Mustafa Barri

Turbulent flow over a backward-facing step. Part 1. Effects of anti-cyclonic system rotation

The effects of rotation on turbulent flow with separation and reattachment are investigated by means of direct numerical simulations. The backward-facing step configuration is rotated about a spanwise axis such that the sudden expansion of the channel is on the pressure side. The upstream flow is a fully developed plane Poiseuille flow subjected to orthogonal-mode rotation, which subsequently detaches from the step corner and eventually reattaches further downstream. The size of the resulting separation bubble with recirculating flow diminishes monotonically with increasing rotation rates and the reattachment distance is reduced from about 7 to 3 step heights. This is ascribed to the augmentation of the cross-stream turbulence intensity in the anti-cyclonic shear layer formed between the bulk flow and the recirculating eddy due to the destabilizing influence of the Coriolis force. The spanwise-oriented vortex cells or roller eddies found in non-rotating shear layers were disrupted by the enhanced turbulence. The flow along the planar wall is subjected to an adverse pressure gradient induced by the sudden expansion. The stabilizing influence of the system rotation in this cyclonic shear layer tends to damp the turbulence, the flow becomes susceptible to flow separation, and a substantial cyclonic recirculation bubble is observed at the highest rotation rates. The resulting meandering of the bulk flow is associated with interactions between the anti-cyclonic shear layer at the stepped side and the cyclonic shear flow along the planar surface. These give rise to enhanced turbulence levels at the cyclonic side in spite of the otherwise stabilizing influence of the Coriolis force. Exceptionally high velocity fluctuations in the spanwise direction are observed in the vicinity of flow reattachment behind the step and ascribed to longitudinal Taylor-Gortler-like roll cells which extend into the backflow region. These roll cells arise from a centrifugal instability mechanism associated with the convex streamline curvature in the reattachment zone.

On the stabilizing effect of the Coriolis force on the turbulent wake of a normal flat plate

The turbulent Karman vortex street behind a flat plate in a rotating fluid is explored by means of direct numerical simulations. The effect of the Coriolis force is often said to be stabilizing on the cyclonic side of the wake and destabilizing on the anticyclonic side. The present computer experiments reveal a more subtle influence of the system rotation. The turbulence is suppressed at the cyclonic side of the wake at Rossby numbers of about unity whereas the cyclonic Karman roller eddies persist. The anticyclonic vortex cells which are blurred by the enhanced turbulence level are scarcely visible. The Strouhal number of this asymmetric vortex shedding is slightly higher than in the absence of rotation. At even higher rotation rates, the three-dimensional turbulence is suppressed also along the anticyclonic side of the wake and a nearly symmetric vortex street is observed at Rossby number of 0.16 with the Strouhal number significantly lower than for the nonrotating wake. At this rotation rate, the anticyclonically shed cells appeared as high-pressure zones. At even lower Rossby numbers, not only the turbulence but also the periodic vortex shedding is suppressed, in accordance with the demarcation line in a Rossby–Ekman number flow regime map.

Asymmetries in an obstructed turbulent channel flow

The asymmetric flow pattern caused by a single thin-plate obstruction in a plane channel has been explored by means of direct numerical simulations. The blockage ratio was 1:2 and the bulk Reynolds number about 5700. In order to mimic an infinitely long channel section upstream of the obstruction, realistic dynamic inflow conditions were provided by a promising technique proposed by Barri et al. [“Inflow conditions for inhomogeneous turbulent flows,” Int. J. Numer. Methods Fluids 60, 227 (2009)]. The fluid downstream of the symmetric obstruction was sucked toward one side where a modestly long region of rather strong recirculating flow was observed. The weaker recirculation bubble formed at the opposite side was 17 times longer than the obstruction height and almost four times the size of the shorter bubble. The overall flow pattern turned out to be rather different from that observed in a similar study of channel flow subjected to periodically repeating obstructions by Makino et al. [“Turbulent structure...

