Active and Natural Suction at Forward-Facing Steps for Delaying Laminar–Turbulent Transition

Abstract

Two-dimensional, direct numerical simulations are used to study the impact of active and natural (passive) suction at forward-facing steps on laminar–turbulent transition. Suction is applied through a gap in front of a step that is located in a flat-plate boundary-layer flow without streamwise pressure gradient. A steady base flow is used with a Mach number of 0.6. Subsequently, Tollmien–Schlichting waves are introduced by suction and blowing at the wall, and their growth over the surface imperfection is evaluated by factors. The investigated step heights are in the range of one to two times the local displacement thickness of the smooth flat plate. Both sharp and rounded step corners are investigated. Cases with and without suction are compared with the smooth flat plate without suction based on factors according to the method. Thus, it is shown that suction is capable to compensate or even overcompensate the negative impact of different steps on laminar–turbulent transition. The work concludes with a configuration that allows natural (passive) suction in front of a forward-facing step. Here, a significant reduction of the factor compared to the smooth flat plate without suction is possible. Further estimations indicate that a net drag reduction may be possible as well.