Problem Description

Solution

COMSOL Multiphysics 3.4

This is described in the COMSOL Multiphysics User's Guide and the COMSOL Multiphysics Modeling Guide. Please start reading from Postprocessing 2D Axisymmetric Models in 3D on page 455 in the COMSOL Multiphysics User's Guide.

 COMSOL Multiphysics 3.3

What you need to do is map your 2D axisymmetric solution onto a 3D geometry. After drawing the 3D geometry, you will use extrusion coupling variables to bring in your solution.

In the example fluid flow model, Axisymmetric_plot_swept_to_3D.mph, the solution computed on the 2D geometry is mapped to the 3D geometry and postprocessed. The 3D geometry is created by revolving the 2D geometry. If you want to study the velocity field and the pressure, there are three variables that need to be mapped: u (the radial velocity), v (the vertical velocity), and p (the pressure). To completely define a mapping, you need to define three coupling variables, one for each of the variables.

The velocity field in a 2D axisymmetric fluid flow model.

When creating an extrusion coupling variable, always start from the geometry you are mapping from. In this example, we want a subdomain extrusion coupling variable with the fluid flow subdomain in our axisymmetric 2D geometry as the source. The corresponding subdomain in the 3D geometry will be our destination.

Between the source and the destination, you need to define a coordinate transformation. Using the general transformation option, you are asked to supply an intermediate coordinate system (x,y) to which both the source and the destination coordinate systems are mapped. With the source transformation x=r, y=z and the destination transformation x=sqrt(x^2+z^2), y=y, you will end up with the axisymmetric coordinates (r,z) corresponding to the cylindrical 3D coordinates (sqrt(x^2+z^2),y).

The velocity field mapped to 3D by the use of extrusion coupling variables.

Step by step instructions on how to set up extrusion coupling variables are available in Axisymmetric_plot_swept_to_3D.pdf. Following the instructions creates the model Axisymmetric_plot_swept_to_3D.mph, which can also be downloaded from the bottom of this knowledge base entry. Opening the model file requires the Chemical Engineering Module, but using extrusion coupling variables only requires COMSOL Multiphysics and the instructions in Axisymmetric_plot_swept_to_3D.pdf are general.

The mapping technique with extrusion coupling variables can be used for more than plotting a 2D axisymmetric solution in 3D. If some of the physics of a non-symmetric coupled problem can be expressed axisymmetrically, the axisymmetric phenomena can be solved in 2D and the non-symmetric ones in 3D. The solutions can be computed simultaneously. Modeling part of the problem in 2D results in huge savings in memory. This is discussed in the last few pages in Axisymmetric_plot_swept_to_3D.pdf.

Related Files

	Axisymmetric_plot_swept_to_3D.mph	1.0 MB
	Axisymmetric_plot_swept_to_3D.pdf	1.3 MB

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