Pipe Flow - CFD Simulation SimFlow Tutorial

The Pipe Flow tutorial serves as an excellent introduction to SimFlow. It primarily concentrates on explaining the fundamental aspects of the software. In this tutorial, we will present the simulation of internal flow through a simple T-pipe geometry (created directly in SimFlow). This simulation will feature a single-phase, incompressible flow, consisting of two inlets and one common outlet.

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:
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Open SimFlow and create a new case named pipe_flow

To build a "Pipe" geometry for the simulation, we will make use of SimFlow’s basic geometry module. This module allows for creating and editing of primitive geometries (box, cylinder, and sphere).

SimFlow also supports importing complex geometries in well known formats such as STEP, STL, IGES, and others. However, in this case, we will use a simple pipe geometry.

For this operation, we will need to create two cylinders, which we will unite in further steps into a pipe. Our initial step will involve adding the first cylinder.

In this step we will create second cylinder.

In this step we will create a pipe geometry by joining two cylinders created in prior steps. For this task we will use an unite tool. This is a geometric boolean operation that will result in a single solid geometry.

The resulting geometry will be displayed in the graphics window.

In this simulation, we are dealing with an internal flow problem. The imported geometry determines the bounds of the fluid domain. To inform the program where the boundaries of the inlet and outlet mesh should be created, we will use the Face Groups.

Face Group is a set of faces that should be distinguished within a geometry. When the geometry containing the face group is used for meshing, an additional boundary is created. The boundary represents the respective face group and inherits its name.

In this step we will create a face group for the second inlet.

In order to choose the outlet, we need to rotate the view so that the other end of the geometry becomes visible.

In the Geometry Panel we can find four Face Groups under the pipe geometry. The first group, named default, contains all non-selected surfaces. These surfaces will become the pipe walls.

The three additional groups represent the inlets and the outlet of the fluid domain. We will rename those groups to make the resulting boundary names more readable.

In order to create the mesh, we need to specify the meshing options used for a given geometry. For the pipe, we will add the boundary layer and refine the mesh near edges by level 1.

For the base mesh, we will use a box geometry. As we would like to create the mesh inside the pipe geometry, we need to make the box size a little bigger than the pipe.

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 pipe we need to place the material point inside the geometry. In this case, we do not need to modify the coordinates of the material point, since the default location is already inside the pipe.

In this step we will initiate the meshing process.

In the meshing panel you may indicate how many CPUs would you like to use for this process. Please note that if you are using SimFlow free version you may only use serial meshing, and you may not create meshes larger than 200'000 nodes.

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After the meshing process is finished the mesh should appear on the screen.

To be able to assign inlet and outlet boundary conditions we have to assign the proper boundary types.

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

We are going to use the standard \(k{-}\omega \; SST\) model to resolve turbulence.

As a fluid material, we will use water.

Selecting material from the Material Database will fill all inputs in the Transport Properties panel. We are still able to overwrite these values at any time.

Now, we will define the inlet and outlet boundary conditions. At the first inlet, we will set constant water velocity.

The second inlet will be defined using pressure inlet condition. For the pressure value, we will use 1 [kPa].

Note, for the incompressible solver, we operate using kinematic properties. And thus instead of using dynamic pressure, we are going to set a value divided by reference density – kinematic pressure \(m^2/s^2\).

At the outlet, we will apply pressure outlet character. The pressure value we will leave default 0. For the incompressible flow, pressure is always relative and we can add any constant (1 atm for example) after computation.

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

Additionally, to specifying the geometry we need to choose which results should be sampled.

Finally, we can start our computation.

Estimated computation time for single-core: 5 minutes

When the calculation is finished we should see a similar residual plot. All the residual values have reached \(10^{-4}\) and the solver has automatically stopped the calculation. The residuals are the measure of how results change between iteration. Reaching a very low value indicates that we have found a steady-state solution and results do not change regardless of further computations.

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

To access more advanced postprocessing functions, use the integrated ParaView software.

A new ParaView window should open. Load results into the ParaView. Be aware that in a steady-state simulation, SimFlow saves only the last two iterations by default. View the last one to see the final solution.

Advanced postprocessing features give us a possibility to display i.e., streamlines. To do that, follow these steps:

To enhance visibility, we can render the streamlines.

We can additionally display the transparent geometry to observe how the streamlines are distributed in relation to it.

The final results contain the transparent geometry and streamlines colored by the velocity.

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