Introduction: boundary conditions for large eddy simulation

TURBULENT flows are of considerable importance in many applications in aerospace engineering. Capturing the dynamics of all relevant scales of motion, based on the numerical solution of the Navier-Stokes equations, constitutes direct numerical simulation (DNS), which is prohibitively expensive in the foreseeable future for most practical flows of interest at moderate-to-high Reynolds number. On the other end of computer simulation possibilities, the Reynolds-averaged Navier-Stokes (RANS) equations, with averaging typically carried out over time, homogeneous directions, or across an ensemble of equivalent flows, have been employed for turbulent flows of industrial complexity. Large eddy simulation (LES) has become the effective intermediate approach between DNS and RANS, capable of simulating flow features that cannot be handled with RANS, such as significant flow unsteadiness and strong vortex-acoustic couplings, and providing higher accuracy than RANS at reasonable cost. New trends also combine RANS with LES to exploit the best features of both approaches in a complementary manner in so-called hybrid RANS/LES.