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Documentation

Tutorial: Fluid Flow through a Centrifugal Pump Using Subsonic Solver

This tutorial demonstrates how to use SimScale to run a centrifugal pump simulation using the Subsonic solver.

pressure particle traces subsonic centrifugal pump
Figure 1: Pressure distribution in the pump

Overview

This tutorial teaches how to:

  • Create a rotating region and extract flow volume for pump simulations.
  • Set up and run an incompressible, steady state simulation.
  • Assign topological entity sets.
  • Assign boundary conditions, material, and other properties to the simulation.
  • Mesh with the automatic meshing algorithm in Subsonic.

We are following the typical SimScale workflow:

  1. Prepare the CAD model for the simulation
  2. Set up the simulation
  3. Create a mesh
  4. Run the simulation and analyze results

1. Prepare the CAD Model and Select the Analysis Type

To begin, click the button below. It will copy the tutorial project containing the geometry into your Workbench.

The following picture demonstrates the original geometry that should be visible after importing the tutorial project.

centrifugal pump geometry for cfd analysis
Figure 2: Imported CAD model of a water pump in the SimScale Workbench.

1.1 Geometry Preparation

Before starting to work using SimScale’s fluid simulator analysis with rotation, we need to make sure to prepare the CAD model to be compatible with this kind of analysis. This tutorial project contains two geometries, shown in the picture below:

creating the flow region for a subsonic centrifugal pump geometry simulation
Figure 3: Original pump geometry (left) and flow volume ready for a simulation (right).

The first one (Original Geometry) consists of the actual pump and its blades. It still requires some preparation before it’s ready to use for CFD simulations.

What we need for a CFD simulation is the second geometry (Centrifugal Pump). It contains the flow region, as well as a volume representing the rotating zone. The steps to make the original geometry CFD-ready are fully described in the following article:

1.2 Create the Simulation

Make sure the Centrifugal Pump geometry is selected for this simulation project:

creating a new simulation for a water pump
Figure 4: Creating a new simulation for the centrifugal pump geometry.

After hitting on ‘Create Simulation’, you will have several simulation types to choose from:

subsonic analysis type
Figure 5: Analysis types available. For this project, Incompressible is chosen.

Choose ‘Subsonic’ as the analysis type and ‘Create Simulation‘.

At this point, the simulation tree will be visible in the left-hand side panel. To run the simulation, it’s necessary to configure the simulation tree entries.

subsonic global settings
Figure 6: Global simulation settings for the centrifugal pump tutorial

The global simulation settings remain as default, as in the figure above. With a Steady-state analysis, we will obtain the equilibrium state of the system, when the flow field no longer changes with time.

Did you know?

Within the pump blades, the flow experiences separation which is effectively captured by the k-epsilon turbulence model.

2. Pre-Processing: Setting up the Simulation

As an overview of the physics, the following picture shows the boundary conditions used in this project:

boundary conditions subsonic centrifugal pump cfd simulation
Figure 7: Overview of the boundary conditions for the centrifugal pump simulation

Did you know?

Velocity Inlet and Pressure Outlet is a very common combination used in CFD simulations as it often results in good stability. This combination permits flow to adjust aiming to assure mass continuity.

2.1 Define a Material

This simulation will use water as a material. Therefore click on the ‘+ button’ next to Materials. Doing this opens the SimScale fluid simulator material library as shown in the figure below:

materials
Figure 8: Library of available materials.

Select ‘Water’ and click ‘Apply’. A window opens up with the water properties. Keep the default values and assign them to the ‘Volume’ that represents the flow region.

water assignment to subsonic centrifugal pump
Figure 9: Assigning water to the flow region. Note that the rotating zone volume is not selected.

Assigning Gravity

Note that there is a section on Model where the gravitational acceleration can be specified. This is useful for cases where gravity plays an important role. For the present tutorial, we will leave it to the default value of 0.

2.2 Assign the Boundary Conditions

In the next step, boundary conditions need to be assigned as in Figure 10. We have a velocity inlet, a pressure outlet, walls, and a rotating zone.

a. Velocity Inlet

After hitting the ‘+ button’ next to Boundary conditions, a drop-down menu will appear, where one can choose between different boundary conditions.

 velocity inlet subsonic centrifugal pump
Figure 10: Choosing velocity inlet boundary condition to apply to the inlet face.

After selecting ‘Velocity inlet’, you can specify the velocity configuration and assign the inlet face. Please proceed as follows:

velocity inlet to pump
Figure 11: Assigning the first boundary condition to a face.

With these settings, a volumetric flow rate of ‘0.004’ \(m^3/s\) enters the domain through the inlet.

