Separation control and efficiency improvement in a 2D diffuser by means of contoured cavities

The performance of a passive control method aimed at reducing and, possibly, eliminating boundary layer separation is evaluated by means of numerical simulation. The passive control, which consists of introducing appropriately-shaped cavities in solid walls, is applied to a plane diffuser. The Reynolds number is such that the turbulence can be neglected (Re=500, based on the diffuser half-width at the inlet section and the inlet velocity on the axis). A configuration characterized by an area ratio of 2 and a divergence angle of 7 degrees is chosen, so that, without the introduction of the control, the flow is characterized by a large zone of steady asymmetrical boundary layer separation. In order to reduce the separated zone and to increase the efficiency of the diffuser, a couple of symmetric contoured cavities is introduced in the diverging walls. An optimization procedure is developed to obtain the cavity geometry that maximizes the pressure recovery in the diffuser and minimizes the boundary layer separation extent. The introduction of the optimal cavities leads to an increase in pressure recovery of the order of 13% and to a strong reduction of the separation extent. This result is due to a favourable modification of the velocity and vorticity fields in the near-wall region. The most important geometrical parameters are also identified and the robustness of the control to small changes in their values is investigated. It is found that the contoured cavities are effective as long as the flow is able to reattach immediately downstream of the cavities.