Synthetic Freestream Disturbance for the Numerical Reproduction of Experimental Zero-Pressure-Gradient Bypass Transition Test Cases

The present work introduces a means of forcing bypass transition within a zero-pressure-gradient smooth flat-plate boundary layer (ZPGSFPBL), suitable for DNS or LES computations for reproducing experimental datasets. In this type of bypass transition, one of the means of forcing transition within the boundary layer is via the introduction of a specific disturbance along the inflow boundary. Following this principle, the current method, introduces synthetic turbulence, generated by the method of Klein et al. (J. Comput. Phys. 186 (2), 652–665, 2003 ), at the inflow, a certain height above the boundary layer, thereby confining it within the freestream. The principle parameter which dictates the transition behaviour is the height above the boundary layer at which the freestream turbulence is injected. By adjusting this parameter, as well as the integral length-scale and the intensity (either estimated or known a priori from experiments), ILES computations providing good agreement with experimental data sets can be achieved upon a variety of grids. This procedure has been validated upon the ERCOFTAC T3A experimental test case (freestream turbulent intensity of 3 % ), where good matching is achieved on streamwise quantities like skin-friction coefficient, shape factor, boundary layer thickness and fluctuating velocity growth rates as well as for profiles of mean velocity and fluctuating velocities in the wall-normal direction.