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CFD TUTORIAL – RIGID BODY MODELING

If you want to get started with the rigid body modeling you should be aware of that the modeling setup, mesh displacement and rigid body motions are depended on the complexity of your system. Maybe a mesh displacement can be restricted to one domain, or a subdomain with sliding mesh is sufficient to model the rigid body? We will help you with these questions and will further recommend that you make a sketch of your system to decide which parts are needed to be moved, this will make the modeling of domains and interfaces in DesignModeler easier.

This tutorial will briefly show three different ways of how to use the Rigid Body 6DOF solver in CFX v.13. We hope this will be a good start if you want to begin using the final release of the CFX Rigid Body solver.

A rigid body is a solid object that moves through a fluid without itself deforming. Its motion is dictated by the fluid forces and torques acting upon it, plus any external forces as gravity and external torque. A rigid body is defined by a collection of 2D regions that form its faces. The rigid body itself does not need to be meshed. Mesh motion is used to move the mesh on the rigid body faces in accordance with the solution of the rigid body equations of motion.

3 examples of the Rigid Body solver will be presented in the following tutorial. The tutorial is not described in detail and will only work as a guidance of how to set up a simulation that includes a Rigid Body.

Recommended prerequisites

- Introduction to CFD with CFX

RIGID BODY EXAMPLE 1

The first example shows a free drop of a thin plate or hatch. The hatch is fixed at one axis and will have a rotational motion. We have modeled a simple plate and generated a surrounding fluid domain. The hatch is enclosed in a cylindrical domain to allow rotational motion of model. See illustration of the domains in Figure 1.The 2D wall boundaries which constitute the hatch surface define the rigid body.

See close view in Figure 2.

Figure 1. The computational domain consists of a box and a cylinder. The cylindrical domain encloses the Rigid Body, i.e. the hatch which is fixed at the x-axis. The walls, which constitute the hatch is visualized in green. A rotation of the modeled hatch will consequently rotate the cylinder.

Figure 2. The walls will define the Rigid Body. The hatch itself is not a domain.

The first thing we will do is to insert a Rigid Body dialog box

. To model the Rigid Body you need to specify the mass and the absolute values of the mass momentum of inertia for the rigid body with respect to a corresponding coordinate system - see Figure 3. Furthermore, you will need to define external force, torque or gravity if it is present. This can be set under the Dynamic setting tab. Mark of for the correct translational and rotational degree of freedom. A typical setup for a thin plate is shown in Figure 3 and 4. You can read about the Rigid Body User interface and the definition of these settings in the ANSYS v.13 Help Guide.

Figure 3. You need to set the total mass of the Rigid Body, and the location, in this case the plate walls named Body. In addition you need to calculate the Mass Moment Of Inertia of the Rigid Body object.

Figure 4. Define external forces. Gravity force is always present in drop cases. For this case the thin plate will only have one degree of freedom, which is a rotational degree about the x-axis.

When you have created your Rigid Body you can go on and define the Fluid Domain. In Basic settings shown in Figure 5 you will need to set Mesh Deformation to Region of Motion Specified. This is needed only in domains where the motion of the Rigid Body will have an influence on the mesh boundaries.

Figure 5. In the Basic Settings tab of the rotating domain, Region of Motion Specified needs to be activated.

For this case we have divided the computational domain into two parts. The inner cylindrical domain will undergo Mesh Deformation when the Rigid Body moves, while the outer rectangular domain will remain steady, the cylindrical domain will therefore be defined as a subdomain, see Figure 6 and Figure 7. The remaining now is to define how the 2D boundaries shall act.

Figure 6. Define the cylindrical domain as a Subdomain. The interface between the rotating and stationary domain will be handled by the sliding mesh feature.

Figure 7. The Subdomain will follow the Rigid Body motion.

The modeling of Mesh Deformation is an important component for solving problems with moving boundaries or moving subdomains. The motion might be imposed, or might be an implicit part of a coupled fluid-structure simulation.

