Numerical Study On The Flat Plate Boundary Layer Bypass Transition Induced By The Cylinder Wake

Transition,referring to the transformation of laminar flow changing to turbulent flow,characterizes the flow state's changes.According to the environmental disturbance's intensity affected by the flow and the path of flow's changing,transition can be roughly divided into two different types: regular transition and bypass transition.The bypass transition received high attention because of its special complexity and important engineering value.In this paper,Large-Eddy simulations of flow past a circular cylinder in the vicinity of a flat plate are carried out to study the bypass transition under the circumstances.Based on the basic idea of LES,the Wall-Adapting Local Eddy-Viscosity Model and Boussinesq hypothesis are introduced into the filtered Navier-Stokes equation,so the N-S equation can be numerical solved completely.The statistical characteristics of the flow field are obtained from the LES,and the validation of numerical algorithm is proved through the comparison of the results of previous experiment results.LESs are carried out for different values of Reynold numbers(Re D)based on the cylinder diameter and free-stream velocity,and different gap to diameter(G/D)ratios.The flow field characteristics of bypass transition are analyzed,including timeaverage flow characteristics,pulsatile flow characteristics and turbulent kinetic energy characteristics.The shedding of the cylinder trailing vortex,the boundary layer structure,the generation of the secondary vortex and the evolution of the secondary vortex downstream interacting with the cylindrical wake or the boundary layer of the plate are studied in detail.The influence of Reynolds number and cylinder height on flow transition is illustrated.