Mesh Generation with Helyx-OS: A Step-by-Step Tutorial

A crucial step in any CFD simulation is creating a good quality mesh. The mesh discretizes your geometry into small control volumes, and the accuracy and efficiency of your simulation depend heavily on its characteristics. Helyx-OS, with its powerful pre-processing tools, makes mesh generation accessible and efficient.

This tutorial assumes you have Helyx-OS installed and have a basic understanding of CFD concepts. We’ll be using a simple example: meshing a basic 2D airfoil for demonstration.

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Step 1: Launching Helyx-OS and Creating a New Project

1. Launch Helyx-OS: Open the Helyx-OS application. You’ll typically see a splash screen and then the main interface.
2. Create a New Project:
   • Go to File > New Project.
   • You’ll be prompted to choose a location for your project and give it a name. For this tutorial, let’s name it airfoil_mesh_tutorial.
   • Click OK.

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Step 2: Importing or Creating Your Geometry

Helyx-OS offers several ways to get your geometry into the meshing environment. You can import existing CAD files or create simple geometries directly.

Option A: Importing a Geometry (Recommended for real-world cases)

If you have a geometry file (e.g., .iges, .step, .stl), you’ll import it.

1. Import Geometry:
   • Navigate to the Geometry module (usually represented by an icon resembling a wireframe cube or a plane).
   • Go to File > Import > Geometry.
   • Browse to your geometry file (for this example, you might have a simple airfoil profile file you’ve downloaded or created).
   • Select your file and click Open.
2. Inspect Your Geometry: Once imported, your geometry will appear in the 3D viewer. Use your mouse to rotate, pan, and zoom to ensure it’s loaded correctly.

Option B: Creating a Simple Geometry (For quick testing)

For this tutorial, let’s briefly touch on creating a simple geometry, though importing is more common for complex shapes.

1. Geometry Module: Ensure you are in the Geometry module.
2. Create Primitive: You can find options to create basic shapes like planes, cylinders, spheres, etc., in the toolbar or under a “Create” menu. For a 2D airfoil, you might import a curve or use a specialized tool if available within Helyx-OS’s geometry capabilities.

For our airfoil_mesh_tutorial, let’s assume you have a simple 2D airfoil curve data you can import or define. If you don’t have one, you can find many airfoil profiles online (e.g., NACA series) and load their coordinate data.

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Step 3: Defining Boundary Conditions and Regions

Before meshing, you need to tell Helyx-OS about the different parts of your geometry and how they should be treated in the simulation.

1. Switch to the Meshing Module: Navigate to the Meshing module. This is usually represented by a mesh icon.
2. Identify and Name Surfaces:
   • In the Meshing module, you’ll typically have tools to select surfaces or edges of your geometry.
   • Select the airfoil surface: Use the appropriate selection tool (e.g., “Select Face,” “Select Boundary”) to pick the surface representing your airfoil.
   • Assign a boundary name: In the properties panel or a dedicated dialog, assign a meaningful name to this surface, such as airfoil_wall.
   • Select the outer boundary (e.g., far-field): Similarly, select the outer boundaries of your domain. These might be a large rectangle or circle enclosing the airfoil. Assign a name like far_field_inlet or outlet depending on your flow direction.
   • Create a Fluid Region: You’ll need to define the fluid volume. Often, you can do this by selecting the faces that enclose the fluid domain. Helyx-OS might have a dedicated tool for creating a “Fluid Region” or “Volume.” Select all the internal faces that constitute your flow domain. Name this region fluid.
   Tip: For 2D cases, you’ll be dealing with edges rather than faces.

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Step 4: Setting Up Mesh Parameters

Now, you’ll define the global and local mesh controls.

