Thesis title: High-Fidelity Numerical Methods for Aeroacoustics Phenomena in Compressible Turbulent Flows
The main objective of this dissertation is to perform high-fidelity numerical simulations of compressible turbulent flows for space applications in order to predict aeroacoustic resonance phenomena. The internal flow developing inside the Solid Rocket Motor (SRM) chamber during the ascent phase of a space launcher might cause severe pressure oscillations that could compromise the payload integrity. The formation of aerodynamic side loads owing to shock-wave/turbulent boundary-layer interaction inside the rocket nozzle might produce the same result. Aeroacoustic resonance phenomena could occur inside the system in both circumstances, producing undesirable forces that could endanger the payload and the launcher structure. As a result, accurate evaluation of the aeroacoustic feedback loop is crucial and should be considered throughout the design phase of such systems.
The endeavors of the community in this research field are motivated by a renewed interest in supersonic flights and space vehicles, as well as the resulting engineering request/demand for high-performance rocket launchers, where the problem of aeroacoustic resonance is present for a considerable part of the launchers’ ascent phase and constitutes a major design constraint. Despite the knowledge of these phenomena has improved over the last decade, the problem of pressure fluctuations in SRM chambers and aerodynamic side loads in rocket nozzles, both related to an aeroacoustic feedback loop, appears to be far more complicated than previously expected, and certain essential physical mechanisms remain unclear. Moreover, given the complex flow conditions and lack of optical access, the experimental campaigns suffer from a lack of flow measurements inside both systems. Therefore, numerical simulations are an important complement to achieve a more deep understanding of these physics, allowing major unanswered issues to be addressed.
The present dissertation adopts high-fidelity numerical simulations based on the Implicit Large Eddy Simulation (ILES) and the Delayed Detached Eddy Simulation (DDES) techniques to explore the aeroacoustic resonance phenomenon in compressible turbulent flows involved in space applications. In particular, the DDES is a hybrid RANS/LES approach for simulating high-Reynolds number flows characterized by massive separation. In this methodology, the attached boundary layers are handled in RANS mode, which reduces the computational effort, while the most energetic turbulent scales of separated shear layers and turbulent recirculating zones are handled by the LES mode.
The potential of ILES is first tested on the ONERA C1xb solid rocket motor configuration, for which experimental and numerical studies are available, with the prime aim of emphasizing the capability of the full-scale 3D approach to capture the aeroacoustic resonance, the transition to turbulence of the coherent azimuthal vortices detached by the propellant grain, as well as its influence on the induced acoustic feedback, quantified through the level of pressure oscillation. The results indicate that the ILES is able to capture the transition to turbulence, the vortex shedding and the aeroacoustic feedback phenomena inside the SRM chamber. The pressure oscillation RMS is in excellent agreement with the experimental data. The Fourier spectral analysis in time reveals that the detached shear layer is locked on the second acoustic longitudinal mode while the vortex shedding frequency is locked on the third acoustic longitudinal one. These results are also confirmed by the space-time correlation analysis.
The study then focuses on the investigation of the aeroacoustic feedback loop and the flow unsteadiness in a 3D sub-scale Dual-Bell nozzle, exploiting the DDES methodology. For this nozzle configuration, experimental data are available. In this situation, the analysis concentrates on the DDES capability to detect and reproduce the unsteady flow features in order to determine the intensity of the side loads inside the nozzle, which are generated by an aeroacoustic resonance through the presence of an asymmetric mode of the wall-pressure fluctuations. For this reason, the impact of the dual-bell inflection point on the aeroacoustic feedback loop is inspected. The numerical data agree well with the experimental results in terms of mean and fluctuating wall-pressure statistics. The frequency spectra are characterized by the presence of a persistent large bump in the low-frequency range associated with an axi-symmetric (piston-like) motion of the shock system and a broad and high-amplitude peak at higher frequencies generated by the turbulent activity of the detached shear layer. The presence of the axi-symmetric mode induces thrust oscillations, as occurs for the internal ballistics of SRMs. The Fourier-based spectral analysis, performed in both time and azimuthal wavenumber space, reveals also the presence of a small first (non-symmetrical) pressure mode and its role in the generation of the aerodynamic side loads.