MEMS flexible thermal flow sensors for measurement of unsteady flow above a pitching wind turbine blade

MEMS (Micro-Electro-Mechanical System) thermal flow sensors have been applied widely in boundary-layer studies and aerodynamic flow sensing due to high spatial resolutions and fast response times as well as minimal interference with fluid flow. In this study, self-made MEMS thermal flow sensors were designed and fabricated on a flexible skin. The steady laminar separation was investigated on two-dimensional LS (1) 0417 airfoil by using thermal flow sensors at various angles of attack with validation by hot wires and flow visualization. The unsteady flow on the pitching airfoil was experimentally investigated to simulate the dynamic stall condition of VAWT (Vertical Axis Wind Turbine). Based on variations of the mean value and standard deviation of the thermal flow sensor signals, nine stages of unsteady flow developing events are identified with further evidence from flow visualization. As the reduced frequency (k) increases, the incipient transition is delayed to higher angles of attack during the pitch-up motion; the re-laminarization is postponed to lower angles of attack during the pitch-down motion. The hysteresis is more pronounced at higher k where the oscillating time scale plays a more significant role in determining the unsteady flow pattern than the convective time scale. The phase difference between transition and re-laminarization was enlarged from Δα≈4.9° for k=0.009 to Δα≈13.5° for k=0.027 at Re=6.3×104.