## Physics

### Solving time dependent models with inconsistent initial values

Solution Number: 1172 Versions: All versionsA common mistake when setting up transient models is to have initial conditions that are inconsistent with the loads and boundary conditions. This occurs most often when running transient fluid flow studies, but the ...

### Using Symmetries in COMSOL Multiphysics

Solution Number: 1038 Versions: All versionsHow 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 ...

### Stabilization Schemes in COMSOL

Solution Number: 1104 Applies to: CFD Module, Chemical Reaction Engineering Module Versions: All versionsHow can I see which stabilization schemes are used by COMSOL Multiphysics in transport physics interfaces such as Single-Phase Flow and Transport of Diluted species? Numerical solutions of transport equations can ...

### My Flux Calculation Seems Wrong

Solution Number: 973 Versions: All versionsI 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 ...

### Avoiding Negative Concentrations

Solution Number: 952 Applies to: CFD Module, Chemical Reaction Engineering Module, COMSOL Multiphysics, Corrosion Module, Electrochemistry Module, Electrodeposition Module, Microfluidics Module, Mixer Module, Subsurface Flow Module Versions: All versionsWhy 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 ...

### Computing Time and Space Integrals

Solution Number: 913 Versions: All versionsHow do I compute integrals over space or time? What if I want to use the result of these integrals elsewhere in my model? The COMSOL Multiphysics architecture allows you to evaluate integrals in both space and time in ...

### How does COMSOL Multiphysics handle complex-valued numbers and problems in the frequency domain?

Solution Number: 878 Applies to: AC/DC Module, Acoustics Module, COMSOL Multiphysics, MEMS Module, Plasma Module, RF Module, Semiconductor Module, Structural Mechanics Module, Wave Optics Module Versions: All versionsHow does COMSOL Multiphysics handle complex-valued numbers and problems in the frequency domain? Should I use the RMS, the peak, or the instantaneous value to specify sources in models being solved in the frequency ...

### What Does Degrees of Freedom (DOFs) Mean in COMSOL Multiphysics?

Solution Number: 875 Versions: All versionsWhat 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 ...

### Modeling of Step Transitions

Solution Number: 905 Versions: All versionsYou may want to define loads, material properties, or boundary conditions that vary abruptly with respect to the solution, or with respect to time. This can, however, lead to a solution which is discontinuous in space ...

### How do I solve a PDE with space derivatives of order higher than two?

Solution Number: 816 Versions: All versionsHow do I solve a PDE with space derivatives of order higher than two? For example, the equation for a function . Introduce names for the second derivatives of u, say . Then the equation can be written . ...