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Shankar Ghosh

Bio: Shankar Ghosh is an academic researcher from University of Minnesota. The author has contributed to research in topic(s): Direct numerical simulation & Turbulence. The author has an hindex of 2, co-authored 5 publication(s) receiving 77 citation(s).
Papers
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Journal ArticleDOI
Shankar Ghosh1, Krishnan MaheshInstitutions (1)
Abstract: Numerical simulations of laser energy deposition in air are conducted. Local thermodynamic equilibrium conditions are assumed to apply. Variation of the thermodynamic and transport properties with temperature and pressure are accounted for. The flow field is classified into three phases: shock formation; shock propagation; and subsequent collapse of the plasma core. Each phase is studied in detail. Vorticity generation in the flow is described for short and long times. At short times, vorticity is found to be generated by baroclinic means. At longer times, a reverse flow is found to be generated along the plasma axis resulting in the rolling up of the flow field near the plasma core and enhancement of the vorticity field. Scaling analysis is performed for different amounts of laser energy deposited and different Reynolds numbers of the flow. Simulations are conducted using three different models for air based on different levels of physical complexity. The impact of these models on the evolution of the flow field is discussed.

63 citations


Proceedings ArticleDOI
Shankar Ghosh1, Krishnan Mahesh1Institutions (1)
07 Jan 2008-
Abstract: Numerical simulations of laser energy deposition in quiescent air and in isotropic turbulence are conducted. The flow field is classified into three phases: shock formation, shock propagation and subsequent collapse of the plasma core. Each phase is studied in detail. Vorticity is found to be generated in the flow at short times due to baroclinic effects and at long times due to rolling up of the plasma core. An explanation is presented for the roll up process. Scaling analysis is performed for different amounts of laser energy deposited and different Reynolds number of the flow. Simulations are conducted using three different models for air based on different levels of physical complexity. The impact of these models on the evolution of the flow field is discussed.

11 citations


Proceedings ArticleDOI
Shankar Ghosh1, Krishnan MaheshInstitutions (1)
08 Jan 2007-
Abstract: The thermal effects of laser–induced plasma on isotropic turbulence are studied using direct numerical simulation. Laminar and turbulent simulations are conducted. For the laminar simulations, a tear–drop shaped blast wave propagates into the background becoming spherical in time. Shock radius and jumps in fluid properties at the shock front are compared to experiment. Both short and long time behavior of the plasma is studied. At short times, baroclinic vorticity is generated as a consequence of propagation of the curved shock through the domain. At long times, the flow field forms toroidal vortex rings, as observed in experiments. For the turbulent simulations, turbulence levels get enhanced in regions of compression across the blast wave and suppressed in regions of expansion in the plasma core.

2 citations


Shankar Ghosh1, Krishnan MaheshInstitutions (1)
01 Dec 2007-
Abstract: Numerical issues associated with simulating strong blast waves in high temperature flows in a turbulent background have been addressed effectively. Spherical energy deposition in both laminar and turbulent flows is considered. A predictor–corrector based shock capturing scheme is implemented along with a base spectral discretization scheme to simulate strong blast waves generated in the flow. The shock capturing scheme is extended for high temperature flows in equilibrium. Formulation of high temperature eigen vector matrices for 3–D Euler equations is presented for a generalized coordinate system. A logarithm formulation of the continuity equation is used to address stability issues related to very low densities in the core. A non-linear limiter has been implemented to eliminate excessive dissipation of the background turbulence due to shock capturing.

1 citations


01 Jan 2005-
Abstract: The thermal eects of plasmas on isotropic turbulence are studied using direct numerical simulations. The temperature ratio of the plasma region to the background region is moderate. Two idealizations of the plasma are considered { spherical and conical. The conical idealization is found preferable in that it approximates the tear{drop shape of the plasma region that is observed experimentally, and produces baroclinic vorticity. The variation of the magnitude of the vorticity with temperature ratio and size of plasma region is examined. A blast wave followed by a region of expansion propagates normal to the axis of the plasma region. The turbulence is observed to be suppressed in the core of the plasma, presumably due to bulk expansion.

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Journal ArticleDOI
01 Jun 1960-Physics Bulletin
TL;DR: This is really two separate books within the same pair of covers, which are devoted to the discussion of similarity and dimensional, methods and their application to a variety of problems in mechanics and fluid mechanics.
Abstract: By L I Sedov London: Cleaver-Hume Press Ltd Pp xvi + 363 Price 105s This is really two separate books within the same pair of covers First of all Chapters 1-3, some 145 pages, are devoted to the discussion of similarity and dimensional, methods and their application to a variety of problems in mechanics and fluid mechanics

