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User Guide: Incompressible flow simulation in centrifugal pumps

This user guide demonstrates how you can use the full Hex-dominant Automatic operation to create a mesh which supports the simulation of rotating systems and set up an incompressible flow simulation in centrifugal pumps.

Geometry preparation

Firstly, the geometry needs to be ‘CFD ready’

The solid volumes should be non-overlapping and should all be touching each other. More specific preparation details are explained here: Cad Preparation.

Basic recommended geometry checks:

  • Ensure that the imported geometry consists of Solid parts and not sheet/surface elements.
  • Remove any small fillets or faces which are insignificant for the analysis.

Rotating region

  • The geometry needs to be prepared in a special way so that a rotating zone mesh can be produced:
  • A cylinder needs to be placed around the rotor. All cells within the cylinder will later be treated as rotating.
  • The cylinder needs to be defined as a solid body.

Import tutorial project into workbench

  • Inside the outer pipe geometry we have a rotating region and an impeller. Hiding the outer pump surfaces, we can see the rotating geometry.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Rotating region in the pump
  • Hiding the rotating region allows us see the impeller inside.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Impeller inside the rotating region
  • As a first step we need to create a new simulation. To create a simulation left click under the Geometries  and then on Create Simulation.

    Create simulation before generating the mesh
  • Select the simulation model. Click on Incompressible.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Select the Incompresible simulation icon
  • keep the default settings of the simulation the same and click on the tick mark to save

Mesh Generation

  •  Left click on the mesh icon to create a new mesh.
  • Choose the Hex-Dominant (only CFD) Algorithm.
  • Change the meshing mode to Material point
  • Keep the sizing option as Automatic sizing and the Fineness as Coarse
  • Specify the number of processors to 16
Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
Automatic mesh sizing options for mesh generation

Background Mesh Box

  • An important parameter is the size of the background mesh box. As this is a internal mesh the size of the background mesh box can be the the default.
  • Expand geometry primitives under Mesh to view the the coordinates of Background base mesh box

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    keep the default background mesh coordinates as it is an internal mesh

Material Point

  • Material point: This is the parameter the algorithm uses to determine wether the mesh is created inside a shape or outside. In this case this point therefore lies within the pump but outside the impeller surface
  • Select the Material Point and give the coordinates as below:
Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
Set the material point coordinates such that the material point lies inside the pump and outside the impeller

Region Refinement

  • A region refinement is used to refine the mesh within a volume.
  • The cylinder around the propeller will define a zone within the cells will be more refined than in the rest of the mesh.
  • Click on the ‘+’ icon next to Refinements and add an region refinement.
  • Select the Refinement mode as ‘inside’ and the Length of refinement as ‘0.001’.
  • Select the rotating region for region refinement as shown below:

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Set up region refinement in the rotating region to get a more refined mesh

Surface Refinement

  • In order to have a finer mesh over the surface of the body, we add surface refinement.
  • Click on Refinements to add a new refinement and select surface refinement. We use 2 surface refinements:
  • On the rotor to refine the cells near the blades and
  • On the cylinder to define the rotating zone.

Refinement for Blade surfaces

  • This refinement enforces that the cells near the propeller surface will get refined.
  • Select the impeller surfaces and define the minimum and maximum length as 0.001.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Set up surface refinement on the blade surfaces

Refinement for the rotating (MRF/AMI) zone:

  • This step is crucial to correctly define the cell zone which will rotate.
  • Add a surface refinement and set the ‘Cell Zone’ option to ‘With Cell Zone’
  • Keep the minimum and maximum levels at 0.001.
  • Select the rotating region for the refinement

    Set up the surface refinement for the rotating region

Boundary Layer Refinement

  • Layer refinements are used to create boundary layers near solid walls.
  • When considering turbulent effects, boundary layer refinement is required in order to obtain a correct solution.
  • Create a new layer refinement and assign all faces of the propeller and the pipe outer surfaces
  • This can be done by hiding the rotating region and using active box selection to select all the surfaces and deselect the inlet and outlet surfaces
  • Select the  Inflate Boundary layer setting and the below values.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Set up the boundary layer refinement and select all the faces on the impeller and the pump outer surfaces
  • Once the mesh is fully set up, the mesh generation can be started.
  • The mesh generated is shown below:

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Mesh generated with 4.7 Million cells
  • After the mesh is generated, use the mesh as the domain for simulation and click on reset the assignments when prompted.
  • Click on generate mesh clip to inspect the internal mesh. Adjust the settings of the normal. Click on generate mesh clip icon.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Generate mesh clip showing the region refinement in the rotating region

Simulation Setup

Materials

  • Assign the standard air material to the fluid domain as shown below:

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Assign the standard air material to the fluid domain

Initial Conditions

  • Default values for initial condition parameters are usually enough. If these parameters estimated correctly, the solution will converge faster.

Boundary Conditions

  • We have to assign three boundary conditions.
  • The pump inlet needs to be assigned an inlet velocity boundary condition and the inlet velocity is as given below:

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Setting up the inlet as velocity inlet
  • The outlet of the pump needs to be assigned a pressure outlet boundary condition.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Setting up the outlet boundary condition as pressure outlet

MRF/Rotiang zone

  • Click on the ‘+’ icon next to rotating zones under Advanced concepts
  • Define the MRF rotating zone and select the rotating region as shown below:
Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
Set the coordinates of the MRF-Rotating zone

Numerics

Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
Change the PBiCG solver to Smooth solver and keep the default settings the same
  • The default settings are usually suitable. We only change the solver from PBiCG to smooth solver for all the variables for smoother convergence.

Simulation

Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
Set up the simulation control settings

The Simulation Control settings define the general controls over the simulation. The following controls should be applied:

Result Control

  • It is important to monitor the convergence of pressure values on the inlet and outlet faces.
  • We monitor the surface average data on the inlet and outlet faces
  • The settings are shown below:
Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
Set up result control to monitor the average pressure at the inlet and outlet faces

Results

  • Click on the ‘+’ icon next to simulation run and start the simulation
  • Check on the convergence of our field variables by clicking on the convergence plot icon under settings

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Convergence plot for the simulation
  • Check to see if the value of pressure on the inlet surface is converged. The value at the outlet surface stays at zero.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Monitoring the average pressure at the inlet surface
  • Click on solution fields icon under the convergence plot icon to open the post processor. Click on results and select pressure to view the pressure field on the entire domain.

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Pressure field throughout the domain
  • It is clearly seen that the inlet pressure is negative implying that the impeller is imparting a pressure head to the fluid.
  • Select the cutting plane in the x direction and select the scalar and vector as velocity to see the velocity contours and vectors
  • The rotating region clearly shows the rotating fluid inside it.
Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
Velocity contours and vectors in a cutting plane
  • We can view the path tracked by the fluid particles by clicking on the ‘+’ icon next to particle traces and selecting the outlet face for seeding as shown below:

    Incompressible flow Simualtion in a Centrifugal Pump using Simscale. Automatic Meshing, Rotating Geometries, Mesh Refinements
    Generate the particle traces by picking the outlet face for seeding
  • Click on velocity to map the scalar and to compute the vector

    Generate the particle traces by mapping them to velocity scalar

 

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