# 5 COMSOL Tricks and Hacks for Beginners

## 1.Out in Log file

In the auto solve mode of COMSOL, the solver automatically selects the step size it needs to arrive at a solution. However, it only stores the results you have specified. “-out” in the log file under the “stepsize” column suggests that the solution has been stored for the specified user-defined time step.

## 2. Resolving Line and Point contact

In the example shown above, I was trying to simulate uniaxial unconfined compression on a scaffold geometry. As you can see, it is a 2D problem involving the ‘Solid Mechanics module’ of COMSOL. After applying the load and executing the simulation, I received the following error.

“Failed to find a solution for all parameters, even when using the minimum parameter step. No convergence, even when using the minimum damping factor. There was an error message from the linear solver. The relative error (3.8e-005) is greater than the relative tolerance. Returned solution is not converged.”

The error occurred because of the stress singularity created due to point-contacts between the fibres in adjacent layers. The solver failed to resolve the unbound stress at the point contacts and this prevented the solution to converge.

In order to resolve the issue with singularity, the geometries were redefined to make them overlap slightly as suggested by the COMSOL user guide. I increased the diameter of the circular scaffolds by 1% without changing the overall height of the construct, allowing for overlap. This indeed helped the solver resolve the problem. Check this COMSOL forum to understand the issue in depth.

## 3. GMRES Iterative Method

General Minimal Residual Method (GMRES) is an iterative method used to solve a non-symmetric system of linear equations. GMRES algorithm is particularly useful to save the computational time and memory expense.

If you are a newbie to COMSOL, you should know that there are two categories of algorithms used to solve systems of linear equations in COMSOL, namely Direct and Iterative methods.

### 3.1. Direct Method

There are direct solvers in COMSOL.

• MUMPS
• PARDISO
• SPOOLES

For a well-conditioned Finite Element problem, these 3 solvers differ in their computation time to arrive at a solution but don’t differ much in terms of their accuracy. While PARDISO is the fastest, SPOOLES happens to be the slowest algorithm among the 3 direct solvers in COMSOL.

MUMPS comes with a computational memory advantage because it supports cluster computing.

## 4. Saving Storage Space

Quite often you may get the error of “Disk error while writing to temporary solution file”. There are some other types of errors associated with the limitations in disk space.

This type of error occurred due to the limitation of storage space in my C:/ drive. Since COMSOL stores the solutions in the C:/temp folder by default during simulations, I suggest you change the file location of the temp file to another drive (say F:/) where you have more than 20 GB free space. You can do so by going to File->Preferences ->Files.

#### SurajPanigrahi

I started this blog for my love of writing and disseminating my views on things that excited me. I don't have a clear direction with this blog. However, you can find content mostly related to higher education, career development, MedTech, finance, and budget travel hacks. Background: After graduating in Mechanical Engineering from IIT (BHU) Varanasi, India, I pursued a dual degree Master's program in Europe (MS in Biomedical Engineering at RWTH Aachen, Germany and MS in Bioengineering at Trinity College Dublin, Ireland). I am currently working as a 'Manufacturing Engineer' in a MedTech company in Ireland.
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