Simulation of Ram-Accelerator Flowfields
AIR FORCE RESEARCH LAB BOLLING AFB DC
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Ram-accelerators are devices that use combustion to accelerate projectiles to high velocities. Shaped projectiles are propelled through an accelerator tube by expansion of combustion products. Ignition of the premixed fuel and oxidizer is shock-induced. Tailoring the shape of the projectile results in improvements in performance. In this research numerical simulations of the reacting flowfield surrounding the projectile are obtained and analyzed. Computational Fluid Dynamics CFD methods are used to solve the extended Navier-Stokes equation set. Chemical kinetics are modeled by a quasi-global reaction mechanism including 10 species and 12 reactions. Flow is assumed to be laminar. The governing equations are discretized using a finite-volume formulation. Inviscid fluxes are calculated using a Roe-TVD scheme, while standard central differences are used for the viscous terms. A modified form of the diagonal implicit method is used to advance the solution from initial conditions to the steady state. Some species mass conservation equations are replaced by elemental mass conservation equations to improve convergence. As a whole, the simulations demonstrate the value of CFD to the aerospace engineering field. Physical models are evaluated, numerical methods are tested, and flow phenomena are analyzed. In the course of the research, the advantages and drawbacks of the quasi-global mechanism are exhibited. Compared to a more detailed model the reaction front is smeared, and the model must be calibrated with established data. However, the quasi-global mechanism offers reasonable accuracy at a significantly lower computational cost than more detailed models. More research must be devoted to the modified diagonal implicit scheme. For some choices of replaced species, the method allows negative mass fractions to develop in the evolution of the simulations.
- Fluid Mechanics