Vertically localised equilibrium solutions in large-eddy simulations of homogeneous shear flow

Resumen

Unstable equilibrium solutions in a homogeneous shear flow with sinuous (streamwise-shift-reflection and spanwise-shift-rotation) symmetry are numerically found in large-eddy simulations (LES) with no kinetic viscosity. The small-scale properties are determined by the mixing length scale l(S) used to define eddy viscosity, and the large-scale motion is induced by the mean shear at the integral scale, which is limited by the spanwise box dimension L-z. The fraction R-S = L-z = l(S), which plays the role of a Reynolds number, is used as a numerical continuation parameter. It is shown that equilibrium solutions appear by a saddle-node bifurcation as R-S increases, and that the flow structures resemble those in plane Couette flow with the same sinuous symmetry. The vortical structures of both lower-and upper-branch solutions become spontaneously localised in the vertical direction. The lower-branch solution is an edge state at low R-S, and takes the form of a thin critical layer as R-S increases, as in the asymptotic theory of generic shear flow at high Reynolds numbers. On the other hand, the upper-branch solutions are characterised by a tall velocity streak with multiscale multiple vortical structures. At the higher end of R-S, an incipient multiscale structure is found. The LES turbulence occasionally visits vertically localised states whose vortical structure resembles the present vertically localised LES equilibria.