Experimental Investigation of Spanwise Periodic Surface Heating for Control of Crossflow-Dominated Laminar-Turbulent Transition

Abstract

Research into laminar-turbulent transition delay in three-dimensional boundary layers is motivated by the possible drag reduction of aircraft with swept wings. Saric et al. discovered a nonlinear transition-delaying mechanism which is now known as "Upstream Flow Deformation" (UFD). Because the effect can be triggered by artificially exciting an exponentially amplified instability mode, it has the potential to be used as an energetically very efficient method for boundary-layer stabilization compared to approaches such as distributed suction and other actuation concepts aimed at reducing the crossflow component of the laminar boundary layer. Saric et al. used discrete roughness elements to artificially excite a stationary instability mode as control mode. Naturally, the stabilizing nonlinear effects strongly depend on initial conditions which motivates the research into actuation concepts better suited for controlling them. Therefore, different actuation methods with a controllable excitation amplitude have already been investigated, such as discrete pneumatic and plasma actuation. In the case of pneumatic actuation, the UFD effect has been successfully employed for transition delay, while no such successful attempt with a plasma actuator is known to the authors. In the present work, an alternative actuation concept based on spanwise periodic surface heating and its effect on the boundary-layer flow field were investigated using the redesigned DLR swept-flat plate experiment at a Reynolds number of Re=1.6*10^6 in the 1MG wind tunnel in Göttingen. An actuator has been developed which can create a spanwise periodic wall-temperature distribution at 8 different chordwise positions to introduce an initial disturbance into the boundary-layer flow field. Hot-wire anemometry in the boundary layer downstream has shown the successful excitation of the control mode and quantified its amplitude depending on the actuation position and the amplitude of the surface-temperature periodicity. For a selected combination of actuation position and amplitude, the impact on the boundary layer downstream of the actuator was investigated in detail. The unsteady boundary-layer flow field was sampled at 14 chordwise, 60 spanwise and 30 wallnormally distributed positions for a duration of 4 seconds each with a sampling rate of 32 kHz. This allows for the quantitative characterization of the relevant stationary and non-stationary phenomena across the complete transition scenario, including the onset of secondary instabilities and subsequent transition itself. Particular attention is paid in the presentation to the development of travelling crossflow instabilities because their inadvertent elevation by other actuation methods has been known to cause the UFD method to fail.