Catalytic Converter - CFD Simulation SimFlow Tutorial

In this tutorial, we will be demonstrating how to model a porous medium. We have simulated the flow through the catalytic converter using the Darcy-Forchheimer porosity model. The inlet velocity has been described by a formula that depends on the coordinates. Second-order velocity discretization (Linear Upwind) has been used to increase the accuracy of the simulation.

SimFlow is a general purpose CFD Software

To follow this tutorial, you will need SimFlow free version, you may download it via the following link:
Download SimFlow

Open SimFlow and create a new case named catalytic_converter

Firstly we need to Download GeometryCatalytic-converter

Since the imported geometry is in STL format, which does not store unit information, we need to confirm the unit in which the model was created. In the selection of unit, we can use the Geometry size label, which displays overall size of the model in each direction. In our case, the model was created in millimeters.

After importing geometry, it will appear in the 3D window.

The imported geometry is made up of a single surface. We need to split it into multiple faces for further processing.

In order to split original geometry into separate objects, we will disable Create Single Geometry options:

To make the resulting mesh boundaries more readable we will rename each geometry face:

In this tutorial, we would like to model a porous zone inside the catalytic converter. To do so we need to define the region where the porous resistance will be applied. For this purpose we will create new box geometry:

In order to create the mesh, we need to specify geometries options used during the meshing process.

Now we will define the base mesh. The box geometry determines the background mesh domain. The model is symmetrical on the XZ plane, so we do not need to mesh the whole geometry. Using the box dimensions we will choose only half of the domain.

Material Point tells the meshing algorithm on which side of the geometry the mesh is to be retained. Since we are considering flow inside the catalytic converter we need to place the material point inside the geometry.

You can specify the point location from the 3D view. Hold the CTRL key and drag the arrows to the destination.

Everything is now set up for meshing

After the meshing process is finished the mesh will appear in the graphics window.

Now we will define the cell zone using box_1 geometry.

The cell zone appears inside the catalytic converter. It is colored on light blue.

We want to analyze internal, incompressible turbulent flow. For this purpose, we will use the SIMPLE (simpleFoam) solver.

For turbulence modeling, we will use the standard \(k-\varepsilon\) model.

We will change the kinematic viscosity to the value of the exhaust gas.

In order to include porosity to our simulation we have to define porosity for the box_1 cell zone.

SimFlow allows users to create named parameters that can be used instead of numbers. This allows to easily change a single property without the need to edit all inputs where a given property is used. In this tutorial, we will define two global parameters that we will use during further setup.

We will set inlet velocity as a parabolic profile. The profile we will define by the formula using U and R parameters. Velocity magnitude near the wall will be equal to zero and the highest value of U will be in the center of the pipe.

To visualize and check velocity distribution we will open ParaView. Firstly we will save the case.

Run the ParaView from the Postprocessing panel.

We will display a velocity magnitude map on the inlet.

Back in SimFlow, we will also update the turbulent mixing length for the ε-equation at the inlet.

In order to give a better initial guess for velocity and pressure fields, we will use the "Potential" initialization feature. This utility solves pseudo potential flow prior to actual calculations.

We would like to monitor how results change during computation. We will enable plotting the average pressure at the inlet during the calculation:

We will also display intermediate results on a section plane.

In this tutorial, we run a steady-state simulation. We are only interested in the latest time step. However, it is advised to keep the previous time step result as a backup on the disk. We will make a backup every 50 iterations.

To speed up the calculation process increase the number of CPUs basing on your PC capability. The free version allows you to use only 2 processors in parallel mode. To get the full version, you can use the contact form to Request 30-day Trial

Estimated computation time for 2 processors: 5 minutes

Go to the p on inlet tab to observe convergence process.

After preliminary calculation, we will continue with a more accurate algorithm. Change velocity discretization to the Linear Upwind scheme to minimize numerical diffusion affecting results of the simulation.

Go back to the p on inlet tab and observe convergence process.

Slices tab appears next to the Residuals tab. Under this tab, we can preview results on the defined section plane.

At the end, we will look at the pressure distribution. The Darcy porous model assumes resistance and hence pressure gradient to be proportional to the velocity in the porous zone. We can observe the effect of increased resistance from the porous zone. The pressure builds up in front of the porous zone and then linearly decreases along the flow direction due to resistance.