Static Mixer - CFD Simulation CFD Software Tutorial

In this tutorial, we demonstrate a feature called Passive Scalar. Passive scalar adds a transport equation to the system of governing equations. The passive scalar does not influence the flow itself but only introduces a marker for tracing fluid transport. This tutorial is used to track the mixing gases entering from 2 different inlets. The flow is still solved as a single phase and Passive Scalar only tracks how fluid travels from one inlet and mixes with the same fluid coming from different inlets. In some cases, this technique can be used to track different fluids, as long as the properties of the fluid are very close to each other (density, viscosity). In postprocessing, you will learn how to create Streamlines in ParaView.

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 static mixer

Firstly we need to Download GeometryStaticMixer

The STL format does not contain the unit information which are defined during the geometry export. If we do not know the exported unit, we can estimate it based on the total size of the model. It is displayed next to Geometry size label. In our case, the default unit meter is correct.

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

In order to create the mesh, we need to enable meshing for the imported geometry.

The imported geometry represents only the mixer blades. By using a cylinder base mesh we will define the pipe housing.

We need to assign individual names to each side of the base mesh. This will allow us to apply different conditions to each side.

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 cylinder we need to place the material point outside the blades.

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

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

Check the mesh quality and cells statistics.

To provide two different fluids into the mixer, we need to split the inlet boundary into two separate ones. We will use additional geometry to mark the selection for the extraction.

Now, using the new geometry we will extract a new boundary from the original inlet.

As the results of the extraction, we should receive two separate inlet boundaries. Both boundaries can be distinguished by the different colors.

With SimFlow, the user can also modify the existing mesh domain. We can extend the volume by extruding a specific boundary. For the purpose of this tutorial, we will extend the mixer tube by extruding the outflow face.

The outlet boundary will be extended by 5 cm and additional mesh will be split into 37 cells in the extrusion direction.

We want to analyze the incompressible turbulent flow. For this purpose, we will use the PIMPLE (pimpleFoam) solver.

In this tutorial, we will consider a laminar flow.

In order to define water properties, we go to the transport properties panel. We will use predefined fluid properties from the material database.

We will use passive scalar to simulate the mixing of two different fluids. Passive scalar adds an additional transport equation to the system of governing equations. Note, the passive scalar does not influence the flow itself but only introduces a marker for tracing fluid transport. The scalar takes a value between 0 and 1 (0 represents clear water, and 1 represents water with air dissolved in it). To control scalar properties we can define either the Schmidt number or custom diffusivity. In our case, we will define custom diffusivity equal 2.0e-09 which corresponds to the water-air mixture.

We will define the constant inlet velocity for both inlets.

Repeat these steps for the second inlet.

For the inlet_scalar1 boundary set the inflow phase value.

Before we start simulation we need to define the initial conditions. We will specify a constant velocity equal to 0.15 m/s which corresponds to the inlet velocity.

During calculation, we can observe intermediate results on a section plane. To add sampling data on a plane we need to define plane properties and also select variables that will be sampled. Note that runtime post-processing can only be defined before starting calculations and can not be changed later on.

Create the next slice above the mixer.

Duplicate the slice once again and move it to the vicinity of the outlet.

Before running computations adjust the time controls in order to capture appropriate time scales of the flow features.

We can control how often results should be saved on the hard drive. We will write the results at the interval of ` 0.1 ` seconds. Note, that only saved data will be available during postprocessing.

To speed up the calculation process, take advantage of parallel computing and increase the number of CPUs based on your PC’s capability. The free version allows you to use only one processor (serial mode). To get the full version, you can use the contact form to Request 30-day Trial

Estimated computation time for serial mode: 30 minutes

Slices tab appears next to Residuals. Under this tab, we can preview results on the defined slice planes. The results preview is available during the calculation and we can track it on a regular basis. The newly calculated time step will be actualized automatically as long as the time selector points to the latest time step.
We want to check if the fluids are mixed at the end of the mixer. We will display scalar1 contribution at each slice in the mixer.

After computations are finished we can do complex visualization of our results with ParaView.

Load the results into the program.

We can visualize the flow by displaying the streamlines.

To show the geometry together with the streamlines we will follow below steps:

This concludes the tutorial, covering all the aspects we intended to showcase. For a finely tuned presentation of the results, you may take advantage of the more advanced features.

In ParaView, you can display streamlines, contour plots, vector fields, line or time plots, calculating volume or surface integrals and create animations.

To familiarize yourself with the ParaView capabilities, it’s worth checking out our video tutorial, Paraview CFD Tutorial - Advanced Postprocessing in ParaView, in which we demonstrate some of the most commonly used post-processing techniques.

Paraview CFD Tutorial - Advanced Postprocessing in ParaView