Delay of Laminar-Turbulent Transition on Swept-Wing with Help of Sweeping Surface Relief

In recent works [1, 2] an effective influence of oblique streaky surface relief to stationary cross-flow vortices amplification was established theoretically and experimentally. In the experimental work [2] the wave trains of stationary cross-flow vortices exited due to artificial 3D localized surface roughness were developed downstream both over the smooth surfaces (as baseline case) and in cases when the model surface was modified with surface relief (comprising elongated strips oriented oblique to the local flow direction). The obliqueness of relief strips with respect to the flow vector was varied and clear influence of the relief obliqueness on cross-flow vortex amplification was found. In particular in case when the surface relief strips were oriented parallel to the leading edge a remarkable cross-flow vortex amplitudes reduction was detected. Even stronger stabilizing effect was observed when the relief strips were oriented in between the leading edge and the local flow direction. Based on the obtained results a new concept of passive swept-wing flow laminarization was suggested. In contrast to experiments [2] performed with hot-wire on 35-degree swept-plate model the present experimental study is performed on 45-degree 3D swept-wing section [RODTRAC]. The distributed leading edge roughness of the swept-wing model provides excitation of unstable cross-flow vortexes the amplification of those leads to "saw-tooth-like" laminar-turbulent transition of 3D boundary layer flow. The surface of wing model was modified by applying of surface relief comprising strips oriented oblique to flow direction. Integral influence of surface relief on laminar-turbulent transition position was visualized with help of infrared thermography. It is proved that application of surface relief with proper orientation (according to findings in [1, 2]) is able to delay cross-flow dominated transition to turbulence on a swept-wing.