the bubble.…”

“…behind a rearward facing step -contain a definite unsteadiness of a frequency much lower than that appropriate to the largest eddies which occur around reattachment (e.g. Cherry, Hillier & Latour 1984;Kiya & Sasaki 1985;Castro & Haque 1987;Heenan & Morrison 1998). There has been considerable debate about the precise nature of this unsteadiness and, as a result of multi-point pressure measurements and a review of the literature, Hudy, Naguib & Humphries (2003) have recently suggested that an inherent absolute instability near the middle of the recirculation region provides the basic driver for the observed oscillations in bubble length.…”

Effects of Freestream Turbulence on the Pressure Acting on a Low-Rise Building Roof in the Separated Flow Region

“…The small-scale turbulence must be characterized to evaluate the roll up of the separated shear layer, the reattachment length, and the strength of the vortices advecting through this region (Melbourne, 1979;Kiya and Sasaki, 1983;Tieleman, 1993). The bubble's average extent, i.e., the mean reattachment length (X R ), is also sensitive to the aspect ratio of the building.…”

Self-Sustained Oscillations of Turbulent Flow in an Open Cavity

“…6, the white and black circles represent the local minima and maxima identified in this way, respectively. At each time point, the [13] (1.42, 0.53) Chang et al [18] (1.42, 0.52) [23]. Moreover, the strong negative pressure fluctuations of the vortical structures and the induced positive pressure fluctuations are responsible for the intermittent pressure fluctuations.…”