Roughness Induced Transition Delay in a Swept-Wing Boundary Layer in Presence of Freestream Disturbances. Part 1. Turbulence Effects

Results of experimental investigations on control of the laminar-turbulent transition location in a 45-degree swept-wing boundary layer by the distributed micro-sized roughness (MSR) elements in presence of freestream perturbations are discussed in this two-part work. Part 1 of this study (the present paper) is devoted to the freestream turbulence effects, while Part 2 (Borodulin et al., 2023) concentrates on consideration of the acoustic waves' effects. The present paper (Part 1) contains: (i) a brief description of previous studies in the field, (ii) description of the experimental setup and techniques of the measurements (based on infrared thermography), (iii) the results of optimization of parameters of the MSR-elements, and (iv) results of studies of freestream turbulence effects on the MSR transition control. The MSR-elements are shown to excite in the boundary layer controlled stationary vortices, which are able to modify significantly both the base flow and its stability characteristics. Variation of the height and the period of the MSR-elements allowed us to find the most stabilizing effect on the boundary layer flow compared to the case without MSR as a reference one. A significant downstream shift in the position of the laminar-turbulent transition caused by the MSR has been detected and documented in detail. The robustness of this flow control method to freestream turbulence has been studied by increasing the turbulence level by means of turbulence generating grids. At enhanced freestream turbulence levels, the laminar-turbulent transition was found to move upstream, while the MSR-elements lost their effectiveness in the transition control.