Ship Hull Response In cylBumpInterIbFoam Tutorial

Mohsen Irannezhad

Marine Technology/Naval Architecture,

Chalmers University of Technology,

Gothenburg, Sweden

2016-12-08

1. Introduction

2. Theory

• Immersed Boundary Method

• IBM boundary conditions

• IBM in FOAM-extend

3. Test Case

• Idea formation

• Workflow

• cylBumpInterIbFoam tutorial modification

4. Results and Discussion

• wave patterns

• Future work

Introduction

Fluid Flow Around An Immersed Body

• Body Fitted Computational Grid

Advantages

Easy boundary condition application

Good for simple geometries

Disadvantages

Complicated geometries difficulties

Moving geometry handling problem

Deforming geometry handling problem

• General Grid Interface, Over-set Mesh,

Mesh Morphing, Immersed Boundary Method

Immersed Boundary Method• History

• Mesh Generation

• Governing Equations

Advantages over body fitted method

Re-evaluating cell types

• Boundary Condition

Problem with fluid equation

Different methods to solve the problem

• Dead-to-Live Cells

Problem when body is deforming or/and moving

Solved by assuming that the cell has the condition of closest live cell for the first time step after change

http://www.tfd.chalmers.se/~hani/kurser/OS_CFD_2015/Hrvo

jeJasak/ImmersedBoundary.pdf

Boundary Condition in IBM

• Continuous Forcing IBM

Direct applied force/source term in governing equations prior to discretization

e.g. momentum equation

Simple and efficient for deforming bodies

Difficult for rigid bodies e.g. ship hull and numerically unstable and inaccurate

• Discrete Forcing IBM

Equations modifications after discretization

Scheme dependency results in more control over accuracy and stability of simulation

Two different methods: Indirect Forcing and Direct Forcing

1. Indirect Forcing DFIBM

continuous source term – immersed body boundary is not sharply represented

2. Direct Forcing DFIBM

not continuous source term – immersed body boundary is sharply represented

Comparison of Methods

CFIBM

Pros

Easy to implement

Good for deforming bodies

Good for moving bodies

Good for defined mechanical force

Good in handling dead-to-live cells

Cons

Accuracy and stability issues

Stiff equation for rigid body

Bad at high Reynolds numbers

Indirect Forcing DFIBM

Pros

Good for rigid bodies

Good in handling dead-to-live cells

More control over accuracy and stability

Cons

More difficult to implement

Bad at high Reynolds numbers

Direct Forcing DFIBM

Pros

Good for rigid bodies

More control over accuracy and stability

good at high Reynolds numbers

Cons

More difficult to implement

bad in handling dead-to-live cells

Immersed Boundary in OpenFOAM

• Indirect Forcing DFIBM in FOAM-extend

Idea is solving equations using boundary values and neighboring live cell values

Moving the boundary from actual geometry to IB cell centers surrounding it

Usual process of solving governing equations

Approximation process is the same for all equations except for pressure

• IB Cell Value Approximation In FOAM-extend

Assumption that all quantities follow a quadratic behavior near the boundary

Using least square weighting function

Both Dirichlet and Neumann boundary conditions

• IBM class implementation in FOAM-extend

Implementation through three classes

• Immersed boundary wall functions in FOAM-extend

Introducing a sample point

Dirichlet & Neumann Boundary Conditions

• Dirichlet BC

Such as no slip condition on the walls

Coefficients are calculated through least square curve fitting

to neighboring stencil cells

• Neumann BC

Such as specified heat flux walls

Introduction of local coordinate system

Coefficients are calculated in same way as Dirichlet BC

Pictures http://www.tfd.chalmers.se/~hani/kurser/OS_CFD_2015/HrvojeJasak/ImmersedBoundary.pdf

IBM Class Implementation in FOAM-extend

• The immersedBoundaryPolyPatch class

Takes care of basic mesh support functions for IBM meshes

• The immersedBoundaryFvPatch class

Supports basic and derive Fv properties of IBM

Recognizes live and dead cells

Calculates the background mesh and surface mesh intersection points, normal and distances

Calculates interpolation matrices used in imposition of boundary condition

• The immersedBoundaryFvPatchField class

Using the interpolation matrices to evaluate boundary condition

Evaluate patch fields for IB patches

Takes care of any boundary updates due to movement or deformation

Immersed Boundary Wall Functions in FOAM-

extend

• Implementation complexity in comparison with body fitted grids

• Sampling point is introduces at distance 1.5 times the grid resolution normal to body

• Calculation of flow properties at that point using neighboring live cells

The mesh resolution should be finer than body fitted approach

One possible solutions is to refine the mesh around the immersed body

Mesh refinement could be performed using refineImmersedBoundaryMesh

utility

Test Case• Idea is simulating the flow around a ship hull with IBM support, using similar case

