Control of Crossflow Transition at High Reynolds Numbers Using Discrete Roughness Elements

Abstract

Transition analysis is performed for a swept wing at a Mach number of 0.75 and chord Reynolds number of approximately , with a focus on roughness-based crossflow-transition control at high Reynolds numbers relevant to subsonic flight. The roughness-based transition control involves controlled seeding of suitable, subdominant crossflow modes to weaken the growth of naturally occurring, linearly more unstable instability modes via a nonlinear modification of the mean boundary-layer profiles. Therefore, a synthesis of receptivity, linear and nonlinear growth of crossflow disturbances, and high-frequency secondary instabilities becomes desirable to model this form of control. Because experimental data are currently unavailable for passive crossflow-transition control on high-Reynolds-number configurations, a holistic computational approach is used to assess the feasibility of roughness-based-control methodology. The potential challenges inherent to this control application, as well as the associated difficulties in modeling this form of control in a computational setting, are highlighted. At high Reynolds numbers, a broad spectrum of stationary-crossflow disturbances have large-enough linear amplification to cause transition, and, while it may be possible to control a specific target mode using discrete roughness elements, the nonlinear interaction between the control and target modes may yield strong amplification of the difference mode and, hence, produce an adverse impact on the transition delay using spanwise periodic roughness elements.