A Preliminary Report on the Zero-Crossing-Rate Technique for Average Shear Measurement in Flowing Fluid

The characteristics of a flush-mounted hot film sensor were investigated in turbulent flows of both water and drag reducing polymer solutions in a 4-inch-diameter pipe. For water flows, a linear relationship was found between the average power supplied to the sensor and the cube root of the wall shear stress. With the addition of polymer additives, the heat transfer rates at a given shear stress were reduced from those found with water alone. Analysis of the heat transfer fluctuations occurring in various flow facilities has shown that the zero crossing rate is related to the wall shear stress and to fluid properties for water, polymer, and air flows. The zero crossing rate is not dependent on hot film sensitivity or contamination. Evaluations of the fluctuation microscale indicated that it had been increased by the addition of drag reducing polymer to the water. Autocorrelation measurements were made of the heat transfer fluctuations, but the limited data for the autocorrelations were not conclusive. Attempts to obtain cross-correlation coefficients between heat transfer and surface pressure fluctuations as measured with a small hydrophone were unsuccessful. The zero crossing rate of the surface pressure fluctuations was found to be related to the local wall shear stress.Naval Ship Research and Development Cente