Showing papers by "Vagesh D. Narasimhamurthy published in 2022"

Perforation effects on the wake dynamics of normal flat plates

Abstract: Abstract The effect of perforation on the wake of a thin flat plate placed normal to the free stream at Reynolds number ($Re$) 250 (based on plate width $d$, and inflow velocity $U_o$) is studied by means of direct numerical simulation. The perforated plate of length $6d$ consist of six equidistant square holes of varying sizes corresponding to porosity $\beta$ (ratio of open area to total plate area) of 0 %, 4 %, 9 %, 12.25 %, 16 %, 20.25 % and 25 %. It is observed that the bleed or jet flow through perforations pushes the shear layer interaction farther downstream with increasing $\beta$. This causes a monotonic decrease in the drag coefficient with increasing porosity, and a sharp fall seeming to begin at $\beta \approx 4\,\%$. On the other hand, the Strouhal number increases with $\beta$ up to 16 % (at $\beta =16\,\%$, loss of flow three-dimensionality leads to a ‘quasi-laminar’ state of flow). This is followed by a sharp fall in the Strouhal number at $\beta \approx 20\,\%$. The behaviour of the large-scale vortical structures in the far wake is influenced by the near-wake behaviour of the bleed flow, where the local $Re$ based on the perforation hole size determines the overall flow three-dimensionality. It is also observed that the jet or bleed flow undergoes meandering instability when pitch separation is equivalent to the hole size (at $\beta =25\,\%$). The low-$Re$ turbulent flow for a non-perforated plate is altered to a transitional state by the presence of perforation. The streamwise vortex pairs (secondary instabilities) become fairly organized as $\beta$ is increased from 0 % to 16 %. The secondary instability at $\beta =16\,\%$ appears similar to mode-B with wavelength ${\approx }1d$. On the contrary, the secondary instability at $\beta =25\,\%$ appears similar to mode-A with a wavelength of ${\approx }2d$.

Direct numerical simulation of planar turbulent jets: effect of a pintle orifice

Abstract: Effects of a pintle-shaped orifice on a planar turbulent jet flow at Reynolds number 4000, based on the inlet bulk mean velocity and the jet width, are studied using direct numerical simulations. Flapping of the jet along with a low frequency modulation of the Kelvin-Helmholtz (KH) instability, in the presence of a pintle-shaped orifice, is observed. To compare the pintle-jet behaviour, a free-jet is simulated as a reference case. The effects of the near-field region on the far-field flow characteristics have been investigated. In both the cases, the KH instability in the near-field influences the far-field jet, whereas the pintle-jet also exhibits a low-frequency flapping. In addition, oblique vortex pattern has been observed in the case of pintle-jet. The far-field flow statistics of the pintle-jet with a top-hat inlet interestingly agree with those of the free-jet with a hyperbolic tangent inlet. Temporal variation of the jet characteristics have been analysed using spatio-temporal plots. In addition, the large and small scale turbulent motion have been studied using three anisotropic invariant maps (turbulence triangles, eigenvalue and barycentric maps). Moreover, that the barycentric map gives a better visualization of the anisotropic behaviour has been observed in the current study.

Shear-layer dynamics at the interface of parallel Couette flows

Abstract: The current article aims to make a detailed analysis of co-flowing plane Couette flows. Particularly, the variation of flow quantities from the turbulent to non-turbulent region is studied. While the enstrophy exhibits a sharp jump, the other quantities (e.g., mean velocity, Reynolds normal stress, and kinetic energy) show a continuous variation across the interface. The budget analysis of Reynolds normal stresses reveal that different terms play their role in turbulence transportation, depending on the Reynolds normal stress under study. The terms production, diffusion, and redistribution play an important role in streamwise Reynolds stress (u'u'). In the spanwise Reynolds stress (v'v'), the diffusion terms play a significant role. In the wall-normal Reynolds stress (w'w') only the redistribution term is significant. The influence of one flow over other in the co-flow state was observed through the additional mean velocity and Reynolds normal stress found in the system compared to a standard plane Couette flow (pCf). Comparing the co-flow system with conventional pCf system, the former exhibits greater vorticity, vortex stretching, and kinetic energy. A detailed analysis on the geometry and topology of flow structures was studied using flow invariants.

Characteristics of wake turbulence generated by a normal flat plate

Abstract: Direct numerical simulations were performed to investigate the wake turbulence behind a thin normal flat plate. At Reynolds number 400, a low-frequency mod ulation 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-K´arm´an (V-K) vortices, while highly coherent V-K vortices were shed in the high-drag regime. Distinct flow structures were observed in these two drag regimes. To identify the dominant modes in the flow, proper orthogonal decomposition (POD) technique was used. 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 was compared with higher Reynolds number of 1200 case. The comparison revealed that the near wake flow is drasti cally different at these two Reynolds numbers, resulting in significant variation in the near wake statistical quantities, but at downstream locations, the variation in the quantities were observed to be similar both qualitatively and quantitatively.