Effect of large streamwise gradients on laminar-turbulent transition - A new numerical approach -

Abstract

One of the key areas for present and future aircraft design is to achieve a significant drag reduction by delaying the transition of the incoming free-stream air from laminar to turbulent state [1]. However, the unavoidable presence of surface irregularities like steps, gaps, humps, etc. introduces regions of localized strong streamwise gradients. These surface irregularities can enhance the linear spatial growth of traveling instabilities, such as TS waves or CF vortices, reducing the expected delay in LTT location. Therefore, it becomes critical to correctly model the interaction of the flow instabilities with the above mentioned surface irregularities. Standard methodologies such as LST (Local Stability Theory) or PSE (Parabolized Stability Equations) cannot handle rapid changes of flow quantities in streamwise direction. Other methodologies, for example HLNS (Harmonic Linearized Navier-Stokes) equations or linear DNS (Direct Numerical Simulations), do not impose any assumption regarding the nature of the flow instabilities in streamwise direction. Consequently, they are computationally very expensive and can restrain the parametric range of surface irregularities to be studied.