Use of Hot-Film Anemometry for Wall Shear Stress Measurements in Unsteady Flows

Hot-wire and hot-film anemometry are widely used in steady flows for instantaneous velocity measurements, and their use has been extended to velocity and wall shear stress measurements in unsteady flows. The technique of hot-film anemometry relies on the Reynolds analogy which relates the diffusion of heat to the momentum exchange. The paper investigates the applicability of the analogy in linearly varying flows. The investigation is a combination of CFD analyses using the Transition SST model and experimental measurements. Results show that, in a linearly accelerating flow, while wall shear stress increases immediately upon the onset of acceleration, heat transfer indicates a relative lag in response. A quantitative analysis of the effects of flow parameters shows that the deviant behaviour is especially pronounced with increasing acceleration and/or reduced initial flow Reynolds number. The initial deviation can be predicted using a non-dimensional parameter based on turbulence timescales and acceleration rate, thereby providing a possible solution to correcting wall shear stress measurements using hot-film anemometry in fast accelerating flows.