CFD Modeling of Rotating Machinery in FLUENT and OpenFOAM

Modeling of Rotating Machinery

• CFD modeling of rotating machinery, such as pumps, fans, turbines, or compressors, is commonly carried out in ANSYS Fluent and OpenFOAM.
• Both software platforms offer robust tools for simulating the rotating behavior of fluid flow around or within machinery components, enabling engineers to analyze and optimize designs effectively.
• You can read the basic of turbomachinery and its CFD modeling

  • Refer the Fundamentals of Turbo-machinery and Governing Laws

  • CFD Modelling of Turbo-machinery

• Here’s a detailed comparison and setup guide for modeling rotating machinery in Fluent and OpenFOAM.

 

Rotating Machinery Modeling in ANSYS Fluent

Key Approaches:

• Moving Reference Frame (MRF):
  • The MRF approach assumes that the rotating zone is stationary relative to the flow equations by adding source terms to represent rotation.
  • This is suitable for steady-state problems, where transient blade-passing effects are not critical.
• Sliding Mesh:
  • This method simulates the actual motion of the rotating part by rotating the mesh and is ideal for transient analysis, capturing detailed interactions between the rotating and stationary zones.
• Mixing Plane (Stage Interface):
  • The mixing plane model averages flow across periodic sections, often used for compressors or pumps with periodic boundaries, creating a more computationally efficient steady-state approximation.

Setup Process in Fluent

1. Geometry and Meshing:
   • Divide the geometry into rotating and stationary zones (e.g., fan blades for rotating, ducting for stationary).
   • Create interfaces between these zones.
   • Mesh the zones separately, ensuring high quality around the rotating parts.
2. Defining Rotating Zones:
   • In the Cell Zone Conditions, define the rotating region with the appropriate Rotation Speed and Rotation Axis.
   • For MRF, apply the frame motion in the rotating cell zone.
   • For sliding mesh, set up the mesh motion settings to rotate at the desired RPM.
3. Boundary Conditions:
   • Set inlet and outlet boundary conditions.
   • For interfaces between rotating and stationary zones, apply an Interface Boundary Condition to allow fluid continuity.
4. Solution Setup:
   • Select a suitable Turbulence Model (k-ε, k-ω, or LES for detailed eddy simulations).
   • If using sliding mesh, set the solution to transient and configure time-stepping controls.
   • For MRF, steady-state is generally used, which can simplify convergence.
5. Post-Processing:
   • Analyze flow features like pressure, velocity profiles, turbulence, and vorticity.
   • For sliding mesh, transient analysis results allow blade-passing effect observation and detailed transient flow patterns.

Advantages of Fluent:

• GUI-driven, simplifying complex setup for rotating zones.
• Wide range of built-in models for rotating flows.
• Direct coupling with optimization tools.

Rotating Machinery Modeling in Open-FOAM

• OpenFOAM is more flexible but requires some expertise in setting up cases and custom scripts for rotating flows.

  • Refer the Basics of OpenFOAM : Open source CFD Solver

• It provides efficient solvers for rotating flows via two primary methods:

Key Approaches:

• Moving Reference Frame (MRF): Implemented with MRFSimpleFoam or simpleFoam with MRF source terms. Suitable for steady-state applications.
• Sliding Mesh (AMI – Arbitrary Mesh Interface): Allows for transient simulations by creating an interface that rotates. This is more computationally demanding but can capture unsteady interactions effectively.

Setup Process in OpenFOAM:

1. Geometry and Meshing:
   • Use blockMesh or snappyHexMesh to create the mesh. Separate the rotating and stationary zones, often defining separate cell zones.
   • In cases with sliding mesh, ensure mesh conformity at the AMI interface.
2. Defining Rotating Zones (MRF):
   • In constant/MRFProperties, define the rotating zones with the rotational axis and speed.
   • Activate MRF source terms in the appropriate cell zones.
3. Setting Up AMI (Sliding Mesh):
   • Configure AMI patches in boundary files to define interfaces between rotating and stationary zones.
   • Set up the rotation speed in dynamicMeshDict, defining the RPM and axis of rotation.
4. Boundary Conditions:
   • In 0 folder files, specify inlet and outlet conditions.
   • For AMI, ensure continuity at the interface by using appropriate boundary conditions like cyclicAMI to maintain flow consistency.
5. Selecting Solvers and Running Simulation:
   • For steady-state MRF cases, use MRFSimpleFoam.
   • For transient sliding mesh cases, use pimpleDyMFoam or pisoFoam for transient calculations.
   • Set up the controlDict to specify time-stepping for transient runs.
6. Post-Processing:
   • OpenFOAM integrates with ParaView for visualizing velocity, pressure, and turbulence. Vorticity and blade-passing effects can be visualized for sliding mesh simulations.

