Measurement of Wall Shear Stress Fluctuation with the Micro-fabricated Hot-film Sensor in a Boundary Layer of a Wall Jet

Abstract

In this study, we developed a hot-film sensor to measure the wall shear stress fluctuation in turbulent flows. The heated element of the sensor, which acts as a sensing part is 10 μm wide and is folded back twice for increasing its electrical resistance to produce heat. In addition, the substrate of the sensor is made up of silicon wafer of 1 μm thickness for increasing the sensor’s temporal resolution, in consideration of the substrate’s heating capacity. Further, the static/dynamic response test and the measurement of the wall shear stress fluctuation in the boundary layer of the wall jet are performed. The results show that the newly developed hot-film sensor responds to the wall shear stress properly and that its roll-off frequency is 1 kHz. In addition, it is found that the power spectrum of the wall shear stress fluctuation has several peaks. However, no distinct peaks are found in that of the streamwise velocity fluctuation near the wall. This implies that the wall shear stress fluctuation is influenced more by the specified flow structure, such as the coherent vortex structure which arises with a constant frequency in a turbulent boundary layer.