Kedar Pathak

Plume dynamics and shielding characteristics of nanosecond scale multiple pulse in carbon ablation

The behavior of ablated plumes produced by nanosecond scale multiple laser pulses typical for carbon ablation is studied in order to understand the plume expansion dynamics and shielding effect of plume with special interest to ionization of plumes. The patterns of a planar plume (typical for channel cutting) and an axisymmetric plume (typical for hole drilling) appear to be quite different. Ionization in carbon plume is estimated using the Saha equation. An iterative procedure is developed to determine the local equilibrium temperature affected by ionization. It is shown that though shielding due to the presence of ionized particles in carbon plume is small, the effect of ionization on plume temperature can be considerable. Shielding effect is calculated for laser pulses with different time intervals between pulses. The effects of high temperature and low density of plume are conflicting and cause shielding behavior to be nonmonotonic. It is shown that the nonmonotonic dependence of the delivered laser e...

Nonlinear model reduction for unsteady discontinuous flows

Abstract We develop a new nonlinear reduced-order model (ROM) based on proper orthogonal decomposition (POD), which can be used for quantitative simulation of not only smooth flows, but also flows with strong discontinuities. The new model is derived using a Galerkin projection of the fully conservative, nonlinear discretized 2-D Euler equations onto the POD basis constructed for each conservative variable. This approach can be interpreted as a variant of the spectral method with a truncated set of basis functions. A system of ordinary differential equations (ODEs) derived using this model reduction technique resembles the major nonlinear and conservation properties of the original discretized Euler equations. The new reduced-order model also preserves the stability properties of the discrete full-order model equations, so that no additional stabilization is required unlike conventional POD-based models that are susceptible to numerical instabilities. The performance of the new POD ROM is evaluated for 2-D compressible unsteady inviscid flows over a wide range of Mach numbers including trans- and supersonic flows with strong shock waves.

Error Minimization via Adjoint-Based Anisotropic Grid Adaptation

A new adjoint-based grid adaptation methodology for solving steady and unsteady problems is presented. In contrast to conventional grid adaptation techniques utilizing the error equidistribution principle, the new approach directly solves an error minimization problem for which grid node coordinates are used as control variables. A minimum of the error functional is found using a gradient method based on the adjoint formulation. The mesh sensitivity derivatives required for solving the error minimization problem are computed using the solution of the corresponding adjoint equations. The key advantage of this formulation is that the adjoint equations are solved only once at each grid adaptation iteration regardless of the number of grid nodes, which makes this approach well suited for mesh adaptation. The new adjoint-based grid adaptation strategy is tested on several benchmark problems governed by the Euler and Poisson equations, and the numerical results are compared with those obtained on uniform meshes with the same number of grid points.

Laser ablated carbon plume flow dynamics under magnetic field

The dynamics of a conducting laser ablated carbon plume flow in the ablation furnace typical for nanoparticle synthesis is investigated by numerical modeling. The effect of magnetic field on the flow is accounted through the Lorentz body force. The study begins with benchmark calculations of two simple test cases, the Kelvin–Helmholtz instability and Hartmann layer. The evolution of plume is then studied for longitudinal and transverse magnetic fields. It is observed that the transverse magnetic fields have more impact than longitudinal fields on plume evolution for this application. Ionization and heat capacity variation in the plume are accounted through the Saha equation and the Shomate equation, respectively. Multiple plume ejections typical for pulsed laser deposition of thin films are also discussed.

Dynamics of Plumes Generated by Local Injection of Ablated Material

Numerical modeling is employed to study the heat transfer modulation between the thermal protection shield and the gas flow that is caused by ejection of underexpanded pyrolysis gases through the cracks in the thermal protection shield. The simulations are performed for an axisymmetric bluff body flying at Mach 7. The influence of the geometry of the thermal protection shield on the heat transfer pattern is studied for two representative shapes. The results are presented for three different flight altitudes (low, ground level; moderate, 20 km; and high, 30 km). At the low altitude, the plume pressure is lower than the pressure behind the detached front shock wave and the plume propagates slowly along the wall surface. At high and moderate altitudes, the plume path (and, consequently, the convective heat transfer between the thermal protection shield and the plume) depends on the plume interaction with the bow shock wave. The effect of viscosity for the plume injection conditions and freestream Mach number considered is found to be negligible at simulated altitudes. The effect of the initial pressure of pyrolysis gas on the plume dynamics is significant. The presence of the blast wave associated with the underexpanded plume alters the heat transfer and increases mixing. Finally, the enhanced heat transfer caused by the emergence of multiple plumes is investigated.