Local global SAW sensors for turbulence

The determination of fluctuating normal and shear stresses on a structure embedded in a turbulent flow is an important problem in fluid dynamics. A novel surface acoustic wave (SAW) sensor is developed to detect the surface forces (wall pressure and wall friction) and the direction of the turbulent flow as a function of position and time on the structure. The sensor is composed of a pair of SAWs having an identical center frequency with surface waves experiencing shear stresses in opposite directions. The difference in SAW velocity is proportional to the shear stress associated with the turbulent flow. The difference between the mean velocity of a pair of the SAWs subject to turbulent flow and the velocity of the SAW in a stationary fluid is proportional to the pressure (normal stress of the flow). The direction of fluid flow is determined through an arrangement of three pairs of SAW sensors in a manner similar to a strain rosette. The SAW sensor can operate over a wide frequency range. Thus, by employing an appropriate center frequency depending on the spatial and temporal resolution required, it is possible to measure simultaneously the fluctuating surface forces and the direction of a turbulent flow both locally and globally.<>