Jeffrey L. Payne

RANS simulations of a simplified tractor/trailer geometry.

Steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations are presented for the three-dimensional flow over a simplified tractor-trailer geometry at zero degrees yaw angle. The simulations are conducted using the SACCARA multi-block, structured CFD code. Two turbulence closure models are employed: the one-equation Spalart-Allmaras model and the two-equation k-ω model of Menter. The discretization error is estimated by employing two grid levels: a fine mesh of approximately 20 million grid points and a coarse mesh of approximately 2.5 million grid points. Simulation results are compared to the experimental data obtained at the NASA-Ames 7×10 ft wind tunnel. Quantities compared include: surface pressures on the tractor/trailer, vehicle drag, and time-averaged velocities in the base region behind the trailer. The results indicate that both turbulence models are able to accurately capture the surface pressure on the vehicle, with the exception of the base region. The Menter k-ω model does a reasonable job of matching the experimental data for base pressure and velocities in the near wake, and thus gives an accurate prediction of the drag. The Spalart-Allmaras model significantly underpredicted the base pressure, thereby overpredicting the vehicle drag.

DSMC and Navier-Stokes Predictions for Hypersonic Laminar Interacting Flows

Direct Simulation Monte Carlo (DSMC) and NavierStokes calculations are performed for a Mach 11 25 deg.-55 deg. spherically blunted biconic. The conditions are such that flow is laminar, with separation occurring at the cone-cone juncture. The simulations account for thermochemical nonequilibrium based on standard Arrhenius chemical rates for nitrogen dissociation and Millikan and White vibrational relaxation. The simulation error for the Navier-Stokes (NS) code is estimated to be 2% for the surface pressure and 10% for the surface heat flux. The grid spacing for the DSMC simulations was adjusted to be less than the local mean-freepath (mfp) and the time step less than the cell transient time of a computational particle. There was overall good agreement between the two simulations; however, the recirculation zone was computed to be larger for the NS simulation. A sensitivity study is performed to examine the effects of experimental uncertainty in the freestream properties on the surface pressure and heat flux distributions. The surface quantities are found to be extremely sensitive to the vibrational excitation state of the gas at the test section, with differences of 25% found in the surface pressure and 25%-35% for the surface heat flux. These calculations are part of a blind validation comparison and thus the experimental data has not yet been re

Navier-Stokes and direct simulation Monte Carlo predictions for laminar hypersonic separation

Axisymmetric direct simulation Monte Carlo (DSMC) and Navier‐Stokes simulations are performed as part of a code validation effort for hypersonice ows. The e owe eld examined herein is the Mach 11 laminar e ow over a 25 ‐ 55-deg blunted biconic. Experimental data are available for surface pressure and heat e ux at a Knudsen number Kn=0.019 based on the nose radius. Simulations at a reduced freestream density (Kn=0.057) are performed to explore the region of viability of the numerical methods for hypersonic separated e ows. A detailed and careful effort is made to address the numerical accuracy of these simulations, including iterative and grid convergence studiesforNavier ‐Stokesandtemporal,grid,andparticleconvergencestudiesforDSMC.Goodagreementisfound between the DSMC and Navier ‐Stokes simulation approaches for surface properties as well as velocity proe les within the recirculation zone for the reduced density case. The results obtained indicate that the failure of earlier DSMC simulations at Kn=0.019 is due to insufe cient grid ree nement within the recirculation zone. Furthermore, it is shown that accurate simulations of the biconic at the experimental conditions with the DSMC method are not yet possible due to the extreme computational cost. Nomenclature d = molecular diameter, m f = general solution variable Kn = Knudsen number based on nose radius, ¸=RN L = characteristic length scale, m n = number density, particles/m 3 p = pressure, N/m 2 , order of accuracy q = heat e ux, W/m 2

Navier-Stokes and DSMC simulations for hypersonic laminar shock-shock interaction flows

