Revisiting end conduction effects in constant temperature hot-wire anemometry

The effect of end conduction losses in constant temperature hot-wire anemometry is studied theoretically and experimentally. A perturbation analysis is employed to determine the dynamic response of a hot-wire together with its constant temperature anemometer (CTA), with unetched portions of the wire (stubs) and prongs taken into account. Results of the model reveal that when the end conduction losses are significant, dynamic sensitivity of the hot-wire-CTA system can be either more or less than the static sensitivity based on the specifications of the hot-wire probe and the CTA. This means that contrary to the commonly accepted scenario, a hot-wire with insufficient length-to-diameter ratio could either amplify or attenuate turbulence energy depending on probe and CTA parameters. Experiments performed in a turbulent boundary layer flow are used to confirm this result. Examination of variance of streamwise velocity fluctuations and energy spectra from the experiments reveal that interaction of the wire filament with the supports through axial conduction results in one or two low frequency steps in the transfer function of the hot-wire-CTA system. These steps can be either positive or negative depending on the hot-wire geometry and damping of the system.