Vagesh D Narasimhamurthy

The 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 behavior, 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 has been analyzed using spatiotemporal 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 behavior has been observed in the current study.

This 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 reveals that the terms playing a key role in turbulence transportation vary 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 another 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 a 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.

On roughening one of the walls in a planar Couette flow, it was reported that turbulence augments near the opposite wall [Javanappa and Narasimhamurthy, "Turbulent plane Couette flow with a roughened wall,"Phys. Rev. Fluids 6, 104609 (2021)]. The current direct numerical simulation work further explores this interesting phenomenon by investigating the flow dynamics and anisotropic nature of turbulence. For roughening, transverse square ribs are placed only on the bottom wall with streamwise pitch separations s = 5 r and 10r, where r = 0.2 h is the rib height and h is the channel half height. The time series of spanwise vorticity fluctuation in the case of s = 10 r shows the presence of coherent Kelvin-Helmholtz-like structures behind the ribs. Phase analysis using Hilbert transform reveals that the flow within the cavity for the s = 5 r case is in-phase, while a phase shift is observed for the s = 10 r case. The visualization of enstrophy production rate (ω i S i j ω j) reveals that regions of intense positive ones are observed to be topologically "sheet-like,"while the regions of negative ones are found to be "spotty."Anisotropy tensors and anisotropic invariant maps are used to explore turbulence anisotropy at both large and small scales of motion. It is observed that anisotropy is reduced in both the cases near the vicinity of roughness.

Direct numerical simulation (DNS) of rib-roughened turbulent channel flow rotating about its spanwise axis, by Narasimhamurthy and Andersson [Turbulence statistics in a rotating ribbed channel. International Journal of Heat and Fluid Flow 51, 29–41. (2015)], is revisited to seek complementary insights into the combined effects of roughness and Coriolis force on the turbulence. Flow in the channel was maintained at a friction Reynolds number, Reτ=400 and the non-inertial reference frame was rotated at different speeds quantified by the dimensionless rotation number, Ro=0, 2 and 6. Both the aforementioned parameters are based on the friction velocity uτ and half-height of the channel h. The channel walls were symmetrically mounted with transverse square ribs with cross-section of side k=0.1h and pitch λ=8k. Rotation causes preferential aligning of the near-wall vortical structures. The Taylor-Görtler–like roll-cells similar to those found in the rotating smooth-channel flows, survive the presence of the transverse ribs, but exhibit transient behavior. Increased transport of turbulent kinetic energy from the pressure side at higher Ro is evident from the variation of the vertical transport velocity Vk. The rotational production rates assume increasingly significant roles in distributing the kinetic energy in different directions. Anisotropy invariant maps and Taylor microscales show that the structure of turbulence is affected by rotation in a significant manner.

This paper investigates the dispersion of tiny inertial particles in the two-dimensional laminar wake from a pair of square cylinders placed side-by-side to each other. The flow Reynolds number (defined based on the cylinder size h and uniform inflow velocity Uc) in our study is fixed at 75. The wake pattern resulting from the interaction between vortices shed from individual cylinders depends on the cross-stream spacing between the cylinders, i.e. spacing ratio s/h. We examine the effect of varying the cross-stream spacing between the cylinders on the body impaction and dispersion of particles in the wake flow over a range of their responses. Three different spacing ratios, s/h = 0.3, 2, and 4 are considered. We find that the impaction efficiency of Stokes number St = 0.1 particles is not sensitive to the spacing ratio. However, the impaction efficiency of St = 1 and 10 particles is dependent on the spacing ratio and increases with the latter. We illustrate how the distribution and clustering of representative inertial particles (i.e., with Stokes number unity) evolve over a vortex shedding cycle uniquely due to the wake flow in each spacing ratio configuration and explain the physical mechanism of particle clustering using backward tracking. The non-uniform clustering of particles with different responses to the wake flow in each spacing ratio configuration is analyzed using the Voronoï diagrams. The spacing ratio effects on the dispersion of particles are quantified in terms of mean statistical quantities such as mean local concentration, clustering intensity, and velocity statistics. The local concentration of St = 1 and 10 particles at streamwise positions away from cylinders is significantly influenced by the wake flow behavior governed by the choice of the spacing ratio. Further, the combined effect of gravity and hydrodynamic drag forces on the impaction and clustering behavior of particles is reported, where the Froude number Fr=0.32. In all the cases, particles exhibit ballistic behavior, showing a significantly reduced tendency to form clusters and a drastic increase in the impaction efficiency.

