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-wave voltage-perturbation test to systematically quantify the frequency response (fs) 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 fs were compared against the dynamic frequency response (f(D)) previously obtained in parts I and II using a known near-wall fluctuating flow field. Although the observed trends for fD and fs were similar, their magnitudes were vastly different, notably for the commercially available hot-film gauges, or which fs was up to five orders of magnitude greater than fD. 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.