Showing papers by "Vagesh D. Narasimhamurthy published in 2019"

DNS of plane Couette flow with surface roughness

Abstract: Direct numerical simulation of fully developed turbulent plane Couette flow (pCf) has been performed to investigate the effect of surface roughness. Flow characteristics in a statistically stationary field are then compared with the smooth pCf data. The present study considers a case, where the bottom wall is fixed, while the top wall is moving with a constant velocity $$U_{\mathrm{w}}$$ in the flow direction. For roughening, square ribs are placed on the stationary wall with a streamwise pitch $$\lambda = 5k$$, where $$k = 0.2h$$ is the roughness height and h is half the height between the walls. In the present study, we examined the flow and pressure fields around the ribs. For the pitch $$\lambda = 5k$$, a single recirculation zone is observed in the cavity between any two consecutive ribs. In addition, the profiles of the mean data in the channel core region at different streamwise locations are shown to collapse with each other, indicating that the flow is quasi-homogeneous in the streamwise direction. Further, counter-rotating secondary roll cells or Taylor–Gortler-like vortices, which evolve temporally and oriented along the flow direction, are also observed in the rough pCf. In addition, the large-scale turbulent structures are distorted into fine-scale motion around the ribs. Furthermore, the skin friction coefficient and the mean pressure fields are qualitatively similar to a rough Poiseuille flow (with ribs only on one wall).

Reynolds number effects in rib-roughened turbulent channel flow

Abstract: A study involving the direct numerical simulation (DNS) of turbulent rough channel flow at Reynolds number $$\hbox {Re}_{\tau }=180$$ is undertaken, to investigate the effects of Reynolds number variation on the various statistical quantities and the near-wall dynamical structures. The definition of Reynolds number predominantly considered in this study, $$\hbox {Re}_{\tau }$$, is based on the wall friction velocity $$u_{\tau }$$ and the half-height of the channel h. The channel walls are roughened by square ribs elongated along the span of the channel. The ribs are arranged at a constant pitch width and in a manner such that the crests face the ones on the opposite wall, i.e., the roughness on the channel is symmetric about the channel centerline. The results obtained from the current DNS are compared with those from two DNSs at a higher Reynolds number of 400, one of them with roughness height $$k^+$$ comparable to that in the current DNS and the other with a higher value of $$k^+$$. The mean streamline patterns are contrasting in the presence or absence of the secondary vortex; the effects on the location of a seemingly persistent saddle point are also observed. Variation of the skin friction coefficient over a pitch width has apparently lent support to the observations. A measure of the roughness function is presented. The near-wall structures in terms of the contours of the fluctuating velocity reveal finer scale structures at the higher Reynolds number, when the value of $$k^+$$ is also high.