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Unsteady shear stress measurements with heated films--analysis and experiment
Abstract
A hot-film sensor, a thin metal film deposited on a solid substrate, is mounted flush with the wall to measure the wall shear stress based on the heat transfer from the heated film in the viscous region. Hot-film sensors are a valuable tool for the measurement of the wall shear stress in steady flow. However, in unsteady flow a serious error is caused by the loss of heat in the substrate which retards the hot-film sensor response. This dissertation extends the use of hot-film sensor to unsteady flow measurements. A numerical calculation is carried out for the transient response of a flush-mounted hot-film sensor in a steady gas flow heated by a step change in the hot-film output flux. This calculation is part of a study of unsteady behavior of hot-film anemometers that involve substrate effects. The numerical method used is called the Unsteady Surface Element method. The numerical results show that the early-time film temperature agrees well with an analytical solution and the steady-state Nusselt number agrees with an existing numerical solution. The spatial average film temperature versus time is presented for several values of the rectangular hot-film aspect ratio. Through an inverse method, a hot-film sensor is investigated numerically for the phase lag and the amplitude reduction in step-like and oscillating flow. Accuracy of the recovered shear stress in these two flow pattern is also examined. Experimental measurements on the response behavior of the hot-film in unsteady flow compare favorably with the numerical simulation results. The distorted shear stress from the hot-film measurement in unsteady flow is recovered successfully by our inverse estimation scheme. A parameter estimation scheme is also applied to obtain the substrate thermal properties of hot-film sensors.
Subject Area
Mechanical engineering|Aerospace materials
Recommended Citation
Liang, Ping-Wen, "Unsteady shear stress measurements with heated films--analysis and experiment" (1992). ETD collection for University of Nebraska-Lincoln. AAI9308185.
https://digitalcommons.unl.edu/dissertations/AAI9308185