What Is a CFD Solver?

A CFD solver is the core computational engine that performs the numerical calculation of fluid flow, heat transfer, turbulence, and other physical processes inside a simulation domain. After the geometry, mesh, boundary conditions, and physical models are defined, the solver applies mathematical algorithms to approximate the governing equations of fluid motion—primarily the Navier–Stokes equations, the energy equation, and turbulence transport equations.

The solver performs the following key tasks:

• Discretizes the equations using the mesh

• Iteratively computes values of velocity, pressure, temperature, and other fields

• Enforces boundary and initial conditions

• Models physical phenomena such as buoyancy, convection, turbulence, and diffusion

• Continues iterating until the solution converges

In essence, the CFD solver transforms a physical HVAC scenario into a stable, repeatable numerical solution that predicts airflow, comfort, and temperature distribution within the space.

The buoyantSimpleFoam Solver

buoyantSimpleFoam is one of OpenFOAM’s primary solvers for incompressible, buoyant flows under steady-state conditions. It is widely used in HVAC, building ventilation, and thermal comfort studies because it captures both forced convection (from mechanical ventilation) and natural convection (driven by temperature differences).

1. Solver Type

• Steady-state: Solves for the final equilibrium flow field

• Pressure-based: Uses pressure–velocity coupling

• Incompressible or weakly compressible air: Suitable for indoor airflows

• Uses the SIMPLE algorithm for iterative convergence

2. Physics Modeled

buoyantSimpleFoam solves the following coupled physics:

• Momentum (velocity) equation

• Pressure equation

• Energy (temperature) equation

• Buoyancy forces based on temperature