M.Sc Thesis


M.Sc StudentAlexander Vaynshtein
SubjectOn the Flow Stability of a Sudden Opening Supersonic Intake
DepartmentDepartment of Aerospace Engineering
Supervisors Full Professors Natan Benveniste
Professor Arieli Rimon
Full Thesis textFull thesis text - English Version


Abstract

A numerical study of a two-dimensional, mixed compression, supersonic intake for a ramjet engine is presented. The simulations were performed with ANSYS? Fluent Inc. code, using an inviscid, time dependent, implicit solver.

Two-stage validations were conducted, solving inviscid and viscous approximations of a shock-tube and a half-diamond obstacle in supersonic stream. The runs are in very good agreement with other computational and theoretical solutions and experimental data.

To estimate the drag contribution of the intake to the general drag of the configuration throughout the boost phase, three different types of conditions were considered. a) Configuration with closed inlet entrance; b) With opened entrance and exit; c) With opened entrance and closed exit. It was demonstrated that the closed entrance configuration provides significantly low drag compared to the others. Moreover, the shock ingestion Mach number for the a-priori closed inlet configuration was found to be significantly lower than for the fully opened one. Once the configuration reaches the 'start' conditions (shock ingestion) and the entrance opens suddenly, a transient process starts. The transition period lasts for few milliseconds until the flow reaches steady state.

A special computational technique was derived to enforce a back pressure in a supersonic hyperbolic flow field. Using this method, the influence of back pressures on the normal shock position along the diverging diffuser was analyzed. The minimal ratio between back and ambient pressures to maintain a subsonic exit (ramjet conditions) was defined, and the total pressure recovery for each pressure ratio was calculated.

To assess the uncertainty in the results caused by various turbulence models and to estimate the mass flow rate needed to be extracted to simulate Euler flow conditions, Navier-Stokes solution (RANS) was run using three turbulence models (Spalart Allmaras, k-ε and k-ω). The test case was a full ramjet conditions flow with back to ambient pressure ratio of 13, ensuring the presence of a normal shock (in the Euler solution). Large differences were found between the viscous and Euler solutions. Significant discrepancies were also found among the viscous solutions themselves, thus insinuating that the shock-wave boundary-layer interaction is of major importance. Therefore, mass flow suction of at least 7.1% in the vicinity of strong shock wave impingement is essential in such internal flows.