5.1 Velocity and Turbulence Measurement

In a turbulent regime, tiny flow structures are responsible for viscous drag increase of aerodynamic surfaces such as wings and engine inlet ducts in an aircraft. These structures are called eddies. Eddies are typically several hundred microns in width and several millimeters in length. The length scale of eddies is broad, ranging from the same size as the width of the flow field to the microscale, where viscous effects become dominant and energy is transferred from kinetic into internal [1].

Beside the small sizes, the short lifetime of eddies also pose a challenge for measurement and control. The lifetime of eddies is on the order of milliseconds [2]. Conventional instruments with low response time are not able to detect such a fast dynamic.

Because of their small size, micromachined devices are suitable for measuring and manipulating eddies in a turbulent flow. In addition to the spatial and temporal advantages, micromachined sensors and actuators can be integrated with microelectronics, which make them "smart." The small size and the batch machining process allow fabricating arrays of such devices, which can give more detailed information about the flow field.

The following sections discuss the design of microsensors for velocity and turbulence measurement. In contrast to the flow sensors in Chapter 8, velocity measurement in this section is referred to as point velocity measurement. The other type of turbulence measurement is shear stress sensing on walls. Although flow sensors, velocity sensors, and shear stress sensors serve different purposes,...