Fluid physics work together with fluid materials. This is where you configure and define how the fluid materials will behave in your simulation. When you click on Fluid Physics, you will find the following settings described below in the properties panel.
Viscous Model
Click on Fluid Physics to view the Viscous Model dropdown in the properties panel.
You can choose from:
RANS (Reynolds-averaged Navier-Stokes)
DES (Detached Eddy simulation)
LES (Large Eddy simulation)
Inviscid flow, governed by the Euler equations
Laminar flow, governed by the Navier-Stokes equations
Turbulence
Click on Fluid Physics to view the Turbulence settings. This is where you can choose and configure Spalart-Allmaras or SST turbulence models. See our Turbulence Models guide for more information.
Note: If you select Laminar flow from the Viscous Model dropdown, Turbulence settings will not show.
Advanced Solver Settings
This is where you can adjust Spatial Discretization and Solution Controls settings.
Spatial Discretization
Define the schemes for handling spatial gradients and convective terms. This setting controls the accuracy of your simulation, noting that options for higher accuracy may lead to solution stability problems. There are three preset selections of settings that you can choose from:
Default provides a balance between robustness and accuracy.
Conservative is the most robust option for your toughest meshes and problems. If necessary, it will sacrifice solution accuracy for robustness.
High Accuracy attempts to be as accurate as possible and avoids most artificial dissipation, which may lead to stability issues.
You can also select Custom and fine-tune the settings. You can choose between first order and second order convective schemes, turn on or off Robust Dissipation, and more. Hover over each property to view the definition.
Solution Controls
This section controls how quickly your simulation will converge, noting that the quicker the desired convergence, the more unstable the process can be.
There are three preset selections of settings that you can choose from:
Default (Conservative) uses settings that emphasize robustness.
Intermediate Performance provides a balance between speed and robustness, up to 2x faster than the default settings.
Aggressive Performance uses settings biased towards speed, up to 3x faster than the default settings.
As you move up from the default preset to the aggressive preset, the robustness of the solution method will decrease, but the potential time to a converged solution will also decrease.
Custom Settings
Click Customize to finely control Spatial Discretization and Solution Controls settings.
Note: We suggest trying the presets described above before using Custom settings.
Transient Incompressible Fluid Simulations
If you are working with an incompressible fluid, you can set up transient simulations using a constant density.
Find the General section in the simulation control panel and change Time to Transient.
Go to your fluid material in the Materials section and change Density Relationship to Constant Density.
Click on your Fluid Physics and find the Spatial Discretization section in the control panel. We recommend choosing either Default or High Accuracy.
In the same section, find Solution Controls and ensure that Apply Suggestion is set to Default. We recommend leaving this setting as-is.
Next, set up your outputs with the appropriate convergence criteria. We recommend creating an output that targets an absolute density residual on the same order of magnitude as a converged steady state simulation.
If one is not available for similar flow conditions, target a relative density residual below 0.005. Additionally, momentum residuals should be below 0.05.
To set up the recommended residuals:
Find the Outputs > Solution Residuals section and uncheck every box except Mass.
Next to Stopping Condition, click on Stopping Condition 1.
Set Tolerance to 0.005.
Click the + icon next to Outputs and select Global. This will create an output called Solution Residuals 2.
Click on Solution Residuals 2 and check only X-, Y-, and Z-Momentum.
Next to Stopping Condition, click Add.
Set Tolerance for the new stopping condition to 0.05.
Scroll down to find the Max Inner Iterations setting. Some time steps can be more difficult to converge, so we recommend allowing up to 40 inner iterations for the best results.