660 citations


Journal ArticleDOI
Xiaochuan Chai1, Krishnan MaheshInstitutions (1)
Abstract: This paper presents a dynamic one-equation eddy viscosity model for large-eddy simulation (LES) of compressible flows. The transport equation for subgrid-scale (SGS) kinetic energy is introduced to predict SGS kinetic energy. The exact SGS kinetic energy transport equation for compressible flows is derived formally. Each of the unclosed terms in the SGS kinetic energy equation is modelled separately and dynamically closed, instead of being grouped into production and dissipation terms, as in the Reynolds averaged Navier-Stokes equations. All of the SGS terms in the filtered total energy equation are found to reappear in the SGS kinetic energy equation. Therefore, these terms can be included in the total energy equation without adding extra computational cost. A priori tests using direct numerical simulation (DNS) of decaying isotropic turbulence show that, for a Smagorinsky-type eddy viscosity model, the correlation between the SGS stress and the model is comparable to that from the original model. Also, the suggested model for the pressure dilatation term in the SGS kinetic energy equation is found to have a high correlation with its actual value. In a posteriori tests, the proposed dynamic k-equation model is applied to decaying isotropic turbulence and normal shock-isotropic turbulence interaction, and yields good agreement with available experimental and DNS data. Compared with the results of the dynamic Smagorinsky model (DSM), the k-equation model predicts better energy spectra at high wavenumbers, similar kinetic energy decay and fluctuations of thermodynamic quantities for decaying isotropic turbulence. For shock-turbulence interaction, the k-equation model and the DSM predict similar evolutions of turbulent intensities across shocks, owing to the dominant effect of linear interaction. The proposed k-equation model is more robust in that local averaging over neighbouring control volumes is sufficient to regularize the dynamic procedure. The behaviour of pressure dilatation and dilatational dissipation is discussed through the budgets of the SGS kinetic energy equation, and the importance of the dilatational dissipation term is addressed.

72 citations


Journal ArticleDOI
Ch. Leela1, Suman Bagchi1, V. Rakesh Kumar1, Surya P. Tewari1  +1 moreInstitutions (1)
Abstract: We present our results on spatio-temporal evolution of laser plasma produced shockwaves (SWs) and hot core plasma (HCP) created by focused second harmonic (532 nm, 7 ns) of Nd-YAG laser in quiescent atmospheric air at f/#10 focusing geometry. Time resolved shadowgraphs imaged with the help of an ICCD camera with 1.5 ns temporal resolution revealed the presence of two co-existing sources simultaneously generating SWs. Each of the two sources independently led to a spherical SW following Sedov-Taylor theory along the laser propagation direction with a maximum velocity of 7.4 km/s and pressure of 57 MPa. While the interaction of SWs from the two sources led to a planar SW in the direction normal to the laser propagation direction. The SW detaches from the HCP and starts expanding into the ambient air at around 3 µs indicating the onset of asymmetric expansion of the HCP along the z-axis. The asymmetric expansion is observed till 10 µs beyond which the SW leaves the field of view followed by a deformation of the irradiated region in the XY-plane due to the penetration of surrounding colder air in to the HCP. The deformation in the XY-plane lasts till 600 µs. The dynamics of rapidly expanding HCP is observed to be analogous to that of cavitation bubble dynamics in fluids.

37 citations


Journal ArticleDOI
Abstract: The Thomson scattering method was applied to quantify the electron number density and temperature of a laser spark formed in argon. The laser spark was generated by focusing a 15 mJ beam from the second harmonic (λL = 532 nm) of a nanosecond Nd:YAG laser with an 80 mm focal length lens. Images of the spark emission were obtained for times between 1 ns and 20 μs after the laser pulse in order to characterize its spatial evolution. The electron density and temperature for the core of the plasma plume at different instants of its evolution were determined from the Thomson scattered spectra of another nanosecond Nd:YAG laser (532 nm, 10 to 60 mJ/pulse). In the time interval between 400 ns and 10 μs between the laser induced plasma and Thomson scattering probe pulses, we found ne and Te to decrease from 4.3 × 1023 m− 3 to 2.4 × 1022 m− 3 and from 50 700 K to 11 100 K, respectively. Special care was paid to the plasma disturbance by the probe laser pulse in Thomson scattering experiments due to absorption of laser photons by electrons through the inverse bremsstrahlung process.

31 citations


Journal ArticleDOI
Abstract: Results are presented on two-dimensional numerical simulation of a decaying laser spark after the air breakdown. Radiation loss is modeled in a finite volume framework for a finite wavelength range of the emitted rays. The redistribution of assumed absorbed energy among different parts, namely the blast wave energy, the radiative heat energy and the left-over energy is quantified. About 5% of the absorbed energy is found to be trapped around the breakdown region after the blast wave moves away. A prediction is made about the actual available energy for ignition of a combustible mixture. A distinct rollup motion around the breakdown region is demonstrated from the simulation, which is in accordance with other numerical and experimental observations. Finally, the evolution of the blast wave radius with time is validated against in-house experimental data.

31 citations


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Related Authors (1)
Krishnan Mahesh

187 papers, 6.8K citations

68% related
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Author's H-index: 2

No. of papers from the Author in previous years
YearPapers
20082
20072
20051