Evolution of stationary crossflow vortices in boundary layers in swept wings

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Evolution of stationary crossflow vortices in boundary layers in swept wings

Joslin, R.D.

The spatial evolution of crossflow-vortex disturbances in a laminar boundary layer on a swept wing is computed by the direct numerical simulation of the incompressible Navier-Stokes equations. A wall-normal velocity distribution of steady suction and blowing at the wing surface is used to generate a strip of equally spaced and periodic disturbances along the span. Three simulations are conducted to study the effect of initial amplitude on both the disturbance evolution and to provide a database for transition-onset prediction methods. As an example application, a theory for a transition model is compared with the direct numerical simulation data. In each simulation, the vortex disturbances first enter a chordwise region that can be described as linear growth; then, the individual disturbances coalesce downstream and either superpose linearly or interact nonlinearly with adjacent modes; finally, the crossflow vortices enter a region that contains strong nonlinear interactions sufficient to generate inflectional velocity profiles. As the initial amplitude of the disturbance increases (roughness size increases), the length of the evolution to breakdown decreases. In the example application, the three simulations collapse onto a single curve, which is an intermittency correlation for a transition model.