Vagesh D. Narasimhamurthy

Bio: Vagesh D. Narasimhamurthy is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Reynolds number & Turbulence. The author has an hindex of 11, co-authored 46 publications receiving 455 citations. Previous affiliations of Vagesh D. Narasimhamurthy include Norwegian University of Science and Technology.

On oblique and parallel shedding behind an inclined plate

Abstract: Three-dimensional wake instabilities in the form of oblique shedding and vortex dislocations in the flow past an inclined flat plate of angle of attack 20° and Reynolds number 1000 have been reported earlier [D. Yang, B. Pettersen, H. I. Andersson, and V. D. Narasimhamurthy, Phys. Fluids 24, 084103 (2012)]10.1063/1.4744982. In the current study, direct numerical simulations were performed to further explore this bifurcation. At lower Reynolds numbers, i.e., well below 525, the three-dimensional wake was found to be stable and in a parallel shedding mode. However, as the Reynolds number increases, it was observed that both parallel and oblique vortex sheddings arose naturally. Vortex dislocations appeared at the juxtaposition of oblique and parallel shedding modes. The velocity signals were analyzed by a wavelet transformation, from which the instantaneous characteristics of three-dimensional vortex shedding were obtained and examined. Results show that the phase difference of shed vortex rollers in the sp...

Cellular vortex shedding in the wake of a tapered plate at low Reynolds number

Abstract: The unsteady near wake behind a linearly tapered plate has been investigated numerically. The tapering made the Reynolds number based on the inflow velocity and the local width of the plate vary from 25 to 100. The wake flow comprised three different flow regimes coexisting side by side. The wake flow was steady behind the narrow end of the plate. Periodic vortex shedding occurred downstream from where the local Reynolds number exceeded 32. Vortex dislocations enabled a cellular shedding pattern with shedding frequency decreasing toward the wide end of the plate. The regular oblique vortex shedding near midspan was subjected to three-dimensional scrambling toward the wide end of the plate which gave rise to streamwise-oriented vortex structures. The Strouhal number was distinctly lower than in the wake of a uniform plate whereas the base pressure coefficient was substantially higher.

Novel features of a fully developed mixing-layer between co-flowing laminar and turbulent Couette flows

Abstract: A new flow configuration has been proposed in which a bilateral mixing-layer exists in the junction between co-flowing laminar and turbulent plane Couette flows. Contrary to a classical plane mixing-layer, the present mixing-layer did neither grow in time nor in streamwise direction. However, the mixing zone varied with the distance from the stationary wall. A direct numerical simulation showed that very-large-scale flow structures were found in the turbulent part of the flow with Reynolds number 1300 based on half the velocity U1 of the fastest-moving wall and half of the distance 2h between the walls. The laminar-turbulent interface exhibited a large-scale meandering motion with frequency 0.014U1/h and wavelength about 25h. Large-scale Taylor-Gortler-like roll cells were observed in the nominally laminar flow region with Reynolds number 260. This tailor-made flow is particularly well suited for explorations of momentum transfer and intermittency in the vicinity of the laminar-turbulent interface.

Simulation of unsteady flow past tapered circular cylinders using an immersed boundary method

Abstract: Simulation of unsteady flow past tapered circular cylinders using an immersed boundary method

Bilateral shear layer between two parallel Couette flows.

Abstract: We consider an unusual shear layer occuring between two parallel Couette flows. Contrary to the classical free shear layer, the width of the shear zone does not vary in the streamwise direction but rather exhibits a lateral variation. Based on some simplifying assumptions, an analytic solution is derived for this shear layer. These assumptions are justified by a comparison with numerical solutions of the full Navier-Stokes equations, which accord with the analytical solution to better than $1%$ in the entire domain. An explicit formula is found for the width of the shear zone as a function of the wall-normal coordinate. This width is independent of the wall velocities in the laminar regime. Preliminary results for a cocurrent laminar-turbulent shear layer in the same geometry are also presented. Shear-layer instabilities are then developed and result in an unsteady mixing zone at the interface between the two cocurrent streams.

Carbon capture and storage (CCS): the way forward

Abstract: Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

Low-Reynolds-number wakes of elliptical cylinders: from the circular cylinder to the normal flat plate

Abstract: While the wake of a circular cylinder and, to a lesser extent, the normal flat plate have been studied in considerable detail, the wakes of elliptic cylinders have not received similar attention. However, the wakes from the first two bodies have considerably different characteristics, in terms of three-dimensional transition modes, and near- and far-wake structure. This paper focuses on elliptic cylinders, which span these two disparate cases. The Strouhal number and drag coefficient variations with Reynolds number are documented for the two-dimensional shedding regime. There are considerable differences from the standard circular cylinder curve. The different three-dimensional transition modes are also examined using Floquet stability analysis based on computed two-dimensional periodic base flows. As the cylinder aspect ratio (major to minor axis) is decreased, mode A is no longer unstable for aspect ratios below 0.25, as the wake deviates further from the standard Benard–von Karman state. For still smaller aspect ratios, another three-dimensional quasi-periodic mode becomes unstable, leading to a different transition scenario. Interestingly, for the 0.25 aspect ratio case, mode A restabilises above a Reynolds number of approximately 125, allowing the wake to return to a two-dimensional state, at least in the near wake. For the flat plate, three-dimensional simulations show that the shift in the Strouhal number from the two-dimensional value is gradual with Reynolds number, unlike the situation for the circular cylinder wake once mode A shedding develops. Dynamic mode decomposition is used to characterise the spatially evolving character of the wake as it undergoes transition from the primary Benard–von Karman-like near wake into a two-layered wake, through to a secondary Benard–von Karman-like wake further downstream, which in turn develops an even longer wavelength unsteadiness. It is also used to examine the differences in the two- and three-dimensional near-wake state, showing the increasing distortion of the two-dimensional rollers as the Reynolds number is increased.

Carbon Capture and Utilization Technology without Carbon Dioxide Purification and Pressurization: A Review on Its Necessity and Available Technologies

Abstract: Carbon capture and utilization (CCU) has attracted increased attention as a means to mitigate and adapt to climate change. CCU technology regards CO2 as a raw material and reduces CO2 emissions. Ho...