Understanding flow surfaces

When you solve the model, the solver:

• Establishes heat paths (conductances) from the surfaces and obstructions to adjacent 3D fluid elements or cells.

• Splits the 3D flow mesh for embedded flow surfaces creating a 3D obstruction in the fluid flow.

Flow surface
(A) Flow surface(B) Surface obstructions (optional)(C) Convection from top and bottom (optional)(D) Convection from the obstructions(E) Surface drag from the obstructions or roughness of the wall friction

Depending on the geometry of your model, matching faces can exist at the interface between the fluid body and the solid body. When you apply a Boundary Flow Surface simulation object at the interface, you must apply the flow surface at face that has the solid mesh.

Understanding the top and bottom surfaces of a flow surface

You can apply different convection and wall friction options for each side of the embedded flow surface. The two sides are designated as the top and bottom surfaces.

If a boundary flow surface contacts the fluid only on one side, that side is designated the top surface.

The top and bottom surfaces are determined by the normal direction of the associated elements. The top surface faces the direction of the element normal. The bottom surface faces the opposite way.

Note:

To display the element normals, use the 2D Element Normals command in the FEM file. This command lets you reverse the element normals if needed.

Flow surfaces and body-fitted fluid mesh

To ensure accurate flow surface modeling when using the body-fitted fluid meshing, avoid misalignment between the flow surface and the fluid elements it contacts. Most importantly, there must be:

• Planar alignment between the flow surface 2D elements and the adjacent faces of the internal 3D fluid elements.

• Linear alignment between the periphery of the flow surface 2D elements and the edges of internal 3D fluid element faces.

If the 2D elements on a flow surface do not align with internal faces of the 3D fluid elements, then the 3D fluid mesh may not adapt to create internal convecting faces. The resulting flow surface may be smaller than expected or may be entirely ignored by the flow solver.

There is no need for the nodes of the 2D elements selected in a Flow Surface simulation object and the 3D fluid mesh to be coincident. However, you should always check for and merge coincident nodes before solving a model that contains a Flow Surface simulation object.

When you specify a Flow Surface simulation object on a curved surface, you should use a smaller 2D mesh size to ensure accurate adapting of the 3D fluid mesh.

Example:

In the following example, the flow surface 2D elements are defined on a curved polygon face which is an internal boundary of the 3D body-fitted fluid mesh. The nodes of the 2D elements are coincident with the nodes of the 3D mesh (A). Although shared nodes are not required, the shared nodes ensure solver recognition of the curved flow surface even if the mesh is coarse. During the analysis, the 3D mesh opens at the flow surface, blocking flow (B).

Flow surfaces and immersed boundary mesh

When you use immersed boundary method, the flow surface 2D elements are completely inside in the immersed boundary mesh for both boundary and embedded flow surface types. There is no need to match or align flow surface 2D elements with immersed boundary cells. The thermal and flow solvers automatically exchange flow surface convection information between them while solving the thermal-flow analysis.

Flow surface mesh (green) and immersed boundary mesh

How do I

Create a flow surface

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Modeling flow surface obstructions

Meshing for flow surfaces

Modeling semi-permeable surfaces

Convection Properties

Inputs to expressions

Fluid meshing

Immersed boundary meshing

Two-way fluid-structure interaction

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Auto-generated expressions

Geometry creation for body-fitted fluid meshing

Geometry preparation for immersed boundary method

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Understanding flow surfaces, Simcenter 3D 2021.1 Series

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