Directional Sensitivity of Cylindrical Hot-Film Probes in Liquids

Abstract

Hot-film probes are mainly used to measure mean and fluctuating velocities in liquids. Until now, however, the directional sensitivity has not been investigated for all probes of interest. Only for wedge-shaped probes this has been done (see F.J. Resch, DISA Inf. No. 14, 1973). The present paper completes the knowledge about the directional sensitivity for cylindrical hot-film probes in liquids.

The effective cooling velocity, u_eff, of a hot-film sensor can be expressed by

$${{u}_{{eff}}} = U{{(co{{s}^{2}}\alpha + {{k}^{2}}si{{n}^{2}}\alpha )}^{{\frac{1}{2}}}} $$

Here Ucosα: is the normal component of the velocity vector U with respect to the hot-film sensor axis and U sinα the tangential velocity component. The influence of the tangential component is described by the so-called k-factor, which is a function of the velocity U and the yaw angle α.

The following results were obtained for two types of cylindrical DISA hotfilm probes: The k-factor for the standard probe 55F06 and for the slanted probe 55F07 is independent of the velocity U and approximately 0.7. For yaw angles α ≲ 50^° the k-factor does not depend on the yaw angle for both types of probes investigated. Only the slanted probe showed a dependence for yaw angles α ≳ 50^°.