Corrections of hot-wire anemometer measurements near walls

Abstract

Applications of hot-wire anemometers to velocity rneaaurements near walls can result in erroneous velocity data owing to additional heat losses to the wall. It is difficult to account for these errors if calibration data are used that were obtained in calibration test rigs without walls. This has been recognized in many studies in which hot-wires were applied to measurements in wall boundary-layer flows and different suggestions for corrections have been given. The present paper summarizes these suggested corrections and points out existing differences. It is also shown that some hot-wire measurements have been performed without any corrections being applied and reasons for this are given. Whereas most of the existing suggestions for wall corrections of hot-wire data are based on experiments, the present approach uses results of a numerical study.Assuming the problem to be two-dimensional and that the wire can be replaced by a line source of heat, a numerical study is carried out for the temperature distribution downstream of the wire, and computations are performed for the heat loss from the wire in presence of the wall. Computations are performed for two Merent boundary conditions representing ideally conducting and non-conducting materials. These different boundary conditions yield large differences in the computed heat losses from the wire, and these explain the existing differences in the experimentally obtained corrections. The numerical study also shows that the large heat losses for conducting walls are due to the distorted temperature distribution in the temperature wake of the wire.Some of the results of the numerical studies were experimentally verified by the authors and a procedure haa been developed to correct instantaneous hot-wire readings for additional heat losses to a wall. For non-conducting walls, the heat losses are much smaller and are negligible for most practical measurements.

Experimental and Numerical Investigation of 2-D Backward-Facing Step Flow

“…The overheat ratio was set at 1)3 so as to minimize the heat conduction loss to the Plexiglas wall (Bhatia et al 1982;Turan & Azad 1987). All hot-wire calibrations, mean-flow measurements, free-stream turbulence levels, r.m.s.…”

Silicon based flow sensors used for mean velocity and turbulence measurements

“…In the innermost region of the profile, y+ = 8 through y+ = 35, the characteristic deviation from the theoretical lines, which has been noted by many Ligrani and Bradshaw (1987), also appears in the present measurements. Due to the very thin plastic laminate of the surface of the flat plate, this plate may be classified as a non-conducting wall, and since the innermost points of the present measurements are located as far from the wall as at y+ =8, no wall corrections are needed accoding to Bhatia et al (1982). The directional sensitivity of the silicon sensors is shown in Fig.…”

The physics of heat transfer from hot wires in the proximity of walls of different materials

“…After Wills [2], Polyakov and Shindin [4] revisited the hot-wire peculiarities near walls of different thermal conductivities (steel, copper and textolite). They showed that the deviation in hot-wire readings is mainly af- [4] was then followed by a numerical study carried out by Bhatia et al [5] for walls representing ideally conducting and nonconducting (adiabatic) materials. Their corrections were found to be independent of the wall friction velocity, u s , when measured data are plotted as DU þ ¼ f ðy þ Þ, and corrections for conducting materials are needed close to the wall, i.e., y þ 6 2:5.…”