Response of a split film probe under electrical perturbations

Abstract

The split film probe is designed for simultaneous measurements of streamwise and transverse velocity components. However, difficulties in obtaining reliable results have been experienced by many researchers. The problems are attributed mainly to static and dynamic cross talk through the substrate. In this paper, the static cross talk is examined in the first part by deliberately operating the film with different overheats. In the second part, both films are statically balanced, one of the films is subjected to an electrical perturbation, the other film will respond to the dynamic perturbation due to the imperfect insulation of the substrate. At low frequencies, the thermal response can be as high as 37%. At high frequencies, the frequency response is a function of the substrate characteristic frequency and the circuit characteristic frequency. The reasons for the thermal response are examined at the low-frequency limit. The dynamical unbalanced resistance generated by the electrical perturbation will set up an instantaneous temperature gradient across the substrate which produces the cross talk. Furthermore, if azimuthal asymmetry exists, the communication between the two films will also appear due to the azimuthal asymmetric temperature distribution.