Simulation of Offshore Wave Impacts with a Volume of Fluid Method Tim Bunnik Tim Bunnik MARIN
Outline Part I: Numerical method -Overview Part II: Applications - Dambreak - Wave run-up - Sloshing loads in LNG tanks - Sloshing effect on ship motions Conclusions 2
From space to earth Professor Arthur Veldman Sloshing onboard spacecraft 1977 start of development at NLR 1995 step to 3D => ComFlo 2005 SloshSat experiments Hydrodynamic d wave loading 1997 cooperation with MARIN 2001 SAFEFLOW JIP => ComFLOW 2004 ComFLOW-2 JIP 2009 ComFLOW-3 JIP 3
PART I Numerical method 4
Modeling issues Focus on prediction of wave-induced pressures and forces short duration momentum driven viscous effects (turbulence) not dominant makes life easy breaking waves air entrapment and aeration moving structures makes life challenging 5
Mathematical model Navier-Stokes equations: compressible flow Fluids: formerly one phase + void currently two phase fluid+compressible air Free surface: geometric reconstruction and displacement contact angle no -slip Compressible Navier-Stokes continuity of stress Incompressible Navier-Stokes 6
Volume of Fluid Nichols and Hirt 1981 Discontinuous color function cell labels Cartesian grid cut cells Sharp interface method within one grid cell Fluid is transferred between cells 7
Free surface: adapted VOF (1) Cell labeling: Empty, Surface, Fluid, Boundary Displacement using VOF method + interface reconstruction with VOF-function Problem: - gain and loss of water - flotsam and jetsam at free surface Solution: combine VOF with a local height function: 8
Further modeling Snorre TLP Incoming waves at inflow (several wave theories) Absorbing Boundary Condition at open boundaries, or Numerical beach 9
PART II Applications 10
Dambreak experiment (green water flow) 11
Dambreak experiment Simulation Experiment MARIN 12
Dambreak: validation Grid refinement 59 x 19 x 17 118 x 38 x 34 236 x 76 x 68 (1 day on PC) Pressure at front side of box Water height in front of box 13
Wave run-up: up: experiment Results for test 202003 Wave height 15.0m Wave period 11.0s Grid of 182x83x55 cell Comparison of wave heights and pressures 14
Wave run-up: up: pressure First column, close to bottom First column, center 15
Sloshing Test setup: Scale 1:10 2D slice DNV oscillator Regular and irregular motions Filling rates 10%, 25%, 70%, 90% Measurement of: Motions Pressures Water heights High speed camera and digital camera recordings 16
Sloshing. Air-liquid interaction 17
Sloshing 10% fill water height 09 pressure 01 18
Sloshing 25% fill irregular water height 01 pressure 09 19
Sloshing air entrapment 20
Effect sloshing on ship motions 4 35 3.5 closed tank open tank 3 roll RAO [rad/m] 2.5 2 1.5 1 Molin, OMAE 2008. This dataset was used for validation of coupled ComFLOW-linear diffraction model 05 0.5 0 2 3 4 5 6 7 8 frequency [rad/s] 21
Effect sloshing on ship motions Linear diffraction analysis for the barge 0.5 0.4 0.3 z axis 0.2 0.1 0 0.4 0.3 0.2 0.1 0 0.1 0.2 0.3 0.4 y axis The sloshing tank is modeled with ComFLOW (several grids) 2D 1-phase simulations (1 cell in longitudinal direction) Simulations on model scale 400 seconds irregular beam waves to determine roll RAO Hs=0.06 06 m Tp=1.6 16s 22
Effect sloshing on ship motions Seastate irr1 Hs=0.06 m Tp=1.2 s Fine grid 23
Effect sloshing on ship motions roll RAO [ra ad/m] Flat roof 16 cm above still water level. Seastate irr1. 4 anysim-comflow very coarse 3.5 anysim-comflow coarse anysim-comflow fine 3 Molin [2008] 2.5 2 1.5 1 0.5 Sea state irr1 Hs=0.06 m Tp=1.2 s Open tank, airgap 16 cm Good agreement Small grid effects 0 3 4 5 6 7 8 frequency [rad/s] 24
Effect sloshing on ship motions Seastate irr3 Hs=0.12 m Tp=1.2 s Fine grid 25
Effect sloshing on ship motions roll RAO [ra ad/m] Flat roof 16 cm above still water level. Seastate irr3. 4 anysim-comflow very coarse 3.5 anysim-comflow coarse anysim-comflow fine 3 Molin [2008] 2.5 2 1.5 1 0.5 Sea state irr3 Hs=0.12 m Tp=1.2 s Open tank, airgap 16 cm Good agreement Small grid effects 0 3 4 5 6 7 8 frequency [rad/s] 26
Summary Numerics Various delicate issues: free surface conditions, surface displacement, mass conservation, Physics Global physics (wave heights, wave loading) is captured well Overall conclusion Basic CFD technology for violent free-surface motion is available, but refinements are still needed 27
Thank you for your attention. Questions? 28