For our case with a falling plate we want the mesh interface of the subdomain to slide on the interface to the rectangular and steady domain. To prevent the nodes on the subdomain interface to move relative to the local boundary frame we need to set the Mesh Motion of the Interface to Stationary. See Figure 8.

Figure 8. The interface nodes will move relative to the local boundary frame.

The Mesh Motion of the 2D wall boundaries which form the hatch surface, i.e. the Rigid Body, will follow the Rigid Body Solution. The same setting will be applied to the Subdomain as well, refer to Figure 7.

No special action is needed to set up the stationary domain, i.e. the rectangular domain. This domain can be modeled as usual. No consideration to Mesh Motion or the Rigid Body solver is necessary.

Set a proper time step for the transient analysis and you can start solving a Rigid Body motion. A movie of the falling hatch is shown below.

RIGID BODY EXAMPLE 2

In the previous example we presented a very simple case to describe the Rigid Body solver; one degree of freedom and Mesh Displacement with stationary nodes. The problem was solved by sliding mesh and GGI at the interfaces. In the next example we will model a buoy at the water surface by use of the Rigid Body solver. The modeling concept is shown in Figure 9.

Figure 9. Waves that are introduced in the domain will further influence the displacement of the buoy.

As for the hatch you’ll need to insert a Rigid Body and define its dynamics. See example in Figure 10. The buoy will have tree degrees of freedom, translation in x- and y direction and rotational about z-axis – see the Dynamics definition in Figure 11.

Figure 10. The Mass and Moment Of Inertia of the buoy needs to be defined.

Figure 11. Definition of Degrees of Freedom of the Buoy.

For this case we have two distinct mesh domains, however, we will introduce only one fluid domain. In the Domain Basic setting tab the multiphase of water and air is created and Mesh Deformation is activated. The free surface multiphase is modeled as usual.

The buoy will be moved in several directions which mean that the mesh nodes will be displaced. To allow mesh deformation you need to define the Mesh Motion on each boundary. The modeled domain is shown in Figure12.

Figure 12. Mesh domains.

The buoy surface is set to follow the Rigid Body motion, which means that the nodes will not be locally displaced. The Mesh Motion on the Fluid Fluid Interfaces between the two mesh domains are set to Conservative Interface Flux - by using this condition there is no constraint that attempts to keep the meshes on either side of the interface together. The motion of nodes in all domains adjacent to the interface influence, and are influenced by, the motion of the nodes on the interface. The definitions are shown in Figure 13 and Figure 14.

Figure 13. The 6DOF solver will predict the buoy behavior. The nodes will not be locally displaced.

Figure 14. The nodes on the domain interface will be displaced, this will in term skew the elements.

The Mesh Motion on the wall, symmetry and pressure outlets can be set to Unspecified – no constraints on mesh motion are applied to nodes. Their motion is determined by the motion set on other regions of the mesh. *

A transient simulation is prescribed and the motion of the Rigid Body and the resulting Mesh Displacement is visualized in in the following movie.

*A wave is introduced in the simulation – for this case the wave is induced by a specified displacement of walls. A description of how to model this displacement can be found in CFX v130 Tutorial 32.

RIGID BODY EXAMPLE 3

A third example is to put constrain on the Rigid Body and Mesh motion. As for the first example you may also here create a subdomain, see Figure 15. The subdomain will in this case follow the Rigid Body motion.

Figure 15. The buoy domain is defined as a subdomain in this Rigid Body example.

Furthermore, we will have to change the setting of the Fluid Fluid Interface to oppose sliding mesh between the interfaces. The Mesh Motion of the Fluid Interfaces are set as Rigid Body Motion, thus the mesh nodes of the subdomain will not move relative to the rectangular domain. The motion constrains of the interfaces are defined in Figure 16 and 17.

Figure 16. The motion constrains of the interface in the rectangular domain.

Figure 17. Mesh motion and Motion Constraints of the Interface of the Subdomain.

The resulting buoy motion with sliding mesh is shown in the movie below.