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Heated anemometry and thermometry in water Lueck, Rolf Gero

Abstract

The characteristics of several types of heated sensors used for measuring oceanic turbulence have been examined. The author measured the steady heat flux from glass coated ellipsoidal microbead thermistors, computed numerically the unsteady heat flux from an ideal constant temperature flat plate, and analyzed the steady flow calibration data of a paralene-c coated plate thermistor as well as the steady and unsteady calibration data of two conical constant temperature hot film anemometers. In order to understand the behaviour of probes alone, thermal models of these sensors have been developed. The models incorporate a Nusselt number governed steady heat flux from the wetted surface and the thermal effects of a surface coating and a supportive substrate. Derived functional relationships between the steady heat flux and the flow rate agree favourably with the available calibration data. The quasi-steady sensitivity of these probes when used as anemometers or thermometers as well as their signal contamination by temperature or velocity are calculated using the functional heat flux relationships. The substrate and the coating reduce the sensitivity to temperature and to speed as well as the ratio of speed-to-temperature sensitivity. The response of sensors is not governed by the Nusselt number when the boundary layer is unsteady. The unsteady response of a sensor to velocity oscillations is governed by its unsteady viscous boundary layer and may increase with increasing frequency over some frequency ranges. The response bandwidth is wider for velocity than for temperature. The ratio of unsteady temperature-to-velocity sensitivity is highest at zero frequency. Frequency response calibrations methods must realistically simulate the sensor's unsteady viscous boundary layer.

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