Vagesh D Narasimhamurthy

We have considered the three-dimensional wake behind a cross formed by two intersecting flat plates using direct numerical simulations. The Reynolds number based on the uniform inflow velocity U0 and the plate width d was 1000. The vortex shedding in the wake was totally suppressed in a 4d wide intersection region and this gave rise to a massive zone of recirculating flow. Quasi two-dimensional vortex shedding with a primary frequency 0.165 U0/d occurred behind the outer branches more than 7d from the intersection. The wake behind the outer branches of the crossing plates closely resembled the wake behind a single flat plate. However, the wake flow in an intermediate region (located between the intersection region and the outer branches) was affected by persistent secondary flows. Further, shear-layer (K-H) instabilities have been observed in this region. The mean wake structure revealed the formation of four symmetrically positioned pairs of swirling vortices close to the intersection corner next to the plate's edges.

The wake behind T-shaped intersecting flat plates has been studied by direct numerical simulations and compared with the wake behind intersecting plates forming a cross. The Reynolds number based on the uniform inflow velocity and the plate width d was 1000. Similar to the cross-plate the vortex shedding was suppressed in a 4d wide intersection region with a substantial base suction pressure reduction. Shear-layer (K-H) instabilities have been observed and its characteristic frequency obtained. In contrast to the cross-plate, a main feature of the mean wake structure behind the T-plate is the formation of two symmetrically positioned swirling vortices close to the internal corners of the T. This was examined by considering pressure contours and the turbulent production terms of mean streamwise vorticity. In spite of some similarities, major features of the wake behind the T-plate turned out to be distinctly different from the wake behind a cross-plate configuration.

A non-planar or a bilateral mixing-layer is studied by means of a series of direct numerical simulations (DNSs). This mixing-layer forms at the interface of two co-current plane Couette flows of different Reynolds numbers. The current DNS study determined the conditions for the onset of shear-layer instability at the interface. The influence of different Reynolds number (of the co-current plane Couette flows) and their Reynolds number ratio on the mixing-layer is studied. A critical Reynolds number of about 500 (or more particularly one of the co-current plane Couette flows must be turbulent) and a Reynolds number ratio greater than 2 is required for the genesis of this bilateral shear-layer instability. Independent of the Reynolds number and the Reynolds number ratio, the temporal evolution of the shear-layer instability followed the same pattern. In addition, the oscillation frequency of the instability was found to increase with increasing Reynolds number and increasing Reynolds number ratio. Further, influence of instability on the local skin friction and the two-point correlation is elaborated on.

Flow over a perforated plate is studied at Reynolds number 250. Effect of the plate aspect-ratio L/d (where, L is length and d is width) is studied by varying L from 1d-12d. Hydrodynamic forces on the plate and the shedding-frequency match for all the cases, though the wake-dynamics is distinct between the lower and upper L/d cases. In the low L/d cases, wake is coherent along the span and has reduced three-dimensionality. Flow here is undergoing wake-transition, where the wavelength of the secondary instability is about 1d. Further, jets from the perforation holes exhibit in-phase oscillation. Another feature is the presence of energetic sub-harmonics, where the wake is experiencing period-doubling bifurcations. In high L/d cases, wake exhibits higher incoherence due to vortex-dislocations and incoherent jet oscillations, which promotes flow three-dimensionality. Present study indicates that an L/d of about six is required for the simulations to be free from numerical effects.

Parallel computations of flow past a perforated plate of porosity 25% at Reynolds number 250 (based on plate width, d and inflow velocity, Uo) is carried out. The effect of aspect ratio is studied with different span-wise lengths of the domain (1d, 3d and 6d). Present results revealed that an aspect ratio of 6d is required to capture the transient wake dynamics. It was found that statistical quantities stemming from aspect ratio 3d and 6d cases agree with each other, though the dynamical behavior of the wake is very different. The signature period doubling effects associated with short constrained domains were visible in the 1d and 3d aspect ratio cases. Enforcing periodic boundary condition along the short span-wise domains may thus adversely affect the flow.

Direct numerical simulations were performed to investigate the wake turbulence behind a thin normal flat plate. At Reynolds number 400, a low-frequency modulation of the near wake was observed, resulting in a low-drag and high-drag regime. The low-drag regime was characterized by less coherent von Kármán (V-K) vortices, while highly coherent V-K vortices were shed in the high-drag regime. Distinct flow structures were observed in the two drag regimes. Proper orthogonal decomposition (POD) was used to identify the dominant structures in the flow. It was observed that the low-frequency modulation of the wake was captured by the third dominant POD mode. The statistical analysis was also performed to study the variation of Reynolds stresses and fluctuating kinetic energy in the wake. The statistical data were compared with a higher Reynolds number case of 1200. The comparison revealed that the near-wake flow is drastically different at these two Reynolds numbers, resulting in significant variation in the near-wake statistical quantities. However, at downstream locations, the variation in the quantities was observed to be both qualitatively and quantitatively similar.

In plane Couette flow (pCf), the transitional regime is characterized by oblique laminar-turbulent bands. These large-scale patterns are periodic in streamwise and spanwise directions. In this study, we perform direct numerical simulations of pCf undergoing a reverse transition from a turbulent state to understand the characteristics of transitional pCf. The nature of the spatiotemporal intermittency in laminar-turbulent bands is explored through a spatiotemporal diagram. The large-scale flow is extracted by removing the small scales using a filtering operation. The large-scale flow vectors are inclined to the streamwise direction, indicating a considerable spanwise velocity in the large-scale flow. These flow vectors are almost parallel to the bands near the laminar-turbulent interfaces. Transitional pCf is different from its fully turbulent counterpart in certain aspects. Unlike fully turbulent flow, counterrotating secondary roll cells are absent in the transitional regime. The secondary flow reveals a zone where streamlines are almost parallel to the spanwise direction everywhere except in the center, where several small vortices exist. Another major difference is the presence of nonzero off-diagonal components of Reynolds stress tensor in transitional pCf. All off-diagonal components are nonzero due to a significant mean spanwise velocity in the flow.