ENGINEERING AND CFD

Hydraulic machinery, such as water turbines, turbo pumps and marine propulsion systems, are frequently affected by cavitation, which occurs when the local static pressure in the liquid drops below the vapor pressure. This causes evaporation and the generation of vapor pockets (cavities). Being advected again into regions of increased pressure, a sudden implosion-like re-condensation takes place. During these collapse events, the pressure level can reach several hundreds to thousands of atmospheres and strong shock waves are emitted. The flow aggressiveness can be severe enough to damage even metal surfaces. When exposed to cavitation over a sustained time, this may eventually lead to the failure of the affected components.

In this project, researchers of the of the Institute of Aerodynamics and Fluid Mechanics at the Technische Universität München (TUM) focus on the numerical investigation of cavitating flow in the context of ship propellers. A key aspect of this project is to develop the ability to assess local flow aggressiveness and to quantify the potential of material erosion. While material erosion reduces the lifespan of ship propellers and the rudder, cavitation also causes a reduction in thrust and efficiency. The propeller performance is degraded, and fuel consumption increased. Furthermore, cavitation induces noise and structural vibrations. To avoid these highly undesirable consequences, an in-depth understanding of the relevant mechanisms is necessary.

In this study, the scientists employ the density-based flow-solver CATUM [1,2] developed at the Institute of Aerodynamics and Fluid Mechanics at the TUM. With this approach, the formation and propagation of complex shock wave systems, crucial for the dynamics of cavitating flow and its impact on materials, are fully resolved. To accurately capture these effects, time steps smaller than nanoseconds are necessary. In contrast, typical characteristic time intervals of the convective mean flow lie in the order of seconds. Furthermore, very fine numerical grids are necessary, in order to resolve even small-scale flow structures. Combining the requirements for a high temporal as well as spatial resolution leads to a substantial numerical effort, demanding massively-parallel, high-performance computing resources. For all studies, the researchers thus employ the Tier-0 system SuperMUC run by the Leibniz Supercomputing Centre in Garching near Munich.

The aim of this project was two-fold. First, the employed model was validated with the investigation of partial shedding cavities experiencing sheet-to-cloud transition [3]. It was shown that a condensation shock phenomenon, only observed recently in experiments, is well reproduced by the method. Furthermore, it was demonstrated that the approach is suitable for the numerical investigation of cavitating ship propeller flow [4] and can be used to assess local flow aggressiveness on ship propellers [5].

References:
[1] http://www.aer.mw.tum.de/abteilungen/gasdynamik
[2] Schmidt, S.J., Sezal, I. H., Schnerr, G. H. and Thalhamer, M. (2008). “Riemann techniques for the simulation of compressible liquid flows with phase-transition at all Mach numbers - shock and wave dynamics in cavitating 3D micro and macro systems,” 46th AIAA Aerospace Sciences Meeting and Exhibit, Reno
[3] Budich, B., Neuner, S., Schmidt, S. J. and N. A. Adams (2015). “Numerical investigation of shedding partial cavities over a sharp wedge,” 9th International Symposium on Cavitation, Lausanne
[4] Budich, B., Schmidt, S. J. and N. A. Adams (2015). “Numerical Investigation of a Cavitating Model Propeller Including Compressible Shock Wave Dynamics,” 4th International Symposium on Marine Propulsors, Austin
[5] Budich, B., Schmidt, S. J. and N. A. Adams (2015). “Numerical Simulation of Cavitating Ship Propeller Flow and Assessment of Erosion Aggressiveness,” 6th International Conference on Computational Methods in Marine Engineering, Rome

Scientific Contact:
Dipl.-Ing. Bernd Budich
Institute of Aerodynamics and Fluid Mechanics, Technical University of Munich
Boltzmannstr. 15, D-85748 Garching (Germany)
e-mail: bernd.budich [at] tum.de