Wave Impact, Body Motion and Overset Grids in STAR-CCM+ Milovan Peri ć

Introduction Features of STAR-CCM+ enabling studies of wave impact and motion of floating bodies Overset grids in STAR-CCM+ Fluid-structure interaction Examples of application Future developments

Enabling Technology Features of STAR-CCM+ enabling studies of wave impact and flow-induced floating body motion: – High-resolution interface-capturing scheme – Wave generation models – Wave damping – Second-order time advancing – Dynamic fluid-body interaction (DFBI) with 6 degrees of freedom – Mesh motion and adaptation techniques: • Mesh morphing • Sliding grids • Overset grids

High-Resolution Interface-Capturing, I HRIC-scheme was developed in late 1990es and resolves a sharp interface by one cell… FLUENT also adopted it (but it is not identical to STAR- CCM+ implementation – less sharp interface)… Control parameters: – Courant number limits: • Below lower limit, pure HRIC is used to transport volume fraction; • Above upper limit, pure 1 st -order upwind scheme is used; • Between the limit the two schemes are blended. – Sharpening factor: • Based on an anti-diffusion model developed by H. Weller • Used to avoid irreversible mixing due to occasional use of upwind scheme (splashing, wave breaking etc.)

High-Resolution Interface-Capturing, II Simulation of sloshing in LNG-tank subject to roll motion: Volume fraction distribution after 101 periods Simulation of wave propagation

Wave Models STAR-CCM+ provides several wave models: – For initialization of volume fraction, velocity and pressure fields; – For a transient inlet boundary condition. Currently available models: – 1 st -order linear wave theory – Non-linear 5 th -order Stokes wave theory (Fenton, 1985) – Pierson-Moskowitz and JONSWAP long-crested wave spectra – Superposition of linear waves with varying amplitude, period and direction of propagation (can be set-up via Excel-file)

Wave Damping Vertical motion is damped by introducing smoothly increasing resistance… The method proposed by Choi and Yoon ( Costal Engineering , Vol. 56, pp. 1043-1060, 2009) has been implemented into STAR-CCM+: w w x sd – Starting point for wave damping (propagation in x -direction) x ed – End point for wave damping (boundary) f 1 , f 2 and n d – Parameters of the damping model w – Vertical velocity component

Time-Accurate Wave Propagation, I Accurate wave propagation requires 2 nd -order time- integration method. Second-order method (quadratic interpolation in time) requires that the wave propagates less than half a cell per time step. First- order scheme is always stable but less accurate… Test case: – Stokes 5 th -order wave – Wavelength 102.7 m – Wave height 5.8 m – Wave period 8 s – Solution domain 4 wavelengths long…

Time-Accurate Wave Propagation, II Wave damping was applied over the last 100 m before outlet... 41 cells per wave length, 11.5 cells per wave height ( Δ x = 2.5 m, Δ z = 0.5 m) 1st-order scheme , 100 Δ t/T (Co = 0.41), after 4 periods 5 cells 2nd-order scheme , 100 Δ t/T (Co = 0.41), after 4 periods 10 cells

Time-Accurate Wave Propagation, III Wave train initialized using Stokes 5 th -order theory Solution domain 1002 m long (8 wavelengths) Damping applied over last 300 m Wave period 8.977 s, wave height 5 m 20 cells per wave height, 80 cells per wavelength Second-order time integration scheme (quadratic backward) Initial wave profile: Damping applied to initial field...

Time-Accurate Wave Propagation, IV Wave profile after 100 s of simulation time (> 11 periods). Note: 1 cell resolution, almost no reduction in amplitude… Essential for accurate prediction of wave propagation: - 2 nd -order discretization in time - Order of 20 cells per wave height and 80 cells per wavelength

Dynamic Fluid-Body Interaction, I DFBI model computes motion of a rigid body (up to six degrees of freedom). Second-order discretization in space and time is used (compatible to discretization in flow domain). Body motion is affected by: – Flow-induced forces and moments (from shear and pressure forces); – Gravity; – Specified external forces and moments; – Couplings (springs, catenaries,…).

Dynamic Fluid-Body Interaction, II Coupled simulation of flow and flow-induced motion of floating bodies: Implicit coupling by updates within outer iteration loop… Applies also to deformable structures (needs compu- tation of body deformation).

