Facundo DEL PIN / Iñaki ÇALDICHOURY / Rodrigo PAZ / / Livermore Software Technology Corporation

Transcription

1 LS-DYNA R R7 : Strong Fluid Structure Interaction (FSI) capabilities and associated meshing tools for the incompressible CFD solver (ICFD), applications and examples Facundo DEL PIN / Iñaki ÇALDICHOURY / Rodrigo PAZ fdelpin@lstc.com / inaki@lstc.com / rpaz@lstc.com Livermore Software Technology Corporation 3 June 2013

2 Outline Remarks about the ICFD solver 1 Remarks about the ICFD solver 2 Types of coupling 3 4 5

3 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

4 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

5 Outline Remarks about the ICFD solver Types of coupling 1 Remarks about the ICFD solver 2 Types of coupling 3 4 5

6 Types of coupling Remarks about the ICFD solver Types of coupling Strong coupling Default coupling for the implicit structural solver. Weak coupling Default coupling for the explicit structural solver.

7 Strong Coupling Remarks about the ICFD solver Types of coupling It is an algorithm in which the fluid and solid solver iterate within each time step until the residuals of all variables at the interface is less than a prescribed error. Multiple instances of the fluid solver or solid solver may be called within each time step. δt solid = δt fluid. Accurate and robust but expensive.

8 Types of coupling Problems specific for strong coupling In general problems that have significant added mass effects will perform much better when solved using strong coupling. In other words when the solid has to do significant work to move the fluid. As a rule of thumb: ρ s ρ f 1 Typical applications: Hemodynamics, where blood and tissue have almost the same density. Thin flexible membranes. Steady state analysis.

9 Weak Coupling Remarks about the ICFD solver Types of coupling It is an algorithm in which the fluid and solid solver are called once per time step. Thus no check for convergence within a time step is performed. Only one instance instances of the fluid solver or solid solver is called within each time step. Time steps for fluid and solid don t need to match. Faster solution, possible less accurate and less robust.

10 Types of coupling Problems specific for weak coupling In general problems where the solid can easily move the flow around it. As a rule of thumb: Typical applications: Aeroelasticity analysis. Solids with high stiffness. ρ s ρ f 1 Heavy solids with minor non-linearity.

11 One way coupling Remarks about the ICFD solver Types of coupling It is a coupling specifically used for rigid bodies with imposed movement. The forces computed in the fluid will not change the state of the solid. It is not a default behavior so it needs to be controlled by the keyword *ICFD_CONTROL_FSI. The fluid problem runs as if there is a velocity boundary condition on the FSI interface. The cost is the same as for weak coupling.

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

13 Default re-meshing behavior By default the solver will only perform re-meshing when an inverted element is detected. This is the most extreme situation since the solver cannot continue with inverted elements. This behavior works for most problems but in some cases it may result in very distorted elements prior to the inversion which deteriorates the solution.

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

15 Checking for error Remarks about the ICFD solver Using the keyword *ICFD_CONTROL_ADAPT the solver will use an error estimator to evaluate the solution and then compare it to an error value that the user specifies. The local mesh size will be adjusted so that the error es kept constant through out the mesh and close the the error specified by the user. It can be combined with *ICFD_CONTROL_ADAPT_SIZE to also take quality in consideration.

16 Making the mesh slide on surfaces Using the keyword *ICFD_BOUNDARY_PRESCRIBED_MOVEMESH we can indicate the meshing algorithm that the mesh on the walls is allowed to move in some direction and is constrained in the others.

17 Outline Remarks about the ICFD solver 1 Remarks about the ICFD solver 2 Types of coupling 3 4 5

18 Rigid body dropping in water As an example to show the different mesh options we will use a free surface problem. A rigid body with lower density than water will drop.

19 Remarks about the ICFD solver Setting up the FSI interface The fluid mesh and solid mesh do not need to be conforming at the interface. The nodes do not need to match. The solid and fluid geometry need to be close enough for the automatic tracking of the interface.

20 Default behavior Remarks about the ICFD solver The solver will re-mesh only if there are inverted elements.

21 Quality and size control Using *ICFD_CONTROL_ADAPT_SIZE The solver will re-mesh when element quality is deteriorated.

22 One way FSI Remarks about the ICFD solver Using *ICFD_CONTROL_FSI with option 2 does not transfer the fluid force to the structure.

23 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

24 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

25 Heart Valve: Strong Coupling The Valve leaflets are modeled with shell elements. Contact uses the Mortar option. Courtesy of Hossein Mohammadi, Mcgill University

26 Wind turbine: Weak coupling ρ solid ρ fluid 4000

27 We have presented two types of linearizations: Weak Coupling Strong Coupling A different approach that is widely used in engineering is to assume that the problem is actually linear.

28 Assuming that the problem is linear The problem becomes much simpler to solve. In many situations for engineering purposes the results are acceptably accurate. Since now the problem is divided into two problems, one for the fluid part and one for the solid part then it becomes easier to handle, namely the fluid group is responsible for the fluid problem and the solid group is responsible for the solid problem.

29 Assuming that the problem is linear 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 mesh and runs the fluid problem to steady state. 3 The fluid stresses together with the mesh is brought back to the solid group which handles the fluid data with scripts to convert it into the input data for the solid solver. 4 The solid solver performs a modal analysis.

30 Assuming that the problem is linear Some facts about this approach Usually communication between fluid and solid groups takes time. Different priorities between the groups. Sometimes the analysis is done on the basis of favor and good will and not as part of a project. Then communication takes even longer. At least two different licenses are used to solve what should be a single problem.