Inputs for Boundary Layer Mesh Generation in Ansys Fluent Meshing Watertight Geometry Workflow

Summary

This video introduces the 'Add Boundary Layers' task in the Ansys Fluent Meshing Watertight Workflow, emphasizing the importance of boundary layers in fluid dynamics. It explains how these thin regions, crucial for capturing skin-friction drag, heat transfer, and other flow characteristics, affect simulation accuracy. The video walks through the key inputs for defining boundary layers, including the number of layers, growth rate, transition ratio, first layer height, and aspect ratio. Properly configuring these settings ensures precise modeling of fluid flow in CFD simulations, particularly in complex, turbulence-sensitive areas.

Takeaways

• 😀 Boundary layers in fluid dynamics are important for understanding fluid flow behavior, skin-friction drag, heat transfer, and aircraft wing stall.
• 😀 The 'Add Boundary Layers' task in Ansys Fluent Meshing is used to define the mesh in boundary layer regions and is part of the Watertight Workflow.
• 😀 The task starts by importing a mesh file into Fluent, where the Watertight Geometry Workflow is automatically set up.
• 😀 The 'Add Boundary Layers' task becomes active when the setting is set to 'Yes' and several input options are made available.
• 😀 The 'Name' field auto-populates based on the chosen offset method, helping identify the boundary layer settings.
• 😀 The four offset methods available—smooth-transition, last-ratio, aspect-ratio, and uniform—affect how the boundary layer mesh cells are generated.
• 😀 The Number of Layers input defines how many boundary layers to create. A typical range for external flows is 10-20 layers for accurate simulation.
• 😀 The Growth Rate input determines the thickness of each subsequent boundary layer cell relative to the previous one (default is 1.2).
• 😀 The Transition Ratio defines how smoothly the last boundary layer meshes with the volume mesh, with values close to 1 ensuring a smoother transition.
• 😀 The First Height (for Last-Ratio or Uniform) and First Aspect Ratio (for Aspect-Ratio) inputs are critical for setting the first boundary layer cell's thickness and ensuring accurate turbulence modeling.

Q & A

What is the role of boundary layers in fluid dynamics?

Boundary layers are thin, viscosity-dominant regions formed along the surface of a solid body when fluid flows past it. These layers play a significant role in determining skin-friction drag, heat transfer, and phenomena like aircraft wing stall.

Why is it important to accurately capture the boundary layer flow in CFD simulations?

Accurately capturing boundary layer flow is essential because it significantly affects the results of skin-friction drag, heat transfer, and other flow behaviors. If the boundary layers aren't well-resolved, the simulation may fail to model critical physical phenomena.

What task in Ansys Fluent Meshing defines the boundary layer mesh?

The 'Add Boundary Layers' task in Ansys Fluent Meshing is responsible for defining the meshing requirements in the boundary layer region.

What are the four offset method types available in Ansys Fluent for boundary layer mesh generation?

The four offset method types are smooth-transition, last-ratio, aspect-ratio, and uniform. Each method influences how the first cell layer is generated near the solid boundary.

What is the default number of boundary layers set in Ansys Fluent, and what does it depend on?

The default number of boundary layers in Ansys Fluent is 3. The number depends on the accuracy requirements needed for resolving the boundary layer flow, with external aerodynamic flows often requiring 10-20 layers.

What does the growth rate input control in the boundary layer meshing process?

The growth rate input controls the ratio of the thickness of the next boundary layer cell to the previous one. It determines how quickly the boundary layer cells grow as they move away from the surface.

What growth rate is typically used for turbulent flow simulations like LES?

For scale-resolving simulations such as Large Eddy Simulations (LES), a lower growth rate of 1.1 is typically used to ensure that the chaotic aspects of the boundary layer flow are sufficiently resolved.

What does the transition ratio input determine in the boundary layer meshing process?

The transition ratio defines the ratio between the height of the mesh in the last boundary layer and the first cell in the volume fill. A smaller transition ratio indicates a larger jump between the boundary layer mesh and the volume mesh, while a higher value means the last boundary layer cell size is comparable to the adjacent volume mesh.

Why is the 'first height' value critical in boundary layer mesh generation?

The 'first height' value, which is the thickness of the first boundary layer, is critical, especially in turbulent flow simulations. Most turbulence models require a specific first cell height, often defined by the y+ variable, to ensure accurate simulation results.

How does the aspect-ratio offset method calculate the first boundary layer thickness?

The aspect-ratio method calculates the first boundary layer thickness by using the ratio of the local surface mesh size to the height of the first cell in the boundary layer mesh. The surface mesh size is already known, so the algorithm computes the thickness based on this ratio.