Experiments in Fluids manuscript No. (will be inserted by the editor) Unsteady Wall-Shear Measurements in Turbulent Boundary Layers Using MEMS

Abstract Fluctuating skin friction is measured in two and three dimensional turbulent boundary layers using a MEMS sensor and a wall-wire as reference. Skewness, flatness and spectra of the turbulent skin friction are presented to demonstrate the potential and limitations of the MEMS sensor. The measured turbulence intensi-ties of the order of 0.4 are in general agreement with a number of experimental and DNS studies. However, the fluctuating quantities measured with this MEMS sensor, operated at an over heat ratio of 1.3, is shown to depend on the Reynolds number or mean skin friction. There-fore, such a high over-heat ratio, which was proven to dramatically increase the accuracy of mean skin friction measurements in a previous study by the authors [21], may not be appropriate for the measurement of fluctu-ating wall-shear with MEMS sensors, particularly at low mean shear values.