Structure of local pressure-driven three-dimensional transient boundary-layer separation

The separation of a flat-plate, laminar boundary layer under the influence of a suddenly imposed three-dimensional external adverse pressure gradient was studied computationally by time-accurate numerical solution of the incompressible Navier-Stokes equations. The separation decay was then investigated by impulsively removing the pressure gradient. The development and decay of the separation structure were compared with experimental results reported by other investigators for the same geometry. The periodic vortex shedding of the three-dimensional separation was described in terms of a Strouhal number based on the freestream velocity and Blasius boundary-layer momentum thickness at the location where separation occurs. The characteristic Strouhal number of 0.0136 during the separation development from the computation compared favorably with 0.0134 from the experiment. When the adverse pressure gradient was impulsively removed, the boundary layer returned to an attached boundary layer much faster than the time required for the separation development.