On the frequency response of wall transfer probes

The time response of wall transfer probes may significantly affect the interpretation of the physical phenomena involved. The transfer function between the heat or mass flux at the probe surface and the wall shear is analysed in the frequency domain. Space variation induced by the finite propagation velocity of a fluctuating wall shear is considered, as frequently encountered in various two-phase flow systems. The analysis indicates that the response is determined by two parameters-the non-dimensional frequency and the non-dimensional fluctuation velocity. The amplitude and phase corrections, which are to be applied to pseudosteady model calculations are numerically calculated. It is found that as the wall shear fluctuation is slowed down the probe frequency response significantly deteriorates. The sensitivity of the response to the fluctuation velocity reduces as the Prandtl or Schmidt numbers increase, indicating a superiority of electrochemical probes over themal probes.