Ignition overpressure and jet noise at launcher start-up numerical simulation
At solid rocket motor (SRM) start-up, an ignition overpressure (IOP), characterized by a strong positive peak and a large underpressure, is generated during the pressure build-up. Afterwards, the hot supersonic jet emits a strong jet noise rich in high frequency components. The present study focuses on the devising of a simulation methodology dedicated to these phenomena. The framework is that of the computational platform CEDRE, and simulations are compared to experimental measurements from a small scale SRM. Large-eddy simulations (LES) are carried out to model both phenomena. All relevant numerical choices are based on validation test-case results. The methodology applied to the IOP computation consists in a direct approach (where both the generation and the propagation of the wave up to the experimental sensors are simulated). The phenomenon resulting in the generation of the IOP are deduced from this computation (piston-like effect from the supersonic jet initially developing) including heat transfer from the jet. After-burning in the first milliseconds of the simulation is found to be of prime order regarding IOP amplitude and its interaction with the jet. A hybrid approach is applied to jet noise computation, combining LES near-field with far-field sound radiation by solving Ffowcs Williams & Hawkings (FWH) equations. The aerodynamic field thus obtained is in good agreement with literature, and hence encourages to apply this methodology to the IOP computation. This final application underlines the IOP non-linear behaviour and lessen its interaction with the jet to a level closer to experimental observations.