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🎓PhD Defense : Mark NOUN:”Prediction and mitigation of cavity instabilities resulting from fluid-structure interactions “

  Wednesday 10 January 2024 at 14h30

  Conference Room - CERFACS       Organized by Nathalie BROUSSET    

YOU TUBE LINK : https://youtube.com/live/QZ67Guq7dF4?feature=share

Abstract :

Complex unsteady phenomena within rotor/stator cavities of space turbopumps have gained notoriety because of their propensity to induce vibration issues that are clearly detrimental to the operation of the engine. This problem has indeed rendered the development and operation of rocket engines a formidable undertaking. These dynamics, referred to as 'pressure bands', are a consequence of a self-sustained oscillatory motion of the working fluid, thereby engendering a coupling with the solid structure posing a paramount risk to the operation of the turbopump and the structural integrity of its components. Understanding and predicting the source of 'pressure bands' in a multiphysics context is the primary objective of this thesis. For instance, this work provides a numerical and theoretical investigation of forced vibration problems in enclosed rotating flows as well as fluid-structure interaction problems with a focus on hydrodynamic and aeroelastic instabilities. Note that these flows are inherently three dimensional due to the presence of boundary layers on the impeller, stator and cylindrical shroud. Consequently, at high Reynolds numbers, the flow instability is manifested through coherent axisymmetric and/or spiral structures that can be affected by dynamic loads generated either by the rocket or the turbopump itself. Experiments have shown that axial cavity flows also exhibit a different type of instability that lead to a flutter-like phenomena of the rotor. Both problems are addressed in this work using Large Eddy Simulation, an unsteady CFD approach, in conjunction to multiple predictive numerical strategies. All tools show that the underlying dynamics of the flow can be retrieved contrarily to steady approaches like Reynolds Averaged Navier-Stokes Simulations (RANS) that failed in the past to predict such phenomena. Thanks to LES flow only prediction, the flow instability inside a reduced scale hydrogen turbopump is retrieved and has the potential of coupling with the rotor as well as the acoustics of the cavity. To address this problem, a structural mechanics code based on the finite element method is developed to perform modal analyses as well as elastodynamic calculations. Thanks to all these numerical tools, forced vibration problems are first investigated using a bluff body configuration where a 'lock-in' phenomenon is identified whenever a vortex shedding frequency converges to the forced vibration frequency. This first content of this study is later extended to enclosed rotating cavity flows where the vibration of the rotor causes a shift in the hydrodynamic modes and in some cases, a total suppression of these modes. Following these flow only responses and to go further, the structural mechanics solver is further developed and coupled to the LES code thanks to a numerical coupling chain that allows to solve fully unsteady and fully coupled fluid-structure interaction problems. The adopted coupling strategy is first successfully validated through two test cases: a vibrating beam immersed in a still fluid demonstrating that the fluid viscosity dampens the structure motion and brings it back to its initial position, and a Vortex Induced Vibration (VIV) case where a Kármán vortex street sheds from a rigid square and causes large amplitude vibrations of an elastic plate. The coupled solver is then used to simulate the fluid-structure interaction between the rotor disk and working fluid of the turbopump. Results confirm the vibroacoustic coupling between the fluid, rotor disk and cavity obtained by experiments. This multiphysics simulation also allowed the calculation of the necessary amount of damping to stabilize such system demonstrating the capability of the developed coupling. To finish, a Global Linear Stability Analysis (GLSA) framework is detailed and performed to give more insight about the leading eigenmodes and their corresponding growth rate inside such systems.

Jury :

Mme Marlene SANJOSE – Ecole de Technologie Supérieure – Université du Québec – Referee

M. Guillaume BALARAC – LEGI Grenoble – Referee

M.Stéphane AUBERT – Ecole Centrale Lyon – Examiner

M. Antoine DAZIN – ENSAM – Paris – Examiner

M. Laurent GICQUEL – CERFACS – Advisor

M. Gabriel STAFFELBACH – CERFACS – Co-advisor
M. Pavanakumar MOHANAMURALY – CERFACS – Co-advisor