Expansion ratio effects on the separated shear layer and reattachment downstream of a backward-facing step

Abstract

Experiments were carried out to study the behavior of the incompressible turbulent separated shear layer and subsequent reattachment, downstream of a backward-facing step in a channel. The main objective of the study was to determine the effect of the expansion ratio on the development of the mean velocity and turbulence intensity in the shear layer and on the evolution of wall static pressure downstream of the step. The step height-to-upstream channel height ratio was varied between 0.5 and 2.13 while all inlet conditions were kept constant. Both hot-wire anemometry and frequency shifted laser Doppler anemometry were used for the velocity measurements. The Reynolds number based on free stream velocity and channel height upstream of the step was 16,600. The expansion ratio was found to have a particularly strong influence in the development of the turbulent, separated shear layer. Larger step heightto-inlet channel height ratios lead to higher turbulence intensities and faster growth of the unstable shear layer. As a result of this, shorter normalized reattachment lengths occurred with lager expansion ratios. For all the expansion ratios studied, the mean reattachment lenght was uniform along the spanwise direction except very near the side walls. List of symbolsCp H h P Re

PIV measurements in a turbulent wall jet over a backward-facing step in a three-dimensional, non-confined channel

“…These results indicated that there was a non-uniform shear layer thickness near the side walls and that the spanwise uniformity of the mean flow was limited to approximately a 50% depth of the channel. This observation was also found in Ötügen [46]. Indeed, Fig.…”

“…In our experiments, the turbulence intensity of the incoming wall jet reaches approximately 24% of the reference velocity U m . This level was higher than that of the duct flow in the studies of Stevenson et al [58], Vogel and Eaton [61] and Ötügen [46] and strongly enhances the crossstream diffusion momentum [28]. Finally, it is necessary to note that the turbulent wall jet over a backward-facing step in this study is characterized by three length-scales, i.e., the vertical height y m of the inner region, the jet half width Y 1=2 for the wall jet flow, and the step height h for the backward-facing step flow.…”

“…It is observed that the shear layer vorticity thickness growth rates from Ötügen et al [47] are at the same order of magnitude as those obtained our experiments but they remain smaller because of their lower starting value. We argue that the differences in the upstream flows, i.e., the plane turbulent channel and the confined top wall in Ötügen [46] and Ötügen et al [47], yielded the discrepancy in the comparison. However, in both the results, the shear layer growth rates from z=L z ¼ 0:24 increased with a larger spanwise distance from the middle channel, and the difference in the growth rate was greater downstream of the step.…”

“…9 illustrates the primary mechanism for the three-dimensionality of the flow in the vicinity of the wall, where the streamwise vortex patterns are generated at the junction of the side walls and the step corner. The longitudinal vortices develop along the side walls, interact with the secondary flow pattern, and result in the rapid growth of the spanwise non-uniformity of the velocity with the axial distance [46]. Fig.…”

Effects of upstream roughness and Reynolds number on separated and reattached turbulent flow

“…The backward facing step (BFS) has been thoroughly investigated using both numerical simulation and experimental methods. [8,[10][11][12] Comprehensive reviews of previous research on BFS and other separated flows have also been undertaken in the past, [4,13] and a more recent study by Nadge and Govardhan [14] provided a vivid summary of past research activities. Detailed investigations of forward facing step (FFS), on the other hand, are less numerous.…”