MEMS-based pressure and shear stress sensors for turbulent flows

From a fluid dynamics perspective, the introduction of microelectromechanical systems (MEMS) has considerably broadened the spectrum of workable experiments. A typical MEMS sensor is at least one order of magnitude smaller than traditional sensors used to measure instantaneous flow quantities such as pressure and velocity. The microsensors can resolve all relevant scales even in high-Reynolds-number turbulent flows, and arrays of microsensors make it feasible, for the first time, to achieve complete information on the effective small-scale coherent structures in turbulent wall-bounded flows. In this paper we focus on the use of MEMS for the diagnosis of turbulent shear flows and survey the status and outlook of microsensors as used for measurements of fluctuating wall pressure and wall shear stress, two quantities which we deem particularly difficult to measure with conventional probes. For both wall pressure and wall shear stress sensors, we give general background, design criteria and calibration procedure. Examples of measurements conducted with MEMS-based sensors are provided and the minute devices are compared to their larger cousins.