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.

Turbulent plane Couette flow with a roughened wall

Abstract: Using direct numerical simulations, we studied the effects of roughness in a turbulent plane Couette flow where one wall is rough and the other one is smooth. Two different roughness configurations are simulated, and the results are compared with the case where both walls are smooth. Upon roughening a wall, roughness would generate vortical structures near the vicinity of the roughness elements and enhance turbulence locally. In the current study, however, we report that turbulence is enhanced near both the rough and the smooth walls.

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 ${\approx }2d$.

Reynolds number effects in rib-roughened turbulent channel flow

Abstract: A study involving the direct numerical simulation (DNS) of turbulent rough channel flow at Reynolds number $$\hbox {Re}_{\tau }=180$$ is undertaken, to investigate the effects of Reynolds number variation on the various statistical quantities and the near-wall dynamical structures. The definition of Reynolds number predominantly considered in this study, $$\hbox {Re}_{\tau }$$, is based on the wall friction velocity $$u_{\tau }$$ and the half-height of the channel h. The channel walls are roughened by square ribs elongated along the span of the channel. The ribs are arranged at a constant pitch width and in a manner such that the crests face the ones on the opposite wall, i.e., the roughness on the channel is symmetric about the channel centerline. The results obtained from the current DNS are compared with those from two DNSs at a higher Reynolds number of 400, one of them with roughness height $$k^+$$ comparable to that in the current DNS and the other with a higher value of $$k^+$$. The mean streamline patterns are contrasting in the presence or absence of the secondary vortex; the effects on the location of a seemingly persistent saddle point are also observed. Variation of the skin friction coefficient over a pitch width has apparently lent support to the observations. A measure of the roughness function is presented. The near-wall structures in terms of the contours of the fluctuating velocity reveal finer scale structures at the higher Reynolds number, when the value of $$k^+$$ is also high.

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...