Modelling

Which units of physical quantities are used in simFlow?

simFlow uses OpenFOAM® solvers and therefore you must use SI units. It is important to remember that it also applies to dimensions of the geometry, which must be scaled to meters.

I am simulating incompressible flow using incompressible flow solver. Why do I have to specify pressure values in a strange unit instead of Pa?

Incompressible flow solvers in OpenFOAM® operate using a kinematic pressure instead of a physical pressure in order to compute Navier-Stokes equation without explicitly showing the constant fluid density in the equation.

Kinematic pressure is defined as pressure divided by fluid density and it is expressed in m2/s2. Always input kinematic pressure values when using incompressible flow solvers in simFlow to obtain correct results. When you analyze the results, remember that the pressure field in Paraview displays values of kinematic pressure. Therefore, you must multiply the values of kinematic pressure by the fluid density in order to calculate the pressure field expressed in Pa.

Forces acting on the geometry calculated during the simulation are way off what I expected. Did I do something wrong?

There are two main possibilities for incorrect values of forces:

  1. Your model was created in other units than meters.
  2. In case of incompressible solvers you need to multiply displayed values of monitored forces by fluid density.

What are calculation methods of: forces and force coefficients that can be monitored?

These forces are calculated by pressure integration over the selected boundaries in the Forces tab, in the Monitors panel.

If you select the checkbox Monitor Coefficients, the values of forces are divided by the reference area and reference dynamic pressure. Results of this operation correspond to force coefficients, and they are shown in the graph instead of values of the forces.

The ‘Mesh’ button is disabled and I cannot remesh the geometry. How to solve this problem?

The Mesh button is disabled if you have result files in your case directory. In order to remesh the geometry, you have to go to the Run panel and click the Reset Calculations button.

I want to simulate a conjugate heat transfer problem but solvers appropriate for this purpose are disabled in the list of available solvers. Why?

Conjugate heat transfer solvers require creating a mesh region for a solid, and another mesh region for a fluid region. Therefore, they can be chosen only if you created a mesh subregion.

Mesh created in simFlow is stored in the default region. Once you create a mesh, go to the MESH panel, right-click on the default region and choose the Make sub-region option. Next, you can move the Point in a proper location and create a mesh for the other region. Then, it will be possible to select one of the conjugate heat transfer solvers.

I can’t calculate ‘Wall Heat Flux’, because the button in the ‘Postprocessing’ panel is disabled.

Wall heat flux currently can only be calculated for compressible solvers. If you are using Boussinesq approximation, this option will not be available.

I constantly get a ‘nan’ error in the simFlow console. Why?

If you can see a line in the simFlow console similar to:

smoothSolver: Solving for Ux, Initial residual = nan, Final residual = nan, No Iterations 1000 or other containing the ‘nan’ word,

it means that your simulation diverged. It has nothing to do with simFlow, but is caused by the solver you are using and your settings. What you need to do is to click the Reset Calculation button in the Run panel, verify your boundary conditions and numerical settings and rerun the simulation.

Which files should I include when sending a sample case?

If you have a problem with your simulation, please delete all the folders containing the result files from the case directory, which are named according to time-step or iteration number. Then, compress the case directory and upload it to a cloud or hosting provider, for example, using Dropbox or Google Drive, and provide us with a link to the archive. Please do not send large files to us as attachments to your emails, as we may not receive them due to technical limitations. You can use the contact form on simFlow home page.