Parametric study with multiple flow rates

Subsonic analysis type allows parametric studies with the velocity boundary condition. When you select Flow rate as the velocity type while assigning velocity to an inlet or outlet face then you can input multiple flowrates at once either directly or by uploading a file.


parametric flow rate inlet subsonic solver simscale
Figure: 12: Parametric study in Subsonic analysis is possible by simulating multiple flow rates at once

Enter multiple flowrates through the specify value icon as highlighted which opens up a new window to input the values in a table format. Hit ‘Apply’. Don’t forget to assign the inlet face.

This will help initiate multiple runs with different flow rates assigned at once in parallel giving the user a huge time advantage. In this tutorial, we encourage you to try this functionality.

b. Pressure Outlet

Create a new boundary condition, this time a ‘Pressure outlet’ and select the outlet face. Make sure (P) Gauge Pressure is set to a fixed value of ‘0’ \(Pa\).

subsonic pressure outlet bc centrifugal pump
Figure 13: Assign the second boundary condition to the outlet face

c. Walls

In subsonic analysis, all the surfaces that act as walls are automatically treated likewise by the solver itself. Walls outside the rotating zone receive a no-slip condition and walls inside the rotating zone are treated as rotating walls unless you specifically pick the faces inside the rotating zone that you want to be static.

2.3 Advanced Concepts: Creating a Rotating Zone

In the simulation tree, please expand Advanced concepts. Click on the ‘+ button’ next to Rotating zones and select an ‘MRF Rotating Zone’. Define the MRF zone as shown:

subsonic mrf rotating zone
Figure 14: Rotating zone parameters. All entities within this rotating zone will be rotating at 350 rad/s.

Did you know?

Based on the initial input at the start of the simulation, Subsonic automatically assigns the rotating region as MRF or AMI zone. No additional input is required when setting up the rotating zone.

This article provides more information about the difference between MRF and AMI Rotating Zones.

2.4 Simulation Control

Don’t worry about the numerical settings for the subsonic simulations, as their default values are optimized.

Open the simulation control settings and change the Number of iterations to ‘600’.

subsonic simulation control
Figure 15: Simulation control settings, defining a steady-state simulation with 600 seconds

Keep the remaining settings as default. To know more about how to control the simulation read in detail here.

2.5 Result Control

Result control allows you to observe the convergence behavior at specific locations in the model during the calculation process. Hence, it is an important indicator of the simulation quality and the reliability of the results.

a. Forces and Moments

For this simulation, please set a ‘Forces and moments’ control to the impeller as demonstrated in the picture below:

forces and moments on subsonic centrifugal pump
Figure 16: Forces and moments control for the impeller.

b. Area Average

To save time, there’s a pre-saved topological entity set for the Impeller surfaces. Finally, click on the ‘+’ button next to Surface data. Create an ‘Area average’ for the inlet and the outlet:

surface data
Figure 17: Monitors set at the inlet and outlet faces – Area average

c. Pressure Difference

To get the pressure difference between the inlet and the outlet directly set up Pressure difference curves as well. This result control item creates a plot each for the different flow rates with respect to iterations and also creates a pressure curve for the turbine.

subsonic pressure differencce
Figure 18: Monitors set at the inlet and outlet faces – Pressure difference

3. Mesh

To create the mesh, we recommend using the Automatic mesh algorithm, which is a good choice in general as it is quite automated and delivers good results for most geometries.

In this tutorial, a mesh fineness level of 1 will be used for demonstration purposes. If you wish to undertake a mesh refinement study, you can increase the fineness of the mesh by sliding the mesh to higher refinement levels.

subsonic mesh settings
Figure 19: Automatic mesh settings

Did you know?

The automesher creates a body-fitted mesh which captures most regions of interest using physics based meshing.

If you are using a the manual mesher, you can learn how to set up different parameters in this Subsonic manual meshing documentation page.

4. Start the Simulation

Now you can start the simulation. Click on the ‘+’ icon next to Simulation runs. This opens up a dialogue box where you can name your run and ‘Start’ the simulation.

new run
Figure 20: Set up ready to run simulations

While the results are being calculated you can already have a look at the intermediate results in the post-processor by clicking on ‘Solution Fields’. They are being updated in real-time!

It takes roughly 5-8 minutes for the simulation to finish.

run in progress subsonic parametric study
Figure 21: Under Solution fields, you can also access intermediate result sets while the simulation is running.

SimScale has a built-in post-processing environment, which can be accessed by clicking on ‘Solution Fields’, as in Figure 25:

accessing post processor simscale
Figure 22: Accessing the SimScale’s integrated online post-processor

Did you know?

With SimScale, it’s easy to get the pump curve for your geometry. You can run multiple simulations in parallel, only changing the flow rate (in the velocity inlet boundary condition).

For more information about pump curves, please check out this blog post.

