Flow and mass transfer measurements for a flat plate of finite thickness in pulsating flow

Abstract

Laboratory measurements were made of flow and mass transfer over a blunt flat plate of finite thickness, which is placed in a pulsating free stream, U-infinity = U-0(1 + A(0) cos 2 pi f(p)t). Low turbulence-intensity wind tunnel experiments were conducted for small and moderate Reynolds numbers, 770 less than or equal to Re-H less than or equal to 8000. Pulsation was generated by means of an acoustic speaker. The majority of experiments were carried out in the ranges of f(p) = 20.0-80.0 Hz and A(0) less than or equal to 0.15. Flow properties were measured by I-type and split-film probes. Mass transfer rates were measured by employing the naphthalene sublimation technique. The present results for non-pulsation flows (A(0) = 0.0) were shown to be consistent with the published data. For pulsating approach flows, results are provided for the distributions of the wall static pressure, the longitudinal mean velocity and turbulent intensity, and the Sherwood number, Sh, as a function of the stream-wise distance x* measured from the leading-edge separation point. As A(0) or f(p) increases, the time-mean reattachment length is reduced significantly. This implies that the height and length of the separation bubble shrink simultaneously; the position where C-p is recovered moves upstream, and the minimum value of C-p decreases; the reverse flow is intensified; and a substantial augmentation of turbulent energy is discernible. In the separation bubble, the effect of pulsation on Sh is conspicuous. Sit decreases monotonically from the separation point to the minimum value near the secondary separation point, and Sh increases appreciably with increasing x*, after passing the secondary separation point to the maximum value at the reattachment point: and afterward, Sh decreases. The secondary separation point and the position where Sit has a maximum move further upstream, as A(0) or f(p) increases. At large Re-H, the relative influence of pulsation on Sh weakens. (C) 1998 Elsevier Science Ltd. All rights reserved.