Numerical simulation of hydrodynamic wave loading by a compressible two-phase flow method

Abstract

Hydrodynamic wave loading on and in offshore structures is studied by carrying out numerical simulations. Particular attention is paid to complex hydrodynamic phenomena such as wave breaking and air entrapment. The applied CFD method, ComFLOW, solves the Navier–Stokes equations with an improved Volume-of-Fluid method to track the movement of the free surface. A local height function keeps the surface sharp (no ‘flotsam and jetsam’). Application of two different fluid models, single-phase (only liquid) and two-phase (liquid and compressible gas) is presented, the latter model being capable of simulating bubbles of entrapped gas.Treatment of the density around the free surface is found highly critical for obtaining an accurate fluid distribution and velocity field. A newly-developed gravity-consistent density averaging method is applied to prevent spurious velocities around the free surface. The convective terms are approximated by a compressible, symmetry-preserving second-order upwind discretization. Time integration, using second-order Adams–Bashforth, is carried out with a generalization of the familiar pressure-correction method, in which the full acoustical part of the flow equations is treated implicitly.Numerical results are validated against experimental data for two test cases. As an example of internal wave loading, liquid sloshing dynamics are validated with experimental results for a 1:10 scale LNG tank section. In particular, the experimental pressure signal during a moment of air entrapment is compared with one-phase and two-phase flow simulations. The simulation of external wave loading is validated with data from an experiment with wave run-up against a 1:50 scale semi-submersible offshore structure. The test cases show that modeling of two-phase effects can be beneficial for simulating hydrodynamic wave loading.