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

Experiments were performed for the first time using the electronic square-w ave voltage-perturbation test to systematically quantify the frequency resp onse (fs) of near-wall hot-wire probes subjected in turn to varying magnitu des of convective velocity, different substrate materials and changes in wa ll-substrate temperature. In addition, quartz-substrate hot-film gauges wit h various thicknesses of quartz coating were also tested. Results of fs wer e compared against the dynamic frequency response (f(D)) previously obtaine d in parts I and II using a known near-wall fluctuating flow field. Althoug h the observed trends for fD and fs were similar, their magnitudes were vas tly different, notably for the commercially available hot-film gauges, or w hich fs was up to five orders of magnitude greater than fD. This signifies that there are possibly inherent differences between square-wave voltage-pe rturbation and velocity-perturbation tests for quantifying the frequency re sponse of a hot-wire/hot-film system. These differences are then analysed i n relation to the equation of a CTA unit put forth by Freymuth.