Browsing by Author "Hassan A. Hassan, Committee Member"

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Numerical Simulation of Scramjet Combustion in a Shock Tunnel
(2005-12-09) Star, Jason Blue; Jack R. Edwards Jr., Committee Chair; Hassan A. Hassan, Committee Member; D. S. McRae, Committee Member; William L. Roberts, Committee Member
Three-dimensional computational simulations of reactive flowfields within a hydrogen-fueled scramjet-like geometry experimentally tested in a free piston shock tunnel are presented. The experimental configuration (Odam and Paull, AIAA Paper 2003-5244) involves injection of hydrogen fuel into the scramjet inlet, followed by mixing, shock-induced ignition, and combustion. The predictions for both fuel-off and fuel-on conditions were observed to be sensitive to the choice of the wall temperature boundary conditions. The best comparison with experimental data were achieved through the implementation of an approach that involves a simplified conjugate heat transfer model that couples the heat conduction through the wall with the heat conduction of the fluid within the boundary layer. This approach is able to predict thermal loads on the walls of the scramjet model due to shock wave interactions and due to heat release. As such, it is able to more accurately represent the physical temperature response of the engine model. Also shown to produce very good agreement with the statistically-steady experimental data was the isothermal ghost-cell boundary condition, which is based on a simplification of the time-dependent conjugate heat transfer boundary condition. This simplified boundary condition assumes a linear temperature distribution within the wall based on the effective depth that an applied heat load would penetrate, thus, it also allows the actual wall temperature to vary in response to the applied heat load. Results for fuel-off simulations showed that the solution generated by a steady-state simulation implementing the isothermal ghost-cell wall boundary condition was very comparable with the statistically-steady solution obtained from a fully transient simulation with coupled heat conduction within the walls. When integrated in a fully time-accurate manner, the fuel-on simulations showed a striking sensitivity to the modeled rate of air ingestion into the engine. For experimental data that showed steady combustion, the transient simulations resulted in either a steady combusting solution or a progression toward engine unstart, depending on the modeled rate of air ingestion. Also, for experimental data that showed an unsteady thermal choking event leading to eventual unstart, the transient simulations were able to predict both unstart and steady combustion, once again depending on the air ingestion rate. In all cases, the modeled air ingestion process is an approximation of the actual experimental process, in that uniform conditions are imposed as linear functions of time over the inlet plane. The computational results also provide some support for a 'radical-farming' hypothesis, proposed to explain the ability of the hydrogen-air mixture to auto-ignite at relatively low inlet contraction ratios.
A Reconstructed Discontinuous Galerkin Method for the Compressible Euler Equations on Arbitrary Grids
(2010-04-20) Luo, Luqing; Hong Luo, Committee Chair; Hassan A. Hassan, Committee Member; Jack R. Edwards, Committee Member; Zhilin Li, Committee Member
A reconstruction-based discontinuous Galerkin (RDG) method is presented for the solution of the compressible Euler equations on arbitrary grids. By taking advantage of handily available and yet invaluable information, namely the derivatives, in the context of the discontinuous Galerkin methods, a polynomial solution of one degree higher is reconstructed using a least-squares method. The stencils used in the reconstruction involve only the von Neumann neighborhood (face-neighboring cells) and are compact and consistent with the underlying DG method. The resulting RDG method can be regarded as an improvement of a recovery-based DG method, in the sense that it shares the same nice features, such as high accuracy and efficiency, and yet overcomes some of its shortcomings such as a lack of flexibility, compactness, and robustness. The developed RDG method is used to compute a variety of flow problems on arbitrary meshes to demonstrate its accuracy, efficiency, robustness, and versatility. The numerical results indicate that this RDG method is third-order accurate at a cost slightly higher than its underlying second-order DG method, at the same time providing a better performance than the third order DG method, in terms of both computing costs and storage requirements.