Grid Adaptation in Fluid Grid can be adapted to a moving body by several methods: – Morphing, – Embedded rigid body motion (a combination of rotation with a sliding interface and translation), – Overset grids. An example of grid adaptation using morphing for a prescribed deformation of a body.

Overset Grids, I Multiple regions (background, overset)… Each region is associated with one grid continuum (any grid type). Overset grid interface needs to be set for regions, overset boundary on overset grids… Cells are automatically grouped into active and passive . Active cells along interface to passive cells refer to donor cells at another grid instead of the passive neighbours on the same grid... The first layer of passive cells next to active cells are called acceptor cells...

Overset Grids, II Variable values at acceptor cells are expressed via variable values at donor cells using an interpolation function. Background N 1 , N 2 , N 3 – grid Neighbors from the same grid; N 4 , N 5 , N 6 – Neighbors from the overlapping grid. Overset grid

Overset Grids, III In the overlapping zone, cells should be of comparable size (recommendation). Between two walls belonging to different grids, there should be at least 4 cells to enable coupling (requirement). The overset grid should not move more than one cell per time step (recommendation). Store separation Pitching foil in a channel

Fluid-Structure Interaction Coupled computation of flow and flow-induced deformation of structure: implicit coupling is essential… Having both solvers in a single code (even if different methods, like FV and FE) is a big advantage – communication via memory… A finite-element solver for structures (3D, beams, plates, shells, membranes) is being developed in STAR- CCM+… Implicit coupling to ABAQUS is available since V 7.04 – a great increase in robustness compared to explicit coupling (exchange once per time step). Explicit coupling with other FE codes is also possible. Solution mapping available for non- conformal grids…

Examples of Application Stokes wave slamming against a jack-up platform in North Sea (GL) Shallow water wave slamming against an offshore structure (DNV) Simulation of hurricane damage to a platform in the Gulf of Mexico (Chevron; FSI, coupling to ABAQUS) Simulation of ship bow and stern slamming (whipping, springing; GL) Lifeboat launching into waves (DNV, FEDEM, UMOE, NORSAFE, CFD- Marin…) Other applications of overset grids

Wave Impact on Jack-Up Platform CD-adapco in collaboration with GL

Wave Impact on Offshore Structure Simulations by DNV (published at OMAE2012 Conference)

Hurricane Damage on Oil Platform Evidence of damage on a platform after it was hit by a hurricane Deformation in a simulation: good agreement with field observation… Coupled simulation of flow using STAR-CCM+ and deformation of platform structure using ABAQUS. Simulation by CD-adapco Engineering Services for Chevron. Published at OMAE2012 Conference.

Ship Slamming in Waves Container ship in waves Comparison of predicted and measured mean pressure over limited area at two bow locations (analysis by GL).

Effects of Ship Deformation, I Analysis of whipping phenomena at GL: Green water on deck after one slamming event (upper), and Comparison of measured and computed accele- rations in bow region for a rigid and an elastic ship structure (lower). Analysis by GL

Effects of Ship Deformation, II Analysis by GL

Lifeboat Launching Into Waves, I Simulation by H.J. Morch, CFD Marine; Experiment by Norsafe AS

Lifeboat Launching Into Waves, II Initial wave position varied by 20 m (drop from 32 m height). Following wave (180 ° ) Wavelength ca. 220 m, wave height 13.5 m, water depth 33.5 m The questions to be answered: When is the load on the structure the highest? When are accelerations the highest?

Lifeboat Launching Into Waves, III Pressure at one monitoring point for different hit points, 180 °

Parametric Study With Overset Grids, I Flow around a body at different angles of attack A horizontal section through both grids (only active cells are shown). Total number of cells: ca. 1 million Vertical section through the two grids (only active cells are shown).

Parametric Study With Overset Grids, II -30° -15° 0° 15° Velocity distribution in a section 30° parallel to bottom wall for different angles of attack. Steady-state solutions.

Parametric Study With Overset Grids, III Residual history from the computation of flow around a body in a wind tunnel at different angles of attack: time step 1000 s, rotation 15° per time step, standard k- ε turbulence model, under-relaxation 0.9/0.1/0.9 for velocities/pressure/turbulence, wind speed 40 m/s

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