Immersed boundary method available in FOAM-extend-4.0

Ship hull geometry, DTC-scaled hull available in OpenFOAM

Two fluids and a structure interaction

• Similar case is cylBumpInterIbFoam tutorial in immersedBoundary tutorials

Two fluids and a rigid body (ibCylinder) interaction

VOF approach for free surface simulation

Idea Formation

• Use DTC-Scaled hull geometry and use it as a ”bump”

• Place and fixed the hull in a 3D tank with a water level depth

• Introduce different boundary conditions for the tank

• Introduce some simple conditions at inlet

Constant velocity of water condition

Wave propagating condition

• Study the response of the

ship to the introduces conditions

WorkFlow1. Copy cylBumpInterIbFoam tutorial to the run directory

2. Replace the ibCylinder with DTC-hull in triSurface directory

3. Define and create a new volume mesh using blockMeshDict dictionary in polyMesh directory

4. Include immersedBoundaryPolyPatch into boundary in polyMesh directory

5. Refine the volume mesh around the immersed boundary using refineImmersedBoundaryMesh utility and make a new boundary file (optional)

6. Modify properties in constant directory

7. Modify solution control in fvSolution in system directory and continue with…

• 8. Constant Velocity case

Modify setFieldsDict and controlDict dictionaries

Modify boundary fields of alpha1, U and pd

• 8. Wave propagating case

Download, install and utilize swak4Foam library

Make funkySetFieldsDict dictionary and modify the controlDict

Modify boundary fields of alpha1, U and pd with groovyBC

9. Run the case and post process with paraview

Case Setup

• Geometry

DTC-hull available in OpenFOAM as ”.stl ” format (water tight)

Use surfaceConvert utility to make ”.ftr ”

f40NR

mkdir -p $FOAM_RUN

run

cp -r $FOAM_TUTORIALS/immersedBoundary/cylBumpInterIbFoam .

cd cylBumpInterIbFoam/constant/triSurface

rm ibCylinder.ftr ibCylinder.stl

• In OpenFOAM terminal window

OF4x

cp $FOAM_TUTORIALS/resources/geometry/DTC-scaled.stl.gz $FOAM_RUN

run

gunzip DTC-scaled.stl.gz

surfaceConvert DTC-scaled.stl DTC-scaled.ftr

mv DTC-scaled.stl hull.stl

mv DTC-scaled.ftr hull.ftr

Replace the ibCylinder with the hull in consant/triSurface directory

Case Setup

• Volume Mesh Modification

blockMeshDict modified to have two blocks

f40NR

run

cd cylBumpInterIbFoam

gedit constant/polyMesh/blockMeshDict

One block containing geometry and inlet with finer mesh and another block

containing the outlet to the first one

Modify the boundary file by including the geometry in that

blockMesh

gedit constant/polyMesh/boundary

Immersed Boundary Mesh Refinement

• refineImmersedBoundaryMesh Utility implementation (Optional)

Three different level of mesh refinement (ibCells, ibCellCells, ibCellCellFaces)

Refine the mesh by running refineImmersedBoundaryMesh utility with

one of the available levels flag

A 0 directory is produced containing a polyMesh directory

Copy it to the constant/polyMesh directory and remove 0 directory

Case Setup

• Modify properties in constant directory

Gravity g should be modified

• fvSolution and fvSchemes remain untouched interIbFoam solver

Solver for 2 incompressible, isothermal immiscible fluids

using VOF approach

immersed boundary support

Case Setup

Constant Velocity case

• Modify setFieldsDict to introduce water depth of 0.2 m

• Adjust the time step in controlDict

• Modify alpha1, U and pd by including immersed boundary (hull)

• Modify boundary condition according to the case

• Run the case by running interIbFoam

• Post process the results in paraview

Case Setup Wave Propagating case• Download, install and utilize swak4Foam library

Use funkySetFields and groovyBC for wave generation (2nd-order stokes)

Introduce wave as expressions

groovyWaveTank designed to run with interFoam

For alpha1 at inlet e.g. wave with length of 5 m and amplitude of 0.3 m

inlet {

type groovyBC;

valueExpression "(pos().z

Results

Initial condition

Constant Velocity Case

Time step 90

Results

Constant Velocity Case

Time step 120 Time step 120

Results

Wave Propagating Case

wave trough reaches the front part wave peak reaches the front part

Future Work

• Change the simulation to turbulence

• Introduce moving of the hull, possibly by pitchingPlate tutorial and

interDyMIbFoam solver

• Utilizing the forceSectional utility available in swak4Foam library to calculate

the forces on the body

• Introduce other boundary conditions e.g. wave from side and etc.

Thank You