Flow past a normal flat plate undergoing inline oscillations

The flow past an inline oscillating normal flat plate has been considered with the view to explore the variety of wake phenomena which arise even at the low Reynolds number (Re) equal to 100 based on the free stream velocity and the width of the plate. The three-dimensional Navier-Stokes equations were integrated in time over a wide range of excitation frequencies and amplitudes. A wake flow regime map was produced on the basis of the 24 computer simulations. For a certain excitation amplitude, the wake vortex shedding is first antisymmetric at low excitation frequencies fe. When fe is increased the wake first becomes chaotic and thereafter turns into a symmetric shedding mode, for instance the S-II mode with a binary vortex pair on each side of the wake. If fe is increased even further, more complex symmetric wake patterns may occur before the wake ultimately turns into chaos. Symmetric wakes are thus only observed in a band of intermediate excitation frequencies and then with the dominating flow frequen...

Anomalous turbulence in rapidly rotating plane Couette flow

Turbulent flows in rotating frame-of-reference are of considerable interest in a variety of industrial, geophysical and astrophysical applications. In these flows, the system rotation induces additional body forces, i.e. centrifugal and Coriolis forces, acting on the turbulent flow, so that the momentum mechanism becomes more complicated. The present doctoral thesis concerns the system rotation influence on turbulent shear flows. To this end, direct numerical simulations (DNSs) have been performed in order to investigate the effect of system rotation on different turbulent flow configurations including plane Couette flow, sudden expansion flow and wake behind a flat plate. In addition, the PIV measurements for rotating plane channel and sudden expansion flows have been carried out in order to support the numerical simulations results. A realistic turbulent inflow boundary condition is needed in order to perform direct numerical simulations for spatially developing flows subjected to system rotation such as rotating sudden expansion flows. The cost effective method is introduced based on a separate (precursor) simulation in order to establish fully turbulent inflow including wide ranges of length and time scales needed in the direct numerical simulations. On the other hand, the characteristic features of the rotating turbulent flows were investigated by means of three-dimensional flow visualization, two-point correlations and transport for the individual second-moments of the velocity and vorticity fluctuations.

DNS of Orifice Flow with Turbulent Inflow Conditions

Direct numerical simulation has been performed to study flow through an orifice at a bulk Reynolds number of 5700 and a blockage ratio of 1:2. In order to mimic an infinitely long channel section upstream of the obstruction, realistic dynamic inflow conditions were provided by a promising technique proposed by Barri et al. (2009).

Asymmetric vortex shedding in the turbulent wake of a flat plate in a rotating fluid

A numerical experiment of the turbulent flow behind a parallel-sided plate subjected to solid body rotation along the spanwise direction is performed. The Reynolds number \( Re = U_\infty d / \nu\) based on inflow velocity and width of the plate equals 750, while the goal Rossby number \( Ro_g = U_\infty /2 \Omega d\) varies from 1 to 0.16. The work provides an insight into several salient features of the spanwise rotating wake flow and revealed fascinating effects exerted by the Coriolis force on the wake flow turbulence.

Massive separation in rotating turbulent flows

Turbulent flow over a backward facing step can be of great assist to demonstrate the effect of system rotation on separating flows. Depending on the magnitude and orientation of the imposed background vorticity 2Ω relative to the mean flow vorticity ω in the rotating frame of reference, a variety of different flow phenomena may occur. Cyclonic (anti-cyclonic) rotation if mean vorticity vector is parallel (anti-parallel) to the system rotation vector. Cambon et al.[1] and Metais et al.[2] indicated the stabilization effect in the cyclonic rotation regimes in terms of reducing turbulence level and decreasing the momentum interchange (compared to the situation with no rotation). On the other hand, the destabilization effect associated with increase in momentum interchange dominates the moderate anti-cyclonic rotation regimes. Beyond a certain rotation rate the anti-cyclonic regimes begin to restabilize.