Laterally converging duct flows. Part 4. Temporal behaviour in the viscous layer

Abstract

Since insight into entropy generation is a key to increasing efficiency and thereby reducing fuel consumption and/or waste and – for wall-bounded flows – most entropy is generated in the viscous layer, we examine the transient behaviour of its dominant contributor there for a non-canonical flow. New measurements in oil flow are presented for the effects of favourable streamwise mean pressure gradients on temporal entropy generation rates and, in the process, on key Reynolds-stress-producing events such as sweep front passage and on the deceleration/outflow phase of the overall bursting process. Two extremes have been considered: (1) a high pressure gradient, nearing ‘laminarization’, and (2), for comparison, a low pressure gradient corresponding to many earlier experiments. In both cases, the peak temporal entropy generation rate occurs shortly after passage of the ejection/sweep interface. Whether sweep and ejection rates appear to decrease or increase with the pressure gradient depends on the feature examined and the manner of sampling. When compared using wall coordinates for velocities, distances and time, the trends and magnitudes of the transient behaviours are mostly the same. The main effects of the higher pressure gradient are (a) changes in the time lag between detections – representing modification of the shape of the sweep front and the sweep angle with the wall, (b) modification of the magnitude of an instantaneous Reynolds shear stress with wall distance and (c) enlarging the sweeps and ejections. Results, new for both low and high pressure gradients, are the temporal behaviours of the dominant contribution to entropy generation; it is found to be much more sensitive to distance from the wall than to streamwise pressure gradient.