Meshing in CFD

In Computational Fluid Dynamics (CFD), meshing is the process of dividing the simulation domain into many small, discrete cells that the solver uses to approximate the governing equations of fluid motion. Instead of solving the Navier–Stokes equations analytically, CFD solves them numerically over each small cell in the mesh.

A mesh can consist of:

• Hexahedral cells

• Tetrahedral cells

• Polyhedral cells

• Prismatic layers near walls

The quality and structure of the mesh directly affect:

• Accuracy of velocity and temperature predictions

• Numerical stability

• Convergence speed

• Ability to resolve important flow features such as boundary layers, jets, and recirculation zones

In HVAC simulations—where indoor airflow is complex—high-quality meshing is essential for capturing diffuser jets, thermal plumes, stratification, and comfort metrics such as PMV, PPD, DR, and air age.

tensorHVAC-Pro automates this step, but understanding the underlying meshing tools helps users appreciate how the geometry is discretized.

blockMesh

blockMesh is a basic mesh generator that ships with OpenFOAM. It creates simple, structured meshes by dividing the domain into one or more hexahedral blocks.

Key Characteristics of blockMesh

• Produces structured meshes

• Cells are perfectly aligned and follow a strict grid pattern

• Excellent for simple geometries such as cubes, rooms, channels, ducts, or rectangular volumes

• Very fast and easy to compute

• Defined using the blockMeshDict file

• Supports mesh grading (cell refinement in a given direction)

Limitations

• Cannot handle complex STL geometries (objects, furniture, diffusers, curved surfaces)

• Geometry must be decomposed into simple block shapes

In HVAC applications, blockMesh is often used only to create a background mesh, which is later refined using snappyHexMesh.

snappyHexMesh

snappyHexMesh is OpenFOAM’s advanced, automatic, STL-based meshing utility. It is designed to generate meshes around complex geometries—such as rooms with furniture, humans, AC diffusers, ducts, windows, and detailed HVAC systems.

How snappyHexMesh Works

The meshing process happens in three main steps:

1. Castellated Mesh Generation

   • Starts from a blockMesh background

   • Refines cells near STL surfaces

   • Removes cells inside or outside regions (depending on settings)

2. Surface Snapping

   • Mesh conforms to the STL shape

   • Cell vertices are moved to match the geometry

   • Produces accurate surfaces around imported objects

3. Layer Addition (Optional)

   • Generates boundary layer cells on walls

   • Required for resolving near-wall velocity gradients

Key Strengths

• Handles complex indoor geometries effortlessly

• Automatically refines around:

  • Furniture

  • Equipment

  • Diffusers

  • Occupants

• Accurately captures small features

• Allows region-based refinement

• Produces high-quality hex-dominant meshes

Why snappyHexMesh is ideal for HVAC

Indoor airflow involves:

• Multiple objects

• Irregular room layouts

• Detailed boundary surfaces

• Buoyancy and recirculation

snappyHexMesh can refine around these complexities automatically, producing meshes that resolve flow behavior around obstacles and comfort-critical regions.

Meshing Workflow in tensorHVAC-Pro

tensorHVAC-Pro wraps these meshing tools with an easier interface:

• Users only choose Coarse, Medium, or Fine mesh

• Geometry is automatically cleaned and refined

• Objects are auto-processed based on STL names

• SnappyHexMesh handles all complex features

• Minimal or zero geometry cleanup needed

This dramatically simplifies the preparation phase, making CFD more accessible without compromising physical accuracy.