Advantages of Open-FOAM:

• Fully customizable and open-source, which is advantageous for research and complex custom models.
• Efficient with computational resources for large-scale simulations on clusters.

 

Comparison of Fluent and Open-FOAM for Rotating Machinery

Feature	ANSYS Fluent	OpenFOAM
Ease of Use	User-friendly GUI, easy setup	Requires manual setup, scripting knowledge
Rotating Frame Options	MRF, Sliding Mesh, Mixing Plane	MRF, AMI (Sliding Mesh)
Solver Types	Steady and transient with multiple turbulence models	Steady (MRF) and transient (AMI)
Turbulence Models	Comprehensive, with LES and DES options	Flexible, customizable (LES, DES available)
Cost and Accessibility	Commercial license	Open-source
Post-Processing	Integrated (Fluent and CFD-Post)	ParaView for OpenFOAM

• Both Fluent and OpenFOAM provide strong capabilities for rotating machinery simulations.
• Fluent is advantageous for its comprehensive GUI, making it accessible and highly efficient for engineering applications, especially with a focus on ease of use and advanced built-in models.
• OpenFOAM is highly customizable, ideal for research and complex cases where cost is a factor and scripting knowledge is available.

Modeling of Turbomachinery

Basics of Pumps and Its CFD Modeling

Frame motion model in Ansys Fluent

• In ANSYS Fluent, the Frame Motion model allows the simulation of rotating or moving frames of reference, typically used for components like fans, propellers, pumps, or any rotating machinery.
• This approach is beneficial because it enables you to simulate the effects of rotation without needing to model the actual motion of each blade. Instead, the fluid equations are solved in a rotating reference frame, making it computationally efficient while capturing rotational effects.

Steps to Set Up Frame Motion in ANSYS Fluent

1. Geometry and Meshing:
   • Divide the computational domain into a rotating zone (for the moving frame) and a stationary zone (surrounding fluid or static components).
   • Mesh the geometry, ensuring a high-quality mesh in the rotating zone, as it impacts the accuracy of the solution. The mesh does not need to rotate; instead, only the frame of reference will have rotational properties.
2. Define Rotating Zones:
   • In the Fluent interface, identify the rotating domain (such as the area around the fan blades or impeller) as a separate cell zone.
   • Set this zone to be a rotating reference frame by specifying the rotational speed and axis of rotation.
3. Assign Boundary Conditions:
   • Define boundary conditions for both the rotating and stationary zones. Typical boundary conditions may include velocity inlets, pressure outlets, or wall conditions.
   • Specify interfaces between rotating and stationary zones to allow fluid interaction between them, enabling proper data transfer at the zone boundaries.
4. Selecting a Turbulence Model:
   • Choose an appropriate turbulence model, typically a k-ε or k-ω model, which is well-suited for rotating flows. For more complex cases or higher accuracy, you may consider using LES (Large Eddy Simulation) or DES (Detached Eddy Simulation).
5. Set Frame Motion in Cell Zone Conditions:
   • Go to Cell Zone Conditions in Fluent, and select the rotating cell zone.
   • Enable the Frame Motion option, and enter the Rotational Speed in rad/s or RPM. Define the Axis of Rotation to match the physical rotation direction of the fan or rotating part.
6. Configure Solution Controls:
   • Set under-relaxation factors and convergence criteria according to the stability and requirements of the rotating frame model.
   • For complex rotating systems, adjust solution controls like the Courant number to enhance stability, as rotating reference frames can add complexity to the convergence process.
7. Post-Processing:
   • After solving, you can analyze flow parameters such as pressure and velocity contours, streamline plots, and velocity vectors.
   • Use contour and vector plots to visualize the rotation effects and examine parameters like vortex formation, pressure distribution, and potential zones of recirculation.

Key Considerations

• Moving Reference Frame (MRF) vs. Sliding Mesh:
  • The MRF model assumes a steady-state approach where the rotating frame is static relative to the flow, meaning it does not capture transient effects. It’s often used for steady-state simulations in applications where only the average rotation effect is needed.
  • The Sliding Mesh model simulates the actual rotation of the geometry, capturing transient interactions. It’s suitable for cases where blade-passing effects and unsteady interactions are critical.
• Compatibility with Multi-Reference Frame (MRF):
  • Frame motion can be combined with MRF setups, where each rotating component has its own reference frame. This is useful in systems with multiple rotating parts, like multiple fans or turbine blades.

Using the frame motion model effectively allows you to model rotating flows with high efficiency and precision, reducing computational costs while still capturing critical flow details for performance assessment and design optimization.