# 技术支持知识库

## Physics Solutions

1172: Solving time dependent CFD simulations

A common mistake when running transient fluid flow studies with COMSOL Multiphysics is to start the simulation with conflicting boundary conditions. Let's take a simple example to illustrate this issue. We will compute...
Version: All versions - Platform: All Platforms

1165: Updating older models to use new Crosswind Diffusion formulation

In COMSOL 4.3b (released May 2013) the Crosswind diffusion formulation was updated for all flow interfaces supporting Crosswind diffusion. This means that models solved with the new formulation, i.e in COMSOL 4.3b and...
Version: 4.3b,4.4,5.0,5.1 - Platform: All Platforms

1104: Stabilization schemes in COMSOL

How can I see which stabilization schemes are used by COMSOL Multphysics in transport physics interfaces such as Single-Phase Flow and Transport of Diluted species? Numerical solutions of transport equations can...
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1038: Using symmetries in COMSOL Multiphysics

How do I take advantage of the symmetries in my COMSOL model? Using Symmetries to Reduce Model Size By using symmetries in a model you can reduce its size by one-half or more, making this an efficient tool for...
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973: My flux calculation seems wrong

I am solving a diffusion-like problem in COMSOL Multiphysics and discovered that boundaries that are defined with insulation/symmetry conditions have an integrated value of the normal component of the flux that is far...
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952: Avoiding negative concentrations

Why do I get negative values of the concentration in my diffusion, convection and reaction model? Why do I see values that go below, or above, their lowest, or highest, possible values based upon the boundary...
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913: Computing Time and Space Integrals

How do I compute integrals over space or time? What if I want to use the result of these integrals elsewhere in my model? This is a general description of how to use Integral operators. A more thorough walkthrough on...
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905: Smoothed step transitions in COMSOL Multiphysics

When you provide a function, coefficient or material property as a step function to COMSOL Multiphysics, you may experience convergence problems. This is because COMSOL Multiphysics expects the solution to be...
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878: Should I use the RMS, the peak, or the instantaneous value to...

Should I use the RMS, the peak, or the instantaneous value to specify sources for electromagnetic fields? How can I specify the phase angle? The COMSOL Multiphysics formulation uses complex fields when solving...
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875: What does degrees of freedom (DOFs) mean in COMSOL Multiphysics?

What does degrees of freedom (DOFs) mean in COMSOL Multiphysics? The solution time and memory requirements are strongly related to the number of degrees of freedom in the model. It is often desirable to be able to...
Version: All versions - Platform: All Platforms