Transition Mechanisms in Separation Bubbles Under Low and Elevated Freestream Turbulence

Abstract

Through numerical simulations, this paper examines the nature of instability mechanisms leading to transition in separation bubbles. The results of two direct numerical simulations are presented in which separation of a laminar boundary layer occurs over a flat surface in the presence of an adverse pressure gradient. The primary difference in the flow conditions between the two simulations is the level of freestream turbulence with intensities of 0.1% and 1.45% at separation. In the first part of the paper, transition under a low-disturbance environment is examined, and the development of the Kelvin-Helmholtz instability in the separated shear layer is compared to the well-established instability characteristics of free shear layers. The study examines the role of the velocity-profile shape on the instability characteristics and the nature of the large-scale vortical structures shed downstream of the bubble. The second part of the paper examines transition in a high-disturbance environment, where the above-mentioned mechanism is bypassed as a result of elevated freestream turbulence. Filtering of the freestream turbulence into the laminar boundary layer results in streamwise streaks which provide conditions under which turbulent spots are produced in the separated shear layer, grow, and then merge to form a turbulent boundary layer. The results allow identification of the structure of the instability mechanism and the characteristic structure of the resultant turbulent spots. Recovery of the reattached turbulent boundary layer is then examined for both cases. The large-scale flow structures associated with transition are noted to remain coherent far downstream of reattachment, delaying recovery of the turbulent boundary layer to an equilibrium state.

Numerical study of the primary instability in a separated boundary layer transition under elevated free-stream turbulence

“…LES of Yang and Voke 2 for a separated shear layer on a flat plate with a semi-circular leading edge and LES of Abdalla and Yang 3 on a flat plate with a sharp leading edge proved vigorously for both cases that the free shear layer in the bubble is inviscidly unstable via the KH mechanism. Similar mechanism was reported by McAuliffe and Ya r a s 5 who performed DNS of a separation bubble on a flat plate in adverse pressure gradient with low incoming disturbances. Many experiments have been carried out to study separated boundary layer transition at low free-stream turbulence level.…”

“…Several numerical studies have considered separated boundary layer transition under vanishingly low environmental disturbances (Spalart and Strelets, 1 Yang and Voke, 2 Abdalla and Yang, 3,4 and McAuliffe and Yaras 5 ). Direct Numerical Simulation (DNS) of Spalart and Strelets 1 for a bubble induced by an adverse pressure gradient over a flat plate under very low incoming disturbances revealed a wavering behaviour of the separated shear layer gradually moving away from the wall with formation of Kelvin-Helmholtz (KH) vortices.…”

Effects of free-stream turbulence intensity on laminar separation bubbles

“…Further downstream in the later stages of transition (see Fig. 2.1), nonlinear interactions begin to occur and the shear layer is observed to roll-up into spanwise oriented vortices [16,18,31] which are shed at the most amplified frequency of the separation bubble [17,18,26]. The enhanced momentum exchange with the free-stream induced by these vortices enables mean flow reattachment [18].…”

“…The initial growth of these disturbances is primarily two-dimensional and nearly exponential, and can be modelled by linear stability theory [13][14][15]. As the disturbances continue to grow, non-linear interactions begin to occur, and the shear layer rolls up into coherent spanwise oriented vortices [16][17][18]. As these vortices convect downstream, the enhanced momentum exchange with the free-stream enables mean flow reattachment [17,18].…”

Transition Models for Turbomachinery Boundary Layer Flows: A Review

“…As with natural transition, mean-flow turbulence accelerates and controls the growth of the Kelvin-Helmholtz vortices and makes the process less sensitive to other disturbances. The mechanisms of separation-induced transition were studied by experiments, by LES and by DNS by many researchers [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Generally, the free shear layer, once sufficiently turbulent, reattaches forming a separation bubble.…”