Shankar Ghosh

Bio: Shankar Ghosh is an academic researcher from University of Minnesota. The author has contributed to research in topics: Direct numerical simulation & Turbulence. The author has an hindex of 2, co-authored 5 publications receiving 77 citations.

Numerical simulation of the fluid dynamic effects of laser energy deposition in air

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.

Numerical Simulation of Laser Induced Breakdown in Air

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.

Direct numerical simulation of the thermal effects in plasma/turbulence interaction

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.

Numerical simulation of strong blast waves in high temperature turbulent flows

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.

Direct numerical simulation of the thermal eects of plasmas on turbulent o ws

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.

Similarity and Dimensional Methods in 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

Dynamic k-Equation Model for Large Eddy Simulation of Compressible Flows !

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.

Control of Early Flame Kernel Growth by Multi-Wavelength Laser Pulses for Enhanced Ignition

Abstract: The present contribution examines the impact of plasma dynamics and plasma-driven fluid dynamics on the flame growth of laser ignited mixtures and shows that a new dual-pulse scheme can be used to control the kernel formation process in ways that extend the lean ignition limit. We perform a comparative study between (conventional) single-pulse laser ignition (λ = 1064 nm) and a novel dual-pulse method based on combining an ultraviolet (UV) pre-ionization pulse (λ = 266 nm) with an overlapped near-infrared (NIR) energy addition pulse (λ = 1064 nm). We employ OH* chemiluminescence to visualize the evolution of the early flame kernel. For single-pulse laser ignition at lean conditions, the flame kernel separates through third lobe detachment, corresponding to high strain rates that extinguish the flame. In this work, we investigate the capabilities of the dual-pulse to control the plasma-driven fluid dynamics by adjusting the axial offset of the two focal points. In particular, we find there exists a beam waist offset whereby the resulting vorticity suppresses formation of the third lobe, consequently reducing flame stretch. With this approach, we demonstrate that the dual-pulse method enables reduced flame speeds (at early times), an extended lean limit, increased combustion efficiency, and decreased laser energy requirements.

Dynamics of laser induced micro-shock waves and hot core plasma in quiescent air

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.