Deformable DFBI Bodies

In Simcenter STAR-CCM+, the DFBI framework allows you to simulate the motion of a rigid body in response to external forces. In FSI simulations, you can use DFBI in conjuction with the solid stress solver to model deformable or partially deformable DFBI bodies. The DFBI module calculates the overall rigid body motion, while the solid stress solver computes the local deformations.

For more information on DFBI, see Dynamic Fluid Body Interaction. In the context of FSI, the external forces acting on the DFBI body are exerted by the fluid continuum.
The combination of FSI and DFBI allows you to model the following scenarios:
Fully deformable DFBI bodies
Consider the motion of a marine vessel. The vessel moves rigidly in response to the fluid forces and rigid body contact accelerations exerted by the surrounding fluid. If you model the vessel as a fully deformable DFBI body, the vessel is also allowed to deform under the fluid forces and rigid body contact accelerations.
Partially deformable DFBI bodies
Consider a hydrofoil attached to a marine vessel. The system hydrofoil-vessel moves rigidly in response to the surrounding fluid forces and rigid body contact accelerations. The hydrofoil can also deform under the fluid forces and rigid body contact accelerations, while the rest of the vessel is assumed to be rigid.
Deformable components with prescribed motion
Consider a rotating propeller attached to a marine vessel. The system propeller-vessel moves rigidly in response to the surrounding fluid forces and rigid body contact accelerations. Additionally, the propeller has prescribed rotation motion and can deform under the fluid forces and rigid body contact accelerations. Only the propeller is deformable—the rest of the vessel is assumed to be rigid.

The general workflow for FSI simulations still applies (see Modeling Fluid-Structure Interaction (FSI)). The following steps present the additional settings and motion models that are required to combine deformable solids and DFBI bodies in FSI simulations.

  1. Define the fluid and solid regions and create the required contact based interfaces. For the fluid-structure interface, set Type to Mapped Contact Interface. See Defining the Regions Layout.
    You typically require one solid region for the deformable solid and one or more fluid regions. For example, for the two-way coupled simulation of a propeller on a marine vessel, you can create a solid region for the deformable propeller, a fluid region for the fluid around the vessel, and an additional fluid region for the fluid around the propeller. This setup allows you to use different DFBI motion models for the two fluid regions. For example, you can use the DFBI Rotation and Translation motion for the fluid around the vessel and the DFBI Morphing motion only for the fluid around the propeller.
  2. Define physics continua for the fluid and the solid regions and activate appropriate physics models. For both the fluid and the solid continua, select the Implicit Unsteady model.
    For guidelines, see Flow and Energy and General Workflow for Stress Analysis.
  3. In the solid continuum, activate the Flexible DFBI Motion model.
  4. Specify whether the fluid continuum and the solid continuum are one-way fluid to structure or two-way coupled, see Modeling Fluid Structure Interaction (FSI) for more information. For solid continua associated with fully or partially deformable DFBI models, only two-way coupling is available.
    For one-way fluid to structure coupling, the effect of the solid deformations on the surrounding fluid are neglected. For two-way coupling, the effect on the fluid is also considered. See Modeling Fluid-Structure Interaction (FSI).
  5. Select the Regions > [Solid Region] > Physics Conditions > Flexible DFBI Motion Option node and set Flexible DFBI Motion Option to one of the following:
    • DFBI - Deformable body—for the simulation of fully deformable DFBI bodies.
    • DFBI - Partially deformable body—for the simulation of DFBI bodies with deformable components.
    • DFBI - Deformable moving attachment—for the simulation of DFBI bodies with deformable components that have prescribed rotation or translation motion.
  6. Define the fluid and solid materials and the fluid boundary conditions.
  7. For the solid, define the following constraints, depending on the selected Flexible DFBI Motion Option:
    Flexible DFBI Motion Option Loads and Constraints
    DFBI - Deformable body None.
    DFBI - Partially deformable body For the solid surface that is attached to the 6-DOF body, define a surface segment that constrains the surface in all directions (you can use the Fixed constraint method). See Applying Constraints.

    The constrained surface remains fixed, while the remaining free surfaces are allowed to deform.

