Session: 09-02-04 WEC Numerical Modeling

Paper Number: 81447

81447 - Solutions to Wave Damping Over Time in CFD RANS Simulations Due to Exponential Generation of Numerical Turbulence 

In ocean engineering CFD URANS simulations are high-fidelity analysis fundamental to evaluate hydrodynamics loads during extreme events, which are usually overestimated by BEM software; this leads to an oversized structure. To describe multiphase flows, the most used model in CFD simulations is VOF (Volume of Fluid) which consists in creating an interface between the different fluids and assigning to each cell a percentage of each phase. Unfortunately, the model has a well know bias in the ocean engineering community which increase with simulation time and affects the wave propagation. To ensure correct spatial propagation of wave motion it is necessary to use a very low CFL (Courant-Friedrichs-Lewy) factor. This causes a noticeable increase in computational time and speed up the generation of artificial turbulence in the domain. In fact, it is known that with small timesteps the problem of wave propagation becomes progressively ill-conditioned, and the pressure field presents "checkerboard" modes. The problem is intrinsic in the VOF model and cannot be solved by refining the mesh or increasing the order of accuracy of the solvers. The numerical turbulence spreads in the whole domain and dampens the waves over time after several wave periods (20-50).

With this work we analyze the effect of a threshold on the turbulent viscosity in order to reduce the production of numerical turbulent kinetic energy. We investigated this problem in an empty semi-2d tank by monitoring the wave probes over time. The same test was repeated in the 3d domain first in a pure propagation case with and without limiter and then with a floating body, to verify that the kinematics before the damping occurs, is not altered by the limiter and to study how the presence of a floating body speed up the generation of numerical turbulence.

Presenting Author: Pietro Casalone Politecnico di Torino - MOREnergy Lab

Authors:

Pietro Casalone Politecnico di Torino - MOREnergy Lab
Oronzo Dell'edera Politenico di Torino - MOREnergy Lab
Marco Fontana Politenico di Torino - MOREnergy Lab
Giuliana Mattiazzo Politenico di Torino - MOREnergy Lab
Beatrice Battisti Politenico di Torino - MOREnergy Lab