Evaluation of cntTSP for Boundary Layer Transition Detection in Low Speed Flow

Abstract

Recently, low-Reynolds-number flight vehicles have attracted attention such as a micro air vehicle (MAV) and a Mars airplane. For the experiments conducted at lowReynolds-number conditions under the atmosphere, the flow of the wind tunnel must be low speed. It is difficult to measure the pressure field on the wing surface using pressuresensitive paint (PSP) under the low-speed conditions because the pressure fluctuation on the wing surface is very small. The TSP can be used for the boundary layer transition detection even in low-speed conditions. The visualization principle of the TSP measurement is based on the temperature fluctuation caused by the different heat transfer coefficient in the laminar and turbulent flows. When the model surface is heated, the heat transportation is facilitated and the temperature fluctuation on the model surface is increased. A cntTSP is an advanced TSP technique which uses carbon nanotubes (CNT) as a thin electric inner heater for the TSP. The CNT layer can continue to supply constant heat to the model surface even during the measurement. Therefore, the cntTSP allows the dynamic visualization on a moving airfoil, which requires a long-time measurement. In this study, the cntTSP was applied to an oscillating airfoil in order to visualize the dynamic boundary layer transition in the low-speed condition. We succeeded to detect the moving boundary layer transition positions on the oscillating airfoil in the wind tunnel test. A hysteresis of the boundary layer transition position was observed in pitching-up and pitching-down procedures. It was assumed that the hysteresis of the transition position was caused by the unsteadiness of the aerodynamic phenomenon. However, it was also indicated the hysteresis includes the delay of the time response of cntTSP at the same time. Thus, we conclude that the time response is a technical problem of cntTSP and the evaluation of the time response is required.