    DFBI - Deformable moving attachment
To define the 6-DOF body:
  1. Right-click the Tools > Motions node and select New > DFBI Morphing.
  2. For the fluid region surrounding the solid, select the [Fluid Region] > Physics Values > Motion Specification node and set Motion to DFBI Morphing.
    Although the DFBI Morphing motion is not required in one-way fluid to structure coupled analyses, you can only define 6-DOF bodies when a DFBI motion is assigned to a region. You can change the fluid region motion at a later stage.
  3. Right-click the DFBI > 6-DOF Bodies node and select New Body > 3D > Continuum Body.
  4. For the 6-DOF body, [Body 1], set Body Surface to the fluid boundaries that represent the surface of the 6-DOF body (for example, the surface of the vessel, vessel-hydrofoil, or vessel-propeller system), including the FSI interface with the solid boundary. Define the motion of the body using the relevant nodes and properties.
    For instructions, see the relevant sections under DFBI Workflow.
Depending on the selected Flexible DFBI Motion Option and FSI Coupling Method, you require different combinations of motion models for the fluid and solid regions:
  • The solid region must follow the 6-DOF rigid motion. For deformable moving components, the solid region also has prescribed rotation or translation motion (for example, a boat propeller has prescribed rotation). If the fluid and solid continua are two-way coupled, the solid region also requires the Solid Displacement motion, which updates the solid mesh based on the displacements calculated by the solid stress solver.
  • The fluid region must follow the 6-DOF rigid motion. Additionally, its boundaries can deform based on the solid displacements at at the fluid-structure interface.

For more information, see Motion Models Configuration.

Start by creating the 6-DOF rigid motion from the 6-DOF body:

  1. Right-click the 6-DOF Bodies > [Body 1] node and select Create Body Motion.
    Simcenter STAR-CCM+ adds a motion node, [Body 1]-Motion, under Tools > Motions. This motion represents the calculated 6-DOF body motion and can be assigned to regions like any other motion model.
  2. Based on the selected Flexible DFBI Motion Option and FSI Coupling, complete the motion setup as follows:
    Flexible DFBI Motion Option FSI Coupling Steps
    either DFBI - Deformable body or DFBI - Partially deformable body Two-Way (one-way coupling is not available)
    1. Right-click the [Body 1]-Motion > Superposing Motions node and add the Solid Displacement motion.
    2. Select the Regions > [Solid Region] > Physics Values > Motion Specification node and set Motion to [Body 1]-Motion > Solid Displacement.
    DFBI - Deformable moving attachment One-Way Fluid to Structure
    1. Right-click the Tools > Motions > [Body 1]-Motion > Superposing Motions node and add a superposing rotation or translation motion that defines the movement of the attachment.
    2. Define the properties of the rotation or translation motion.
    3. Select the Regions > [Solid Region] > Physics Values > Motion Specification node and set Motion to [Body 1]-Motion > [Rigid Motion].
    4. Select the Regions > [Fluid Region] > Physics Values > Motion Specification node and replace DFBI Morphing with [Body 1]-Motion > [Rigid Motion].
    Two-Way
    1. Right-click the Tools > Motions > [Body 1]-Motion > Superposing Motions node and add a superposing rotation or translation motion that defines the movement of the attachment. Define the properties of the rotation or translation motion.
    2. Right-click the [Body 1]-Motion > Superposing Motions > [Rigid Motion] > Superposing Motions node and add the Solid Displacement motion.
    3. Select the Regions > [Solid Region] > Physics Values > Motion Specification node and set Motion to [Body 1]-Motion > [Rigid Motion] > Solid Displacement.
    As Fully Deformable DFBI Bodies are based on the equilibrium of forces between the solid stress body and the 6-DOF model, when specifying the FSI coupling you must account for the force consistency as stated in the FSI Solution Strategy. Ensure that both the FSI Coupling Specification matches the fluid force setting on the DFBI body and that the FSI Fluid Load Ramping settings match the ramping specifications defined in the DFBI body. As well as these requirements you must also ensure that the FSI traction is not clipped.
    For information on how to set up the FSI coupling see Fluid Structure Interaction General Workflow.
    For information on how to create superposing motions and the required coordinate systems, see Adding a Superposed Motion to a 6-DOF Body.
  3. For two-way coupled continua, define the morphing settings at the fluid boundaries:
    1. Expand the [Fluid Region] > Boundaries node.
    2. For all deformable fluid boundaries including their adjacent interface boundaries, select the Physics Conditions > Six DOF Morpher Specification node and set Method to Six Dof Body.
      Simcenter STAR-CCM+ automatically sets the Deformation Specification condition to Solid Stress. With this setting, the morpher takes into account the deformation of the contacting solid when morphing the fluid boundaries.


    3. For the case of DFBI - Deformable moving attachment (deformable components with prescribed motion) only, select the Physics Values > Morpher Rigid Boundary Motion node and set Rigid Motion to [Body 1]-Motion > [Rigid Motion].
  4. Complete the simulation setup as required.
    For fully deformable DFBI bodies, Simcenter STAR-CCM+ provides the option to automatically populate the inertial properties of the 6-DOF body according to the values that are determined on the solid stress side. See Import Inertial Properties.
    For information on general FSI setups, see Modeling Fluid-Structure Interaction (FSI).