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.
Wetzel, Joseph M.; Killen, John.
A Preliminary Report on the Zero-Crossing-Rate Technique for Average Shear Measurement in Flowing Fluid.
St. Anthony Falls Laboratory.
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