Qualification: 
Ph.D, M.Tech
Email: 
g_nandu@cb.amrita.edu

Dr. Nandu Gopan is presently an Assistant Professor in the Department of Aerospace Engineering at the Amrita School of Engineering, Coimbatore. He received his Doctoral degree (2014) and Master’s degree (2010) from the National Institute of Technology Calicut.

Publications

Publication Type: Journal Article

Year of Publication Title

2020

Nandu Gopan and Alam, M., “Symmetry-breaking bifurcations and hysteresis in compressible Taylor–Couette flow of a dense gas: a molecular dynamics study”, Journal of Fluid Mechanics, vol. 902, p. A18, 2020.[Abstract]


Molecular dynamics simulations with a repulsive Lennard-Jones potential are employed to understand the bifurcation scenario and the resulting patterns in compressible Taylor–Couette flow of a dense gas, with the inner cylinder rotating ( ωi>0 ) and the outer one at rest ( ωo=0 ). The steady-state flow patterns are presented in terms of a phase diagram in the ( ωi,Γ ) plane, where Γ=h/δ is the aspect ratio, h is the height of the cylinders and δ=Ro−Ri is the gap between the outer and inner cylinders, and the underlying bifurcation scenario is analysed as a function of ωi for different Γ . Considerable density stratification is found along both radial and axial directions in the Taylor-vortex regime of a dense gas, which makes the present system fundamentally different from its incompressible analogue. In the circular Couette flow regime, the stratifications remain small and the predicted critical Reynolds number for the onset of Taylor vortices matches well with that of its incompressible counterpart. The emergence of asymmetric Taylor vortices at Γ>1 is found to occur via saddle-node bifurcations, resulting in hysteresis loops in the bifurcation diagrams that are characterized in terms of the net circulation or the maximum radial velocity or the axial density contrast as order parameters. For Γ≤1 with reflecting axial boundary conditions, the primary bifurcation yields a single-vortex state which is connected to a two-roll branch via saddle-node bifurcations; however, changing to stationary (no-slip) endwalls yields a new state, which consists of two large symmetric vortices near the inner cylinder coexisting with an irregular pattern near the stationary outer cylinder. It is shown that the endwall conditions and the fluid compressibility play crucial roles on the genesis of asymmetric and stratified vortices and the related multiplicity of states in the Taylor-vortex regime of a dense gas.

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2017

Nandu Gopan and Alam, M., “Oblique Shock Waves in Granular Flows Over Bluff Bodies”, EPJ Web of Conferences, vol. 140, p. 3053, 2017.

2016

Nandu Gopan, “Rupture of Nanoscaled Water Sheets in the Presence of an Applied Electric Field”, Fluid Dynamics Research, vol. 48, p. 61426, 2016.[Abstract]


Understanding the behaviour of water sheets is relevant in numerous areas, such as thin film coating and atomisation. The rupture of planar liquid sheets are interesting due to the fact that they are objects of co-dimension 1. Previous work seems to suggest that a generic route to liquid structure fragmentation is via liquid sheets. The interplay between inertia, surface tension and viscosity is crucial in determining the dynamics of liquid sheets at a macro scale. At the nanoscale, where thermal fluctuations are expected to play a dominant role, the dynamics become more interesting. The stability and rupture dynamics of nanoscaled water sheets, at constant temperature, are studied using constrained molecular dynamics (MD) simulations. The SPC/E potential with long range electrostatics is used to simulate water molecules. The effect of an applied electric field on the stability of the nanoscaled water sheet forms the focus of this study. The effect of the initial configuration is studied by changing the random seed values used for velocity initialisation. The effect of sheet thickness on the rupture dynamics is also explored. It is seen that when large electric fields (5 V/nm) act across very thin sheets (1 layer), then breakup into multiple ellipsoidal structures is a possibility, and the response of the fluid structure to the applied electric field is non-linear. Furthermore, it is seen that Taylor's predictions for the critical electric field intensity, based on classical electro-hydrodynamics for the onset of instability in macroscopic drops, scales surprisingly well for the case of nanoscaled sheets.

