Effect of Pressure Gradient on Local Excitation of Boundary-layer Instability Due to Free-stream Turbulence and Micro Surface Roughness

The prediction and control of the laminar-turbulent transition is crucial to the designs of vehicles, turbines, etc. The initial condition of transition depends on the exciting process of boundary-layer instability, which is the key to implement its prediction and control. The current researches confirm that the exciting process of boundary-layer instability, namely receptivity, is affected not only by different types of free-stream disturbances and shape parameters of surface roughness elements, but also by the pressure gradient of mean flow. Hence, we study the effect of pressure-gradient on local excitation of boundary-layer instability under the interaction of the low-level, isotropic free-stream turbulence and micro surface roughness in this work. The numerical results reveal the pressure-gradient effect on the receptive process and the group speed of excited wave packets in the Falkner-Skan boundary layer. The favorable/adverse pressure gradients (FPG/APG) are found to be able to promote/suppress the excitation and subsequent evolution of Tollmien–Schlichting (T-S) waves. Then the relations of the pressure gradient with the amplitude, growth rate, wave number, phase speed and shape function of excited T-S waves are studied.

Keywords:

Pressure gradient , Receptivity , Boundary Layer , Roughness element