Deformed Geometry vs. Moving Mesh
You can set up mesh deformation in two different ways, depending on the intended behavior of the model: either the deformation represents a fundamental change in the shape of the geometry, or it represents the shape change induced by deformation of solid materials. COMSOL Multiphysics refers to the former as Deformed Geometry and the latter as Moving Mesh functionality. Technically, the difference is that Deformed Geometry functionality deforms the material frame mesh relative to the geometry frame mesh, while Moving Mesh functionality deforms the spatial frame mesh relative to the material frame mesh. Both types of deformation can be used at the same time, one on top of the other.
The features and interfaces that let you specify the deformation of the material and spatial frame meshes, respectively, are the same in both cases, but are found in different places in the Model Builder tree. You can control the mesh deformations either using features under Definitions>Moving Mesh and Definitions>Deformed Geometry in a Component, or by adding a Deformed Geometry () or Moving Mesh () interface from the Mathematics>Deformed Mesh branch () in the Model Wizard or Add Physics window.
The features available in the Definitions branch are slightly less general than some available in the corresponding interfaces. But the ones under Definitions have the advantage that physics interfaces can be aware of and interact with them, something which is important, e.g., for fluid structure interaction (FSI) and rotating machinery or mixer applications.
When using Deformed Geometry functionality, the material does not follow the change in shape. Deformation of the geometry boundaries therefore corresponds to addition or removal of material. When using Moving Mesh features, solid materials follow the mesh deformation and deform in the same way as the mesh. Fluids and gases, however, are added or removed so as to always fill the current shape of the domain — any effects of compression or expansion must be introduced explicitly into the equations.
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Use Deformed Geometry features to study the behavior of different shapes of an original object. In a model with Deformed Geometry, the material never follows a perturbation of the shape. The total mass of the first shape is not the same as the mass for the second, perturbed geometry. Any deformation can be regarded as removal or addition of material.
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Use Moving Mesh features to study how a solid object deforms as the results of physical load, and how fluids in adjacent domains react to displacement of the domain boundaries — for example, how a tank impeller moves a fluid, or how a MEMS switch moves under the influence of an electric field. In a model with Moving Mesh, a solid material follows the mesh deformation. A movement of a boundary can therefore be regarded as bending or punching the original object. Undeformed and deformed solid objects have the same mass, but the total amount of fluid in a domain whose boundaries deform can change.