ls-dyna® r7 : strong fluid structure interaction (fsi...

Remarks about the ICFD solverFSI Coupling

Why are meshing tools important?Applications

Thoughts about linearization

LS-DYNA R© R7 : Strong Fluid StructureInteraction (FSI) capabilities and associated

meshing tools for the incompressible CFD solver(ICFD), applications and examples

Facundo DEL PIN / Inaki CALDICHOURY / Rodrigo [email protected] / [email protected] / [email protected]

Livermore Software Technology Corporation

3 June 2013

LSDYNA ICFD capabilities 9th European LS-DYNA Users’ Conference




Outline

1 Remarks about the ICFD solver

2 FSI CouplingTypes of coupling

3 Why are meshing tools important?Example

4 Applications

5 Thoughts about linearization





Remarks about the ICFD solver

Implicit solver

Incompressible flows

Suitable for the analysis of turbulent flows

Free surface

Embedded meshing tools

Coupled to structural solver

Coupled to thermal solver for conjugate heat





Types of coupling

Outline




4 Applications






Types of coupling

Types of coupling

Strong coupling

Default coupling for the implicitstructural solver.

Weak coupling

Default coupling for the explicitstructural solver.





Types of coupling

Strong Coupling

It is an algorithm in which the fluid andsolid solver iterate within each time stepuntil the residuals of all variables at theinterface is less than a prescribed error.

Multiple instances of the fluidsolver or solid solver may be calledwithin each time step.

δtsolid = δtfluid .

Accurate and robust but expensive.





Types of coupling

Problems specific for strong coupling

In general problems that have significant added mass effects willperform much better when solved using strong coupling. In otherwords when the solid has to do significant work to move the fluid.As a rule of thumb:

ρs

ρf≈ 1

Typical applications:

Hemodynamics, where blood and tissue have almost the samedensity.

Thin flexible membranes.

Steady state analysis.





Types of coupling

Weak Coupling

It is an algorithm in which the fluid andsolid solver are called once per timestep. Thus no check for convergencewithin a time step is performed.

Only one instance instances of thefluid solver or solid solver is calledwithin each time step.

Time steps for fluid and solid don’tneed to match.

Faster solution, possible lessaccurate and less robust.





Types of coupling

Problems specific for weak coupling

In general problems where the solid can easily move the flowaround it. As a rule of thumb:

ρs

ρf� 1

Typical applications:

Aeroelasticity analysis.

Solids with high stiffness.

Heavy solids with minor non-linearity.





Types of coupling

One way coupling

It is a coupling specifically used for rigidbodies with imposed movement. Theforces computed in the fluid will notchange the state of the solid. It is not adefault behavior so it needs to becontrolled by the keyword*ICFD_CONTROL_FSI.

The fluid problem runs as if thereis a velocity boundary condition onthe FSI interface.

The cost is the same as for weakcoupling.





Example

Why are meshing tools important?

All the Fluid-Solid interfaces are Lagrangian. So the fluid meshfollows the deformation of the structural mesh. This allows for exactimpositions of boundary conditions at the FSI interface but it canresult in highly distorted meshes. The fluid solver can re-mesh thefluid domain if needed to improve mesh quality.





Example

Default re-meshing behavior

By default the solver will only perform re-meshing when an in-verted element is detected. This is the most extreme situationsince the solver cannot continue with inverted elements. This be-havior works for most problems but in some cases it may result invery distorted elements prior to the inversion which deteriorates thesolution.





Example

Checking for quality and mesh size

Using the keyword *ICFD_CONTROL_ADAPT_SIZE the solver will checkif all elements satisfy a minimum quality constraint. If not thesolver will re-mesh. The solver will also check if the required meshsize is respected and if it is not it will re-mesh. Using this keywordit will force more frequent re-meshings.





Example

Checking for error

Using the keyword *ICFD_CONTROL_ADAPT the solver will use an errorestimator to evaluate the solution and then compare it to an errorvalue that the user specifies. The local mesh size will be adjustedso that the error es kept constant through out the mesh andclose the the error specified by the user. It can be combined with*ICFD_CONTROL_ADAPT_SIZE to also take quality in consideration.





Example

Making the mesh slide on surfaces


Using the keyword*ICFD_BOUNDARY_PRESCRIBED_MOVEMESH

we can indicate the meshing algorithmthat the mesh on the walls is allowedto move in some direction and isconstrained in the others.




Example

Outline




4 Applications






Example

Rigid body dropping in water


As an example to show the differentmesh options we will use a freesurface problem. A rigid body withlower density than water will drop.




Example

Setting up the FSI interface


The fluid mesh and solid meshdo not need to be conformingat the interface. The nodes donot need to match. The solidand fluid geometry need to beclose enough for the automatictracking of the interface.




Example

Default behavior


The solver will re-mesh only ifthere are inverted elements.




Example

Quality and size control


Using*ICFD_CONTROL_ADAPT_SIZE

The solver will re-mesh whenelement quality is deteriorated.




Example

One way FSI


Using *ICFD_CONTROL_FSI withoption 2 does not transfer thefluid force to the structure.




Example

Input deck overview

Only 9 keywords used for the fluid problem:

ICFD_CONTROL_TIME

ICFD_CONTROL_ADAPT

ICFD_BOUNDARY_NONSLIP

ICFD_BOUNDARY_FSI

ICFD_PART

ICFD_PART_VOL

ICFD_MAT

MESH_VOLUME

MESH_INTERF





Example

Input deck overview

Only 9 keywords used for the fluid problem:

ICFD_CONTROL_TIME

ICFD_CONTROL_ADAPT

ICFD BOUNDARY NONSLIPICFD BOUNDARY FSIICFD_PART

ICFD_PART_VOL

ICFD_MAT

MESH_VOLUME

MESH_INTERF





Heart Valve: Strong Coupling

Courtesy of Hossein Mohammadi, Mcgill UniversityLSDYNA ICFD capabilities 9th European LS-DYNA Users’ Conference

The Valve leaflets are modeled with shellelements.

Contact uses the Mortar option.




Wind turbine: Weak coupling


ρsolidρfluid

≈ 4000





We have presented two types of linearizations:

Weak Coupling

Strong Coupling

A different approach that is widely used in engineering is toassume that the problem is actually linear.





Assuming that the problem is linear

The problem becomes much simpler to solve.

In many situations for engineering purposes the results areacceptably accurate.

Since now the problem is divided into two problems, one forthe fluid part and one for the solid part then it becomes easierto handle, namely the fluid group is responsible for the fluidproblem and the solid group is responsible for the solidproblem.






How it works

1 The solid group finds the need for FSI simulation.

2 The geometry is sent to the fluid group which builds a meshand runs the fluid problem to steady state.

3 The fluid stresses together with the mesh is brought back tothe solid group which handles the fluid data with scripts toconvert it into the input data for the solid solver.

4 The solid solver performs a modal analysis.






Some facts about this approach

Usually communication between fluid and solid groups takestime.

Different priorities between the groups.

Sometimes the analysis is done on the basis of favor and goodwill and not as part of a project. Then communication takeseven longer.

At least two different licenses are used to solve what shouldbe a single problem.


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