Direct Numerical Simulations of Wake-Perturbed Separated Boundary Layers

Gungor A. G., SIMENS M. P., JIMENEZ J.

JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, vol.134, no.6, 2012 (SCI-Expanded) identifier identifier


A wake-perturbed flat plate boundary layer with a streamwise pressure distribution similar to those encountered on the suction side of typical low-pressure turbine blades is investigated by direct numerical simulation. The laminar boundary layer separates due to a strong adverse pressure gradient induced by suction along the upper simulation boundary, transitions, and reattaches while still subject to the adverse pressure gradient. Various simulations are performed with different wake passing frequencies, corresponding to the Strouhal number 0.0043 < f theta(b)/Delta U < 0.0496 and wake profiles. The wake profile is changed by varying its maximum velocity defect and its symmetry. Results indicate that the separation and reattachment points, as well as the subsequent boundary layer development, are mainly affected by the frequency, but that the wake shape and intensity have little effect, and that the forcing is effective as long as the wake-passing period is shorter than the bubble-regeneration time. Moreover, the effect of the different frequencies can be predicted from a single experiment in which the separation bubble is allowed to reform after having been reduced by wake perturbations. The stability characteristics of the mean flows resulting from the forcing at different frequencies are evaluated in terms of local linear stability analysis based on the Orr-Sommerfeld equation. [DOI: 10.1115/1.4004882]