Two- and three-dimensional large-eddy simulations of a transitional separation bubble

In the present study we use two- and three-dimensional large-eddy simulations to examine the role of small-scale turbulence within a transitional separation bubble studied experimentally by Gaster (AGARD Conference Proceedings No. 4, 1966, pp. 813-854). In addition, several large-eddy simulation parameters and models are studied to show their effect on the computations. The inclusion of a small-scale turbulence model in the two-dimensional computations leads to an increase in the time-averaged separation bubble length and a slight reduction in the peak of the pressure coefficient distribution near reattachment. Increasing the filter width or increasing the Smagorinsky coefficient reduces the peak in the pressure coefficient distribution but also decreases the pressure coefficient within the pressure plateau. The two-dimensional LES accurately predicts the time-averaged bubble length of Gaster but does not accurately describe the experimental wall pressure distribution within the bubble. Three-dimensional LES computations allow the generation of vortex shedding and Görtler vortices within the separated region. A computation without a subgrid scale model allows the Görtler vortices to grow in strength and eliminate the boundary layer separation. The application of a subgrid scale model reduces the strength of Görtler vortices and spanwise vortex shedding. This produces a bubble size and time-averaged wall pressure distribution which compare favorably with experiment. Little difference is seen between the results using the constant coefficient and dynamic coefficient models.