A fully developed turbulent channel flow with symmetrically roughened walls is investigated, where the channel walls are roughened with square ribs, elongated along the span of the channel and are spaced uniformly in the streamwise direction at a constant pitch. The effects of Reynolds number variation on the statistical quantities, the near-wall dynamical structures and on the anisotropic nature of turbulence are studied at two Reynolds numbers Reτ= 180 and 400, where Reτ is based on the channel half-height h and the wall friction velocity uτ. Near-wall resolving large eddy simulations (LES) with different grid resolutions are carried out and validated with in-house direct numerical simulation (DNS) data. Turbulence anisotropy at both small and large scales of motion is investigated using anisotropic invariant maps. A variation in the anisotropic behavior of the flow in the near-wall region is noticed, where the flow is found to be more anisotropic at Reτ=180 than at Reτ=400. Also, the anisotropic behavior of the small-scale motions varies from the large-scale motions at Reτ=400. Two-point correlation and phase analysis using Hilbert transform reveals that the flow within the cavity is independent of the flow outside the cavity. The relatedness of the ‘worm-like’ vortical structures with the positive enstrophy production rate (ωiSijωj> 0) is investigated. The regions of positive enstrophy production rate are observed to be topologically ‘sheet-like’ predominantly at a height just above the rib. The regions of negative enstrophy production rate (ωiSijωj< 0) are less dominant, with a topology combination of weakly ‘sheet-forming’ and ‘tube-forming’. The statistical features could be captured by LES with a grid consisting of only one-fifth of the total number of grid points as that in the DNS mesh.

The effect of perforation on the wake of a thin flat plate placed normal to the free stream at Reynolds number 250 (based on plate width, and inflow velocity) is studied by means of direct numerical simulation. The perforated plate of length consist of six equidistant square holes of varying sizes corresponding to porosity (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. This causes a monotonic decrease in the drag coefficient with increasing porosity, and a sharp fall seeming to begin at. On the other hand, the Strouhal number increases with up to 16% (at, 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. 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 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). The low- 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 is increased from 0% to 16%. The secondary instability at appears similar to mode-B with wavelength. On the contrary, the secondary instability at appears similar to mode-A with a wavelength of.

Fully developed turbulent flows through ribbed channels have been simulated using direct numerical simulation (DNS). Square ribs were arranged transversely to the flow on one of the channel walls, and based on their spanwise extent, resulted in two configurations - two-dimensional (2D) configuration resulting from full-span-width ribs and a three-dimensional (3D) configuration, where the ribs extend only up to half the span-width of the channel, leaving the other half smooth. The 3D configuration thus produced a unique problem of coflowing rough and smooth turbulent channel flows. A striking phenomenon has been observed of the secondary roll cells, exhibiting a strong updraft in the smooth half of the channel. The mean velocity profile from the smooth half surprisingly possesses a linear region of constant slope at the channel core. Comparisons were also drawn with DNS of a smooth channel at the same friction Reynolds number 400 and it was found that the roll cells on the smooth half not only affect the bulk flow negatively but also attenuate the turbulence significantly. In spite of having a higher bulk velocity, the rough half of the 3D configuration is more turbulent than the 2D configuration; this has been attributed to the momentum transfer from the smooth half to the rough half. Statistical turbulence quantities, and the production rates of turbulent kinetic energy and enstrophy have been used to arrive at the inferences. In essence, the roll cells buffer the variations between the rough and the smooth halves in the 3D configuration. The instantaneous streamwise velocity fluctuations in the 3D configuration displayed a wavelike form.

A non-planar mixing layer observed between parallel Couette flows by Narasimhamurthy et al. (Phys Rev E 85:036,302, 2012) is considered. Direct numerical simulation is chosen, and simulations are run in order to determine the critical Reynolds number at which the interface between the co-flowing laminar and non-laminar flow becomes unstable exhibiting a meandering motion. The necessary conditions required to trigger the shear-layer instability were also discussed. Different combinations of Reynolds numbers are chosen keeping the Reynolds number ratio between the laminar and non-laminar flows as constant. Preliminary results indicate that the onset of instability occurs, and a meandering motion is observed at the interface when Reynolds number for the non-laminar flow corresponds to 650.

This study assesses different turbulence modeling approaches for simulation of two-phase coaxial annular swirling jet flows. The problem selected from literature for comparison involves an analytical inlet profile for an annular liquid sheet sandwiched between two coaxial annular gaseous jets. The liquid-gas interface is resolved using the volume-of-fluid model with continuum surface force approximation. Reynolds-averaged Navier-Stokes simulations and detached eddy simulations (DES) are conducted to obtain transient multiphase numerical solutions. Different turbulence models explored include the k-ε renormalization group (RNG) with swirl modification, the Reynolds stress model (RSM), RSM with scale-adaptive simulation (RSM-SAS), and DES. Comparisons with the direct numerical results from literature suggest that the k-ε RNG and RSM approaches simulate only the streamwise shear of the liquid jet and are inadequate in capturing the swirling aspect of the jet flow and expected instabilities. DES can predict several expected features such as radial asymmetry, surrounding gas vortices causing jet instabilities, and eventual jet breakup with reasonable accuracy. While RSM-SAS predicts radial asymmetry, some jet instability, and is much more accurate than k-ε RNG and RSM, it fails to predict instabilities as good as DES and does not predict a complete jet breakup. RSM-based methods are found to be computationally very expensive compared to the k-ε RNG model, suggesting DES as the better alternative than RSM methods for such applications if resources are available.