TABLE OF CONTENTS An area where MEMS sensors have considerably broadened the field of study is fluid dynamics. A typical MEMS sensor is at least one order of magnitude smaller than conventional sensors used to measure instantaneous flow quantities such as pressure and velocity [118]. The micromachined sensors are able to resolve all relevant scales, even in high Reynolds number turbulent flows. Due to their small size, the inertial mass and the thermal capacity are reduced. Thus, they can be used for the study of turbulent flows, where a high-frequency response and a fine spatial resolution are essential. The smallest scales of eddies in turbulent flow are in the order of 100 ?m [64]. Arrays of microsensors could make it possible to achieve complete information on the effective small-scale coherent structures in turbulent wall-bounded flows. Applications of turbulent flow study include the optimization of wing sections of aircraft, the minimization of noise generation of vehicles, or mixing enhancement for fluids.
The goal of measuring turbulent flows is to resolve both the largest and smallest eddies that occur in the flow. In order to obtain meaningful results, both wall pressure and wall shear stress need to be measured [118]. The wall shear stress is the friction force that a flow exerts on the surface of an object.
The wall pressure can be measured with the sensors described in Chapter 6. L fdahl et al. [118] recommends that the pressure sensor needs to have a membrane size between 100 100 ?
