Graham V. Candler

Review of Chemical-Kinetic Problems of Future NASA Missions, II: Mars Entries

A number of chemical-kinetic problems related to phenomena occurring behind a shock wave surrounding an object flying in the earth atmosphere are discussed, including the nonequilibrium thermochemical relaxation phenomena occurring behind a shock wave surrounding the flying object, problems related to aerobraking maneuver, the radiation phenomena for shock velocities of up to 12 km/sec, and the determination of rate coefficients for ionization reactions and associated electron-impact ionization reactions. Results of experiments are presented in form of graphs and tables, giving data on the reaction rate coefficients for air, the ionization distances, thermodynamic properties behind a shock wave, radiative heat flux calculations, Damkoehler numbers for the ablation-product layer, together with conclusions.

Data-Parallel Line Relaxation Method for the Navier -Stokes Equations

The Gauss‐Seidel line relaxation method is modie ed for the simulation of viscous e ows on massively parallel computers. The resulting data-parallel line relaxation method is shown to have good convergence properties for a seriesoftestcases.Thenewmethodrequiressignie cantlymorememorythanthepreviouslydevelopeddata-parallel relaxation methods, but it reaches a steady-state solution in much less time for all cases tested to date. In addition, the data-parallel line relaxation method shows good convergence properties even on the high-cell-aspect-ratio grids required to simulate high-Reynolds-number e ows. The new method is implemented using message passing on the Cray T3E, and the parallel performance of the method on this machine is discussed. The data-parallel line relaxation method combines the fast convergence of the Gauss ‐Seidel line relaxation method with a high parallel efe ciency and thus shows promise for large-scale simulation of viscous e ows.

The solution of the Navier-Stokes equations using Gauss-Seidel line relaxation

Abstract Gauss-Seidel line relaxation is used to solve an implicit flux split difference approximation to the Navier-Stokes equations. The flux split approximation is chosen to maximize the weight of the diagonal elements of the block matrix elements that need to be inverted iteratively by the Gauss-Seidel procedure. There are several flux split approximations that can be chosen. However, not all are suitable for viscous flows containing shear or boundary layers. The present paper will illustrate the adverse effects of flux splitting in viscous flow calculations and propose corrections. The numerical procedures will be applied to solve for subsonic laminar flow past a flat plate, turbulent flow past a cone at March 6, and chemical and thermal nonequilibrium flow past a sphere-cone body at March 18.

Subgrid-Scale Models for Compressible Large-Eddy Simulations ?

Abstract:An a priori study of subgrid-scale (SGS) models for the unclosed terms in the energy equation is carried out using the flow field obtained from the direct simulation of homogeneous isotropic turbulence. Scale-similar models involve multiple filtering operations to identify the smallest resolved scales that have been shown to be the most active in the interaction with the unresolved SGSs. In the present study these models are found to give more accurate prediction of the SGS stresses and heat fluxes than eddy-viscosity and eddy-diffusivity models, as well as improved predictions of the SGS turbulent diffusion, SGS viscous dissipation, and SGS viscous diffusion.

Thermal rate constants of the N2+O→NO+N reaction using ab initio 3A″ and 3A′ potential energy surfaces

Theoretical determinations of the thermal rate constants and product energy distributions of the N2+O→NO+N reaction, which plays a crucial role in hydrocarbon air combustion and high temperature air chemistry, are carried out using a quasiclassical trajectory method. An analytical fit of the lowest 3A′ potential energy surface of this reaction based on the CCI ab initio data is obtained. The trajectory study is done on this surface and an analytical 3A″ surface proposed by Gilibert et al. [J. Chem. Phys. 97, 5542 (1992)]. The thermal rate constants computed from 3000 to 20 000 K are in good agreement with the available experimental data. In addition, the dependence of the rate constant on the N2 internal state is studied. It is found that a low vibrational excitation can reduce the rate constant of this reaction by a factor of 3. Also, we investigate the effect of the N2 vibrational state on the product NO vibrational distribution, and it is found that at low N2 vibrational states, the NO vibrational dist...

Data-parallel lower-upper relaxation method for the navier-stokes equations

The lower-upper symmetric Gauss-Seidel method is modified for the simulation of viscous flows on massively parallel computers. The resulting diagonal data-parallel lower-upper relaxation (DP-LUR) method is shown to have good convergence properties on many problems. However, the convergence rate decreases on the high cell aspect ratio grids required to simulate high Reynolds number flows. Therefore, the diagonal approximation is relaxed, and a full matrix version of the DP-LUR method is derived. The full matrix method retains the data-parallel properties of the original and reduces the sensitivity of the convergence rate to the aspect ratio of the computational grid. Both methods are implemented on the Thinking Machines CM-5, and a large fraction of the peak theoretical performance of the machine is obtained. The low memory use and high parallel efficiency of the methods make them attractive for large-scale simulation of viscous flows.

A parallel unstructured implicit solver for hypersonic reacting flow simulation

Publisher Summary The chapter focuses on two aspects of the solver, as well as the parallel performance of the hybrid implicit method. A new parallel implicit solver for the solution of the compressible Navier–Stokes equations with finite rate chemistry on unstructured finite volume meshes is presented in the chapter. The solver employs the data-parallel line relaxation (DPLR) method for implicit time integration along the lines of cells that are normal to the wall. A point-implicit method is used in the regions where surface-normal lines are not constructed. The new method combines the robustness and efficiency of the implicit DPLR method with the flexibility of using unstructured discretizations. The solver employs a low-dissipation pure-upwind numerical scheme based on the Steger-Warming split flux method, as well as a MUSCL-type scheme designed for unstructured discretizations. The DPLR method is superior to other parallel methods, such as matrix-based point-implicit methods designed for chemically reacting hypersonic flow simulations.

Direct simulation methods for low-speed microchannel flows

Large statistical scatter and effective pressure boundary conditions are two critical problems in the computation of microchannel flows with the direct simulation Monte Carlo (DSMC) method. To address these issues, an extension of the DSMC-IP (information preservation) coupled method is developed from the one-dimensional case to the two-dimensional case for microchannel flow. Simulation results in a microchannel flow from DSMC, IP, and numerical and analytical solutions to the Navier-Stokes equations are compared. The DSMC-IP coupled method successfully reduces the large statistical scatter usually obtained with DSMC in such low-speed flow systems. It also provides a suitable implementation of pressure boundary conditions

NUMERICAL STUDY OF HYPERSONIC REACTING BOUNDARY LAYER TRANSITION ON CONES

Hypersonic gas flow over cones is solved using computational fluid dynamics to obtain accurate boundary layer profiles. A linear stability analysis is performed on the profiles to determine the amplification rates of naturally occurring disturbances, and this information is used with the eN method to predict the boundary layer transition location. The effects of free-stream total enthalpy and chemical composition on transition location are studied to give a better understanding of recent experimental observations. Namely, there is an increase in transition Reynolds number with increasing free-stream total enthalpy, and this increase is greater for gases with lower dissociation energies. The results show that linear stability predicts the same trends that were observed in the experiments, but with N=10, it consistently overpredicts the transition Reynolds numbers by about a factor of 2. The results of numerical experiments are presented which show the effect of reaction endo- or exothermicity on disturbanc...

Effect of numerics on Navier-Stokes computations of hypersonic double-cone flows

A systematic study of the effects of the numerics on the simulation of a steady hypersonic flow past a sharp double cone is presented. Previous studies have shown that the double-cone flow is challenging to compute, making it useful for testing both numerical schemes and physical models. We focus on the numerical aspects only and show that the results are very sensitive to the numerical flux evaluation method and slope limiter used