Moving Surface Parts

When thin volumes are set into motion, the mesh requirements in the part itself and in the motion path in the flow can be quite severe. The moving volume must have a mesh that is fine enough to resolve the gradients through its thickness, and the flow path must have a correspondingly fine mesh.

To provide a more convenient method of analyzing the motion of thin objects, the Motion Module supports moving surface parts. This reduces the meshing requirements on both the moving part (because it is a surface part) and the motion path in the surrounding fluid:

On the Materials task dialog, create a surface part by assigning a solid material to the intended surface. The properties of the material and the shell thickness are used to compute the mass of the part, and influence the movement for flow-driven motion. For user-prescribed motion, the physical properties do not influence the motion. On the Motion dialog, change the selection mode to Surface, and select the surface or surfaces that are to move.

Guidelines

The mesh should, however, be fine enough to resolve the pressure gradients on the surface. Likewise, the mesh within the fluid surrounding the moving shell should be fine enough to allow flow to pass around the surface as it moves.

In the following image, the 3D mesh surrounding the moving shell is quite coarse. As the valve opens due to the force of the fluid, very little fluid can pass around it until it has opened about half way.

In reality, fluid would leak past such a valve at the onset of motion, and is shown in the model with a finer mesh:

Keep in mind that the amount of “bookkeeping” necessary to track the motion of a moving surface is similar to that of a moving solid. Because of this, moving surface motion analyses are as resource intensive as moving solid analyses, for a given mesh density. The advantage of moving surfaces is that the mesh in the motion path does not have to be as dense as for a moving solid analysis.