THE EFFECTS OF VELOCITY RATIO ON THE LARGE SCALE COHERENT STRUCTURES IN FREE SHEAR LAYERS

The velocity ratio of a free shear layer has an important influence on the spatial development of the large scale coherent structures in the layer. In this study, numerical simulations are performed to get an insight into this problem. The obtained numerical results agree quite well with those of a linear inviscid stability theory and the available experimental data.

FIVE MODE INTERACTIONS IN A FREE SHEAR LAYER

This study condiders spatially developing free shear layers at moderate Reynolds numbers that are formed by the merging of two free streams initially separated by a splitter plate. To study the nonlinear interactions between the two- and three-dimensional fundamental and subharmonic modes in the spatially developing shear layer, the selected five modes represent the minimum number that needs to be considered. The objective of this study is to investigate the nonlinear effects arising from the interactions between two- and three-dimensional large-scale coherent structures in a free shear layer. Our attention is focused on the energy exchange mechanisms between the various modes, and the effects of the nonlinear evolution of the phases of the interacting modes. It is shown that depending on the initial modal phase angles the downstream growth of the shear layer and the associated local entrainment can be strongly affected by the presence of these nonlinear interactions. This high sensitivity of the shear layer growth is limited to a narrow band of initial phase angle values.

Flow structure and unsteady fluctuation with separation over a two-dimensional backward-facing step

The flow over a backward-facing step(BFS)is a typical separation and reattachment flow.Its flow structures and unsteady mechanisms are still not well explored.In this paper,the global velocity fields of a BFS are obtained by a synchronous particle image velocimetry(PIV)system with Reh=5345(Reynolds number)and Er=2(expansion ratio).Flow structures are distinguished and defined by the fraction of the negative velocity(upn).The reattachment zone(Lr)is quantitatively defined as(u0.9,u/0.1)on the bottom wall.Spatial distribution of the large-scale vortices couples well with the divided flow structures and their temporal evolution presenting four stages(forming,developing,shedding and redeveloping)when travel downstream.The unsteady motions with various low frequencies are well explained by the coherent vortices and flow structures.Among the unsteady low frequency motions,the Kelvin Helmholtz(KH)vortices and the oscillation of Xr(OX)come likely from the free shear layer.The KH vortices contribute to the unsteadiness of the temporal flow,and the OX is the primary response to the vortical fluctuations.

On bypass transition in separation bubbles:a review

Transition from laminar flow to turbulent flow is of great practical interest as it occurs in many engineering flows and often plays a critical role in aerodynamics and heat transfer performance of those flow devices.There could be many routes through transition,depending on flow configuration,geometry and the way in which transition is initiated by a wide range of possible background disturbances such as free-stream turbulence,pressure gradient,acoustic noise,wall roughness and obstructions,periodic unsteady disturbance and so on.This paper presents a brief overview of wall bounded flow transition in general and focuses more on the transition process in the free shear layer of separation bubbles,demonstrating that at elevated free-stream turbulent intensity the so called bypass transition could occur in geometrically induced separation bubbles where the separation point is fixed.