SPATIAL NUMERICAL-SIMULATION OF BOUNDARY-LAYER-TRANSITION - EFFECTS OF A SPHERICAL-PARTICLE

In the present study, the effects of an isolated stationary spherical particle on the transition process in a hat-plate boundary layer are e xamined by a spatial direct numerical simulation. The full three-dimen sional time-dependent incompressible Navier-Stokes equations are integ rated by a time-splitting method and discretized spatially by a high-o rder finite difference/spectral method. A virtual boundary technique d efining the no-slip boundary of a sphere is implemented within the Car tesian geometry of the computational grid. Two numerical simulations w hich consider the effects of the sphere on the boundary layer are pres ented. The subcritical Reynolds number case reveals the appearance of hairpin vortices shed into the sphere wake which decay as they are con vected downstream. The initial interaction of the sphere and the bound ary layer produces a three-dimensional isolated disturbance comprising a wave part and a transient part. The decaying transient part is conv ected downstream at the local mean velocity, while the wave part induc es a decaying Tollmien-Schlichting wave in the flow field. In the seco nd case, an increase in the Reynolds number results in a wedge of inci pient turbulent flow downstream of the sphere. The development of the wake of the sphere is dominated by the appearance of an isolated distu rbance which rapidly breaks down forming a structure resembling a turb ulent spot. It is demonstrated that the transition induced by a sphere in the boundary layer is due to a mechanism related to bypass transit ion.