Sharif Digital Repository

In this work, a high-order accurate gas kinetic scheme based on the compact finite-difference Boltzmann method (CFDBM) is developed and applied for simulating the compressible rarefied gas flows. Here, the Shakhov model of the Boltzmann equation is considered and the spatial derivative term in the resulting equation is discretized by using the fourth-order compact finite-difference method and the time integration is performed by using the third-order TVD Runge-Kutta method. A filtering procedure with three discontinuity-detecting sensors is applied and examined for the stabilization of the solution method especially for the problems involving the discontinuity regions such as the shock. The... 

Often in a vacuum test facility, a supersonic exhaust diffuser uses the energy of the outflow from the nozzle to create and maintain a vacuum condition in the engine test chamber. In this system, the temperature of the motor exhaust gas, which directly hits the diffuser walls, is much higher than the tolerance of the metal body of the diffuser. The purpose of this research is to design a cooling system for thermal protection of diffuser walls of a vacuum simulator using a water-jacket tool. At the beginning of this study, the effective parameters in the design of the water-jacket cooling system were identified. Then, a new algorithm to design and optimize the water-jacket cooling system was... 

In the present study, the effect of the diffuser inlet-to-nozzle exit area ratio (A_d/A_e ) on the gradual starting performance of a second throat exhaust diffuser has been investigated experimentally. Experiments have been carried out with four conical nozzles with different expansion ratios and considering two approaches of gradual increase and gradual decrease of nozzle chamber pressure by a high-pressure air tank. Investigations show that at high area ratios before starting the diffuser, oscillations in the pressure of the test chamber are created due to the instability of the flow at the diffuser inlet. By frequency analysis of the vacuum chamber pressure, it is observed that as A_d/A_e... 

Depending on the geometry and nozzle pressure, increasing the ratio of the diffuser inlet area to the nozzle outlet area can lead to instability in the high-altitude simulator vacuum chamber startup process. This instability results in the formation of harmonic pressure oscillations throughout the diffuser and the vacuum chamber. In this study, a scaled down second throat diffuser with compressed air has been utilized. The performance of the diffuser has been investigated using four conical nozzles with different expansion ratios to simulate instability at area ratios of 1.27, 1.91, 4.1, and 7.81 with rapid nozzle pressure rise. Experiments were conducted by measuring wall pressures along...