5. Post-Processing

5.1 Visualizing the Mesh

Once inside the post-processor, under the Parts Color filter change Coloring to any solid color of choice and then change the render mode to Surfaces with mesh to show opaque surfaces of the CAD model with the mesh grid.

subsonic mesh centrifugal pump
Figure 23: Mesh visualization inside SimScale’s online post-processor

You can use the cutting plane filter to see the inside of the mesh generated:

centrifugal pump cutting plane mesh subsonic
Figure 24: Inspecting the mesh in detail using a cutting plane
  1. Hit the ‘Cutting Plane’ filter from the top ribbon.
  2. Adjust the position until it cuts the rotor.
  3. Adjust the orientation to X axis.
  4. Change the Coloring to some contrasting solid color.
  5. Toggle on Show mesh so that the mesh can be visible.

After a few seconds, you will see a clip showing the inside of your mesh. This mesh looks sufficient for this tutorial.

5.2 Convergence Behavior

With the previously set result controls, it’s possible to assess convergence. As the iterations go on, key parameters are expected to stop changing. At this point, the simulation is considered converged.

So what are the key parameters? For a centrifugal pump simulation, the velocity at the outlet, the pressure at the inlet, and forces onto the impeller are parameters of interest. For example, let’s have a look at the forces and moments plot:

subsonic convergence plot
Figure 25: Forces and moments result control plot to assess convergence for the flow rate 0.0085 \(m^3/s\) (Run 3). The results are very stable.

5.3 Performance Curve

Since we ran multiple simulations in parallel, the pressure difference \(\Delta p\) is plotted with respect to all flowrates. This is available under Result control>Area averages:

centrifugal pump pressure curve simscale
Figure 26: Centrifugal Pump performance curve for the current set of volumetric flow rate inlets

A Pump performance curve defines the range of possible operating conditions for the pump. Pump curves can tell about a pump’s ability to produce flow under a certain pressure head, as well as its efficiency and braking horsepower. Pump curves are essential for optimal pump selection as well as reliable and efficient operation.

5.4 Pressure

subsonic post processor pressure
Figure 27: In the post-processor, you can still use the selection functionalities available in the Workbench.
  1. Under Parts Color, make sure that the Coloring is ‘Pressure’. This way, the pressure levels are displayed on the boundaries.
  2. To make the blades visible, you can select the outer faces by clicking on them.
  3. Once you select the faces, right-click on the viewer and choose ‘Hide selection’.

After hiding the first set of faces, you may have to hide more internal faces before the blades are fully visible. At this stage, right-click on the legend and select ‘Use continuous scale’ to make the color scheme smoother:

subsonic post processing pressure on blades
Figure 28: The pressure visualization on the blades and bottom surface of the impeller reveals a high distribution on the edge towards the exit of the pump, near the exit of the fluid. Results are shown for Run 4 [0.004 m3/s].

5.5 Velocity Vectors

To get a better understanding of what is going on inside the pump, proceed to add a ‘Cutting Plane’ filter:

velocity on plane in centrifugal pump simscale
Figure 29: This cutting plane normal to the X-axis shows the behavior of the flow when exiting the model. (Run 4 [0.004 m3/s].)
  1. Create a ‘Cutting Plane’ filter by using the top ribbon.
  2. Adjust the Position coordinates of the cutting plane to ‘0, 0, 0’. Furthermore, make sure that the Orientation is ‘X’ and the Coloring is ‘Velocity Magnitude’.
  3. Ensure Clip model is toggled on.

You can also toggle on Vectors to check the flow direction of the field in different areas:

velocity on plane with vectors in centrifugal pump simscale
Figure 30: Velocity contours with velocity vectors plotted. In areas where the fluid accelerates, such as the tips of the blades, and the entrance to the outlet, the vectors are also enlarged. (Run 4 [0.004 m3/s])
  • Change the Coloring to ‘Solid color’, and select the black color from the available options.
  • Then set the Scale factor to ‘0.1’ and the Grid Spacing to ‘0.02’.
  • Activate the ‘Project vectors onto plane’ option.

Near the edges of the blades, the acceleration of the flow can be distinguished due to the representation with a warmer color compared to its surroundings. Other configurations for the cutting plane will give you additional information. For instance, you can change the Orientation to ‘Y’

velocity on plane with vectors in centrifugal pump simscale
Figure 31: Adjusting the orientation of the cutting plane around the water pump. (Run 4 [0.004 m3/s].)

With this configuration, you can observe the flow pattern around the blades from a different perspective.

Analyze your results with the SimScale post-processor. Have a look at our post-processing guide to learn how to use the post-processor.

Note

If you have questions or suggestions, please reach out either via the forum or contact us directly.

Last updated: August 30th, 2022

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