1. Global Mesh Settings:
   • Look for options to set the global mesh size or base element size. This controls the general fineness of the mesh. A smaller size means a finer mesh but more computational cost. Start with a moderate value.
   • You might also find options for mesh quality, such as aspect ratio or skewness limits, which Helyx-OS will try to adhere to.
2. Local Mesh Refinements (Optional but Recommended):
   • Refinement around the airfoil: To capture flow features accurately near the airfoil surface (like boundary layers), you’ll want to refine the mesh in this area.
     • Select the airfoil_wall boundary.
     • Look for an option like “Add Local Refinement” or “Boundary Layer Meshing.”
     • Boundary Layer Meshing: This is a powerful tool. You’ll typically specify the number of layers, the first layer height (often related to the desired $y^+$ value for turbulence modeling), and a growth rate.
     • Surface Refinement: You might also be able to apply a finer mesh size directly to the airfoil surface itself.
   • Refinement in wake region (if applicable): If you anticipate a recirculation or wake region behind the airfoil, you might want to apply a finer mesh in that area as well. This often involves defining a bounding box or a volumetric region for refinement.
   • Cell Type: Choose your desired cell type. For 2D, this is typically triangles or quadrilaterals. For 3D, it’s tetrahedrons, hexahedrons, or polyhedrons. Helyx-OS often defaults to a suitable type or allows you to choose.

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Step 5: Generating the Mesh

With your geometry and meshing parameters defined, you’re ready to generate the mesh.

1. Initiate Meshing:
   • Find the “Generate Mesh” button or command. This is usually a prominent button in the Meshing module’s toolbar.
   • Click it.
2. Monitor the Process: Helyx-OS will start generating the mesh. You’ll typically see a progress bar or messages indicating the meshing status. This can take anywhere from a few seconds to several minutes or even hours, depending on the complexity of your geometry and the mesh fineness.

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Step 6: Inspecting and Validating the Mesh

Once the meshing process is complete, it’s crucial to inspect the generated mesh for quality and suitability.

1. Visual Inspection:
   • Rotate and zoom: Thoroughly examine the mesh around your geometry, especially in areas with refinements.
   • Check cell types: Ensure you have the expected cell types (e.g., no inverted or highly distorted cells).
   • Look for gaps or overlaps: Make sure the mesh is contiguous and there are no unintended gaps or overlapping elements.
2. Mesh Quality Metrics: Helyx-OS usually provides tools to analyze mesh quality. Look for:
   • Cell Volume: Check for excessively large or small cells.
   • Aspect Ratio: The ratio of the longest to the shortest edge in a cell. High aspect ratios can sometimes lead to numerical issues.
   • Skewness: A measure of how distorted a cell is from an ideal shape (e.g., a perfect cube or tetrahedron). High skewness can degrade accuracy.
   • Orthogonality: For structured meshes, orthogonality is important.
   Helyx-OS will usually show statistics and can highlight cells that fail to meet certain quality criteria. If you find problematic areas, you’ll need to go back to Step 4 and adjust your meshing parameters (e.g., refine locally, change growth rates, or adjust global size) and regenerate.
3. Boundary Definition Check:
   • Verify that all the boundary conditions you defined earlier are correctly assigned to the respective surfaces/edges.

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Step 7: Saving and Exporting the Mesh

Once you’re satisfied with the mesh quality:

1. Save Your Project: Go to File > Save Project. This saves your Helyx-OS project file, which includes all your geometry, meshing, and setup information.
2. Export the Mesh: You’ll need to export the mesh in a format that your CFD solver can read.
   • Go to File > Export > Mesh.
   • Choose the appropriate file format for your chosen solver (e.g., .unv for Fluent, .foam for OpenFOAM, etc.).
   • Select the location to save the mesh file.
   • Click Export.

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Congratulations!

You have successfully generated and inspected a mesh for your geometry using Helyx-OS. This mesh is now ready to be used as input for your CFD simulations. Remember that mesh generation is often an iterative process. You might need to experiment with different refinement levels and strategies to achieve the optimal balance between accuracy and computational cost for your specific simulation.

Further Steps:

• Solver Setup: Once you have your mesh, you’ll move on to the Solver module in Helyx-OS or your chosen CFD solver to set up the physics, boundary conditions, and simulation parameters.
• Advanced Meshing Techniques: Helyx-OS offers advanced features like inflation layers (for boundary layers), mesh smoothing, and local mesh adaptation. Explore these as your simulation needs become more complex.

Happy meshing!

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