The dynamic response of a hot-wire anemometer: III. Voltage-perturbation versus velocity-perturbation testing for near-wall hot-wire/film probes

Abstract

Experiments were performed for the first time using the electronic square-wave voltage-perturbation test to systematically quantify the frequency response of near-wall hot-wire probes subjected in turn to varying magnitudes of convective velocity, different substrate materials and changes in wall-substrate temperature. In addition, quartz-substrate hot-film gauges with various thicknesses of quartz coating were also tested. Results of were compared against the dynamic frequency response previously obtained in parts I and II using a known near-wall fluctuating flow field. Although the observed trends for and were similar, their magnitudes were vastly different, notably for the commercially available hot-film gauges, or which was up to five orders of magnitude greater than . This signifies that there are possibly inherent differences between square-wave voltage-perturbation and velocity-perturbation tests for quantifying the frequency response of a hot-wire/hot-film system. These differences are then analysed in relation to the equation of a CTA unit put forth by Freymuth.

The thermal characteristics of a hot wire in a fluctuating freestream flow

“…. '', which concurs with the series of experimental findings of Khoo et al (1998Khoo et al ( , 1999 and Chew et al (1998a) that the dynamic response of the hotwire system in the form of the cut-off frequency due to imposed velocity perturbation test (f DV ) is about one to five order of magnitude lower than the square wave electronic perturbation test given as f DS , depending on whether it is the hot wire or hot film. The experiments indirectly support the contention that the thermal response of the hot wire is the overriding factor for determining the overall response of the hot-wire system and not the (electronic) CTA unit per se.…”

“…However, it is not trivial experimentally to generate an accurately known fluctuating velocity field to obtain the true dynamic response. To a large extent the user has invariably relied solely on the manufacturer's specification which is obtained via the standard electronic perturbation tests to determine the cut-off frequency and taken synonymously as the dynamic response frequency of the hot-wire sensor; this is not necessarily so (see Khoo et al, 1999). Strictly, a hot-wire anemometer should first be calibrated in a known flow (usually a steady flow) and then subsequently subjected to imposed fluctuating flow with known amplitude and frequency.…”

Hot-wire spatial resolution issues in wall-bounded turbulence

“…Regardless, it is clear that some of the laboratory data shown in figures 1 and 2 are approaching an effective Reynolds-number ceiling with conventional anemometer technology. Such considerations become especially pertinent when one considers the results of Khoo et al (1999) and Li (2004), both of which have indicated that the actual response of the combined hot-wire sensor/constant-temperature anemometry system could be up to an order of magnitude less than that indicated by a standard sine/square-wave perturbation test. Also Comte-Bellot (1976) and Freymuth (1977b) have discussed the nonlinear response of hot-wire anemometers, both showing that rapid high-magnitude fluctuations can substantially reduce the useful frequency range of anemometers.…”

Phase Resolved Hot-wire Measurements for High-speed Turbomachinery Flows

“…The electronic pulse test is the standard method for estimating the frequency response of the electronic system, but does not necessarily provide an accurate characterization of the frequency response. 7 The term "fluctuation errors" refers to the methods used for solving nonlinear calibration equations using time-averaged quantities. These errors occur because of the non-linear relationship between voltage and the flow quantities of interest.…”