DSMC and Navier-Stokes simulations are performed as part of a code validation effort for hypersonic flows. The flowfield examined herein is the Mach 11, laminar flow over a 25 deg 55 deg blunted biconic for which experimental data are available for surface pressure and heat flux. Considerable effort is made to address the numerical accuracy of all simulations including iterative and grid convergence studies for Navier-Stokes and temporal, grid, and particle convergence studies for DSMC. Simulations of the biconic at a reduced freestream density are performed to explore the region of viability of the numerical methods for hypersonic separated flows. Excellent agreement is found between the DSMC and Navier-Stokes simulation approaches for surface properties as well as velocity profiles within the recirculation zone. The results of the rarefied biconic study indicate that the failure of prior DSMC simulations at the experimental densities is due to insufficient grid refinement within the recirculation zone. Additional DSMC and Navier-Stokes simulations are performed for the blunted 25 deg forecone using fine computational meshes to address discrepancies between the simulations and the experiment for the forecone heating. The results of this highly refined forecone study provide strong evidence for the presence of a bias error in the freestream conditions.  1 † Senior Member of Technical Staff, MS 0825, E-mail: cjroy@sandia.gov, Member AIAA ‡ Member of Technical Staff, MS 0827, E-mail: magalli@sandia.gov, Member AIAA § Principal Member of Technical Staff, MS 0820, E-mail: tjbarte@sandia.gov, Member AIAA # Principal Member of Technical Staff, MS 0825, E-mail: jlpayne@sandia.gov, Member AIAA * Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000. This paper is declared a work of the U. S. Government and is not subject to copyright protection in the United States. Nomenclature Cp specific heat at constant pressure, J/kgK f general solution variable h specific enthalpy, J/kg p pressure, N/m2, order of accuracy q heat flux, W/m2 R specific gas constant, J/kgK (= 296.8 for N2) RN nose radius, m (= 0.00635) r grid refinement factor s specific entropy, J/kgK T translational temperature, K t time, ms u axial velocity component, m/s V velocity magnitude, m/s x axial coordinate, m y radial coordinate, m γ ratio of specific heats μ absolute viscosity, Ns/m2

Bluff-Body Flow Simulations using Hybrid RANS/LES

The Detached Eddy Simulation (DES) and steady-state Reynolds-Averaged Navier-Stokes (RANS) turbulence modeling approaches are examined for the incompressible flow over a square cross-section cylinder at a Reynolds number of 21,400. A compressible flow code is used which employes a second-order Roe upwind spatial discretization. Efforts are made to assess the numerical accuracy of the DES predictions with regards to statistical convergence, iterative convergence, and temporal and spatial discretization error. Three-dimensional DES simulations compared well with two-dimensional DES simulations, suggesting that the dominant vortex shedding mechanism is effectively two-dimensional. The two-dimensional simulations are validated via comparison to experimental data for mean and RMS velocities as well as Reynolds stress in the cylinder wake. The steady-state RANS models significantly overpredict the size of the recirculation zone, thus underpredicting the drag coefficient relative to the experimental value. The DES model is found to give good agreement with the experimental velocity data in the wake, drag coefficient, and recirculation zone length.

A Comparison of Turbulence Models for a Supersonic Jet in Transonic Crossflow

Numerical simulations of a supersonic jet in a subsonic compressible crossflow are conducted using three turbulence models. The numerical results are compared to existing experimental data. The comparisons with the experiment include separation points, reattachment points, and surface pressures in the near jet region as well as mean total pressure and velocity measurements at 10 and 40 diameters downstream of the jet. The simulations employ a finite volume Navier-Stokes code on structured multi-block grids. The turbulence models considered in this study include the Spalart-Allmaras one-equation model, a low Reynolds number A:-e model and the Wilcox /c-co model. The Spalart-Allmaras model gave poor results in the near-field of the jet, but showed excellent agreement with the downstream total pressure and vorticity data. The two-equations models, while giving good results in the jet near-field, tended to over predict both the strength and the lift-off height of the jetinduced vortex pair.

A domain decomposition study of massively parallel computing in compressible gas dynamics

The appropriate utilization of massively parallel computers for solving the Navier-Stokes equations is investigated and determined from an engineering perspective. The issues investigated are: (1) Should strip or patch domain decomposition of the spatial mesh be used to reduce computer time? (2) How many computer nodes should be used for a problem with a given sized mesh to reduce computer time? (3) Is the convergence of the Navier-Stokes solution procedure (LU-SGS) adversely influenced by the domain decomposition approach? The results of the paper show that the present Navier-Stokes solution technique has good performance on a massively parallel computer for transient flow problems. For steady-state problems with a large number of mesh cells, the solution procedure will require significant computer time due to an increased number of iterations to achieve a converged solution. There is an optimum number of computer nodes to use for a problem with a given global mesh size.

Simulation of Hypersonic Laminar Flow Validation Experiments

Numerical simulations of the flowfield of a Mach 11, Kn 0.02 laminar flow of nitrogen over the forecone of a spherically blunted 25/55 deg. biconic are presented. The numerical simulations are performed with a Direct Simulation Monte Carlo (DSMC) and a Navier‐Stokes (NS) code. The discrepancies between that measured and calculated surface quantities and their sensitivity to the free stream conditions are examined.