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.

Analysis of vortex splitting characteristics in the wake of an inclined flat plate using Hilbert–Huang transform

Abstract: Three-dimensional vortex dynamics in the wake behind a flat plate at 20○ angle of attack are explored by means of the Hilbert–Huang transform. While a completely regular vortex shedding is observed at Reynolds number Re = 500 with a distinct shedding frequency and only a single subharmonic frequency, a complex shedding behavior is observed at Re = 525 and above. The low-frequency variations of the energy content and frequency in time are deduced from the Hilbert spectra. The low-frequency modulations along the span of the plate are substantially increased from Re = 525 to Re = 800. The mean marginal spectra reveal that the energy in the low-frequency band increases with increasing Re. A corresponding reduction of the energy content in the high-frequency band is observed at the highest Reynolds number.

Statistical Flow Properties in the Turbulent Wake of a Tapered Flat Plate Placed Normal to the Free-stream

Abstract: Reynolds-averaged statistical flow properties in the turbulent near-wake of a tapered flat plate placed normal to the free-stream were analysed by means of direct numerical simulation (DNS). A taper ratio of 20 was considered in the present study while the Reynolds numbers based on the uniform inflow velocity and the width of the plate at the wide and narrow ends were 1000 and 250, respectively. The tapering gave rise to cellular vortex shedding with randomly occurring vortex dislocations which prohibited a distinction between coherent and incoherent fluctuations. In the current DNS, it was observed that the magnitude of the Reynolds stresses and the production and dissipation rates of fluctuating kinetic energy are much lower than in the wake of a uniform normal flat plate. This reduced magnitude is perhaps due to the fact that the present tapered plate produces a highly incoherent wake, which may result in the reduction of the coherent velocity fluctuations and their overall contribution to the Reynolds-averaged data. In addition to the primary shear-stress term, a secondary shear-stress component was found to co-exist in the present case. The appearance of this secondary shear-stress term, together with the inherent spanwise inhomogeneity and the existence of a distinct secondary flow, was found to drive the fluctuations towards isotropy. In addition, the present DNS surprisingly revealed the appearance of Kelvin-Helmholtz instabilities towards the higher local Reynolds numbers, i.e. beyond 920.

DNS of plane Couette flow with surface roughness

Abstract: Direct numerical simulation of fully developed turbulent plane Couette flow (pCf) has been performed to investigate the effect of surface roughness. Flow characteristics in a statistically stationary field are then compared with the smooth pCf data. The present study considers a case, where the bottom wall is fixed, while the top wall is moving with a constant velocity $$U_{\mathrm{w}}$$ in the flow direction. For roughening, square ribs are placed on the stationary wall with a streamwise pitch $$\lambda = 5k$$, where $$k = 0.2h$$ is the roughness height and h is half the height between the walls. In the present study, we examined the flow and pressure fields around the ribs. For the pitch $$\lambda = 5k$$, a single recirculation zone is observed in the cavity between any two consecutive ribs. In addition, the profiles of the mean data in the channel core region at different streamwise locations are shown to collapse with each other, indicating that the flow is quasi-homogeneous in the streamwise direction. Further, counter-rotating secondary roll cells or Taylor–Gortler-like vortices, which evolve temporally and oriented along the flow direction, are also observed in the rough pCf. In addition, the large-scale turbulent structures are distorted into fine-scale motion around the ribs. Furthermore, the skin friction coefficient and the mean pressure fields are qualitatively similar to a rough Poiseuille flow (with ribs only on one wall).

Numerical simulation of flow past a rectangular flat plate at incidence

Abstract: Two-dimensional numerical simulation was employed to investigate the unsteady separated flow past a rectangular flat plate inclined with an angle of attack of 30 degrees to the mean flow. The rectangular flat plate characterized by a length-width ratio of 50 represents a marine bluff body, and is described by the immersed boundary method in our code. Two different flow cases were compared for the chosen angle of attack. In the first case, to achieve the attack angle, the flat plate was imposed by introducing an inclination relative to the Cartesian grid. In the second case the flat plate was fit to the grid and the incoming flow was inclined. These two cases both have their advantages and disadvantages according to the different grid implementation. The Reynolds number is 750 with the chord length of the plate as characteristic length scale. The results show that these two flow cases give almost the same vortices shedding alternately from leading edge and trailing edge with a shedding frequency corresponding to a Strouhal number of around 0.34. The steady mean flow was obtained by averaging in time.

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...