Rotating mesh

Table of Contents

Overview

Case files can be found here:

Geometry

The geometry for this tutorial consists of

  • room with
    • door
    • window (outlet)
  • desk
  • rotating fan
  • cylinder defining
    • rotating cells
    • the interpolation surfaces between rotating and non-rotating cells
Geometry

Mesh

The mesh is created with snappyHexMesh using STL surfaces located in the constant/triSurface directory. 

surfaceFeatureExtract

The process begins by extracting features from the STL files (dictionary can be found in system/surfaceFeatureExtractDict). 

blockMesh

This command creates an initial block structured mesh with our starting mesh resolution (dictionary can be found in system/blockMeshDict). 

snappyHexMesh -overwrite

This command refines the initial mesh at the STL surfaces as well as the extruded features (dictionary can be found in system/snappyHexMeshDict). Layer addition is not utilized in this tutorial for simplicity purposes. Usual snappyHexMesh settings are utilized for a coarse mesh here. One important entry can be found in the refinementSurfaces sub-dictionary: 

AMI
{
    level     (2 2);
    faceType  boundary;
    cellZone  rotatingZone;
    faceZone  rotatingZone;
    cellZoneInside inside;
}

This defines a cellZone called rotatingZone, which will later define the rotating cells. Additionally we define a boundary, which will be later used to define the interpolation faces between rotating and non-rotating regions. 

renumberMesh -overwrite

This commands restructures the mesh for better calculation performance. 

createPatch -overwrite

This command converts the boundary AMI to an arbitrary mesh interface (hence AMI), where during the simulation the interpolation of fields between rotating and non-rotating cells will take place (dictionary can be found in system/createPatchDict).

After these steps you can visualize your mesh in ParaView:

Mesh
Note
  • Use higher refinement on AMI and fan for better mesh resolution.
  • Use refinement region defined by AMI for uniform mesh resolution within rotating zone.
  • This will however increase your calculation time.

Here you can see the rotating and non-rotating regions of the mesh:

Close-up around moving part

Simulation

Now we move from the folder mesh to the folder case.

Boundary conditions

  • Boundaries in the folder 0 are defined the following way:
    • door: inlet
    • outlet (window): outlet
    • room: wall
    • desk: wall
    • fan: moving wall
  • velocity U
  • kinematic pressure p
    • door: fixedFluxPressure
    • outlet (window): Fixed value with atmospheric pressure at 0 Pa
    • room: fixedFluxPressure
    • desk: fixedFluxPressure
    • fan: fixedFluxPressure
  • turbulent kinetic energy k
    • door: turbulentIntensityKineticEnergyInlet with 5% turbulence intensity
    • outlet (window): Zero gradient (alternative: Inlet outlet)
    • room: kqRWallFunction
    • desk: kqRWallFunction
    • fan: kqRWallFunction
  • turbulent dissipation rate omega
    • door: turbulentMixingLengthFrequencyInlet with 1.2m mixing length
    • outlet (window): Zero gradient (alternative: Inlet outlet)
    • room: omegaWallFunction
    • desk: omegaWallFunction
    • fan: omegaWallFunction
  • turbulent kinematic viscosity nut

Definition of mesh movement

  • Dictionary dynamicMeshDict can be found in constant.
  • Important entries:
    • cellZone rotatingZone; - This was defined in snappyHexMeshDict to define the rotating cells.
      • solidBodyMotionFunction rotatingMotion; - rotating mesh movement
      • origin (-3 2 2.6); - origin of the axis of rotation of cells (can be anywhere on the rotation axis of the fan)
      • axis (0 0 1); - direction of the rotation axis (here: z-direction)
      • omega 10; - angular velocity of rotation in rad/s= (corresponds to ~95.5 rpm)
Note
  • For a realistic fan movement use omega 20;

Additional dictionaries in constant

  • g - gravitational acceleration
  • transportProperties - definition of kinematic viscosity
  • turbulenceProperties - definition of turbulence model (here: k-omega Shear Stress Transport (SST))

Simulation settings

  • controlDict - runtime settings
    • endTime: 1 s (approx. 0.64 s per revolution)
      • writeInterval: 0.1 s
      • maxCo: 1
  • decomposeParDict - parallel setting
  • fvSchemes - various discretisation schemes
  • fvSolution - numeric settings (matrix solvers, tolerances, correctors)

Running the simulation

The simulation employs the pimpleFoam application, in parallel. 

decomposePar

With this command you divide your mesh and your fields onto multiple processors. 

mpirun -np 4 pimpleFoam -parallel > log.pimpleFoam &

With this command you start your simulation on four cores. If you want to use a different number of cores you have to change the number (here: 4) to you choice. Also don't forget to change the settings in system/decomposeParDict!

Results

Here you see the flow after t = 0.64 s.

Velocity magnitude and vectors on a y-normal plane:

Velocity magnitude and vectors

Close-up of velocity magnitude and vectors on a y-normal plane:

Close-up velocity magnitude and vectors
Note
  • For a more developed flow pattern run the simulation until 10-20s.
Contributors to this page Jozsef Nagy
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