Browsing by Author "Dr. D. Scott McRae, Committee Member"

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Algorithmic Enhancements to the VULCAN Navier-Stokes Solver
(2003-08-15) Litton, Daniel; Dr. Jack R. Edwards, Committee Chair; Dr. D. Scott McRae, Committee Member; Dr. Ashok Gopalarathnam, Committee Member
VULCAN (Viscous Upwind aLgorithm for Complex flow ANalysis) is a cell centered, finite volume code used to solve high speed flows related to hypersonic vehicles. Two algorithms are presented for expanding the range of applications of the current Navier-Stokes solver implemented in VULCAN. The first addition is a highly implicit approach that uses subiterations to enhance block to block connectivity between adjacent subdomains. The addition of this scheme allows more efficient solution of viscous flows on highly-stretched meshes. The second algorithm addresses the shortcomings associated with density-based schemes by the addition of a time-derivative preconditioning strategy. High speed, compressible flows are typically solved with density based schemes, which show a high level of degradation in accuracy and convergence at low Mach numbers (M < 0.1). With the addition of preconditioning and associated modifications to the numerical discretization scheme, the eigenvalues will scale with the local velocity, and the above problems will be eliminated. With these additions, VULCAN now has improved convergence behavior for multi-block, highly-stretched meshes and also can accurately solve the Navier-Stokes equations for very low Mach numbers.
Hybrid LES/RANS Simulation of a 10-degree Double-Fin Crossing Shock Flow at Mach 8.28
(2007-02-28) Boles, John Arthur; Dr. Richard D. Gould, Committee Member; Dr. Jack R. Edwards, Committee Chair; Dr. D. Scott McRae, Committee Member
The simulation of a Mach 8.28 10-degree double-fin crossing shock flow using a hybrid large-eddy ⁄ Reynolds-averaged Navier-Stokes (LES⁄RANS) solver is presented in this work. The solver blends a Menter two-equation model for RANS with a Yoshizawa one-equation subgrid model for the LES calculations. The solver uses a flow-dependent transition function based on wall distance and a modeled form of the Taylor microscale. Turbulent boundary layers are initiated and sustained in the inflow region using a recycling⁄rescaling technique applied to the fluctuation fields. The hybrid LES⁄RANS model is tested using both Menter's Baseline (BSL) and Shear Stress Transport (SST) models for the RANS closure. These results are compared to pure Menter BSL and SST RANS results as well as with the experimental data of Kussoy and Horstman(1992). This study concludes that while the hybrid LES⁄RANS model outperforms RANS calculations in the inflow region where the flow is nominally two-dimensional, it significantly overpredicts the wall heat transfer rates in the region of the crossing shock interaction. Possible explanations for this behavior as well as plans for future attempts at solutions to these shortcomings are provided.
Numerical Simulation of the Internal Two-Phase Flow within an Aerated-Liquid Injector and its Injection into the Corresponding High-speed Crossflows
(2005-08-16) Tian, Ming; Dr. Hassan A. Hassan, Committee Member; Dr. D. Scott McRae, Committee Member; Dr. Zhilin Li (Dept. of Mathematics), Committee Member; Dr. Jack R. Edwards, Committee Chair
The current study investigates the flow structures within an aerated-liquid (barbotage) injector, which is designed to facilitate the rapid breakup of a hydrocarbon fuel jet prior to its entering a scramjet combustor, and the spray structures in the corresponding crossflow. Simulations of the transient, three-dimensional, two-phase flow within the "out-in" injector operating at different gas-to-liquid (GLR) mass ratios and in the corresponding crossflow domain have been performed, and the results compared with experimental pressure measurements of the injector and shadowgraph images of the crossflow. The numerical method solves a "mixture" model of two-phase flow using a preconditioning strategy. High-order spatial accuracy and good interface-capturing properties are facilitated by the use of shock-capturing schemes combined with second order TVD methods. Also, an immersed boundary method is used to investigate the probe effects, and a droplet transport model is used in the crossflow simulations to get more details about effect of droplet size. The injector simulation results highlight the effects of mesh refinement and turbulence model on the predicted solutions. The pressure drop across the injector is predicted reasonably well by the computational methodology, and the trend of increasing injector pressure with increasing GLR is captured properly. Predictions of the absolute pressure level within the injector show some discrepancies in comparison with experimental data but agree well with theoretical estimates. The results of the injector simulations with plenum included are consistent with the results of the discharge tube cases. If the centerline pressure is close to the experimental data, the gas mass flow rate at outlet will approach a value below the experimental data. If the gas mass flow rate at outlet approaches the experimental data, then the centerline pressure will be higher than the experimental data, but agrees well with theoretical analyses. The intrusion of the probe has little effect on the flowfield if the probe is contained wholly within the liquid core, but does affect the flowfield if the probe tip is in the two-phase mixing region, instead of the liquid core. The results of crossflow show that the two-phase flow injects into the crossflow, bends towards the streamwise direction, disperses into a spray plume, and initiates a horse-shoe shape structure of the jet in the cross-sectional planes. The result based on the previous injector simulation at a higher inlet gas pressure shows best penetration height prediction among all freestream Mach 0.3 cases. Including the droplet transport model gives a similar spray structure in the X-Z centerplane as that of the mixture model, but gives a different spray structure in the cross-sectional planes. The horse-shoe shaped structure fades away with increases in the droplet diameter size, and the liquid mass accumulates to the X-Z centerplane.