The Structure and Control of a Turbulent Reattaching Flow

Author: Sigurdson, Lorenz Willard

Year: 1986

Degree: Dissertation (Ph.D.)

Advisor: Roshko, Anatol

Committee Members: Roshko, Anatol; Coles, Donald Earl; Leonard, Anthony; Zukoski, Edward E.; Saffman, Philip G.

Option: Aeronautics

DOI: 10.7907/prfm-x404

Abstract

An unsteady and three-dimensional large-scale structure is proposed for the reattachment region of a separation bubble, based on a visualization study of the flow over a plate with a square leading edge and its axisymmetric counterpart, a flat-faced circular cylinder aligned coaxially with the free-stream. The initial free shear layer structures are primarily two-dimensional but evolve into boundary layer type structures as they near reattachment and interact with the wall. Some segments form "loops" which convect away from the wall and downstream, while spanwise adjacent parts convect toward the wall and upstream. The loops are sometimes clearly arranged in a staggered pattern. Their legs form a series of counter-rotating streamwise vortex pairs which bridge the reattachment zone. These observations reconcile apparently contradictory propositions concerning the fate of the structures as they encounter reattachment. The interaction between successive vortices at alternating spanwise locations is fundamental to several flows. The structure of turbulent wakes is also discussed.

An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp- edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge.

In both forced and unforced cases it was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the "shedding" type instability of the entire bubble. The latter results from an interaction with the image vortices due to the presence of the wall. A method of frequency scaling is proposed that correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.

New methods for approximating axisymmetric flows are presented. Transition of shear-layers and separation bubbles is also discussed.

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