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2014

Nandu Gopan and Sarith P. Sathian, “A Langevin Dynamics Study of Nanojets”, Journal of Molecular Liquids, vol. 200, pp. 246 - 258, 2014.[Abstract]


The behaviour of nano-scale jets emanating from a reservoir under the action of an external force is studied using Langevin dynamics simulations. The advantage of employing a Langevin thermostat to maintain the temperature of the fluid reservoir is highlighted. The effect of hydrodynamic screening introduced by the Langevin thermostat is considered. It is seen that the nature of thermostat plays a crucial role in simulating the onset of instabilities in the liquid structures. A plunger action has been chosen to initiate jet generation. Langevin dynamics is seen to be able to model the physics of nano-scale jets quite accurately. The study also shows that the Langevin dynamics simulations are capable of capturing the dynamics of nano-scale liquid jets, as the nanojet simulated by this method is found to behave in close agreement with the theoretical predictions for nano-scale jets.

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2014

Nandu Gopan and Sathian, S. P., “Rayleigh Instability at Small Length Scales”, Phys. Rev. E, vol. 90, p. 33001, 2014.[Abstract]


The Rayleigh instability (also called the Plateau-Rayleigh instability) of a nanosized liquid propane thread is investigated using molecular dynamics (MD). The validity of classical predictions at small length scales is verified by comparing the temporal evolution of liquid thread simulated by MD against classical predictions. Previous works have shown that thermal fluctuations become dominant at small length scales. The role and influence of the stochastic nature of thermal fluctuations in determining the instability at small length scale is also investigated. Thermal fluctuations are seen to dominate and accelerate the breakup process only during the last stages of breakup. The simulations also reveal that the breakup profile of nanoscale threads undergo modification due to reorganization of molecules by the evaporation-condensation process.

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2013

Nandu Gopan and Sarith P. Sathian, “The Role of Thermal Fluctuations on the Formation and Stability of Nano-Scale Drops”, Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 432, pp. 19 - 28, 2013.[Abstract]


The stability of nano-scale drops is an interesting area of research that could revolutionise areas such as drug delivery, printing and nano-scale manufacturing. In this study, the generation and stability of nano-scale sized drops are investigated using molecular dynamics (MD) simulations. It is seen that in the formation and stability of small drops, thermal fluctuations play an important if not dominant role in determining the stability dynamics. The nature of thermal fluctuations is found to be stochastic. The conditions that influence thermal fluctuations in nano-scale drops are also investigated

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2012

Nandu Gopan and Sarith P. Sathian, “Molecular Dynamics Studies in Nanoscale Liquid Structures: Geometry and Thermal Effects on Nanojet Development”, Molecular Simulation, vol. 38, pp. 179-188, 2012.[Abstract]


Conventional macroscopic jet theory relies heavily on experimental correlations which cannot be easily extended to the nanoscale regime. Moreover, the fluid dynamic effects at small length scales and their contribution to the development of nanoscale liquid structures are fundamentally different from their macroscopic counterparts. This coupled with the high spatial and temporal resolution requirements at nanoscale domains make molecular dynamics (MD) an excellent tool for studying such structures. In this study, the formation and breakup of nanojets (NJs) developing from high pressure into vacuum is investigated using MD based on non-Hamiltonian formulations. By ejecting the equilibrated argon atoms through various nozzle geometries and diameters, nanoscale jet flows were generated. The dependence of the jet structure on nozzle geometry and diameter is studied. The influence of geometry on NJ formation is also studied along with issues involved in the equilibration and thermostat coupling parameter. Various thermostats are compared to understand the role they play in MD simulations of liquid nanostructures. Tuning of the thermostat coupling parameter has also been discussed. The jet breakup phenomenon is analysed and a comparative study, vis-à-vis, well-established continuum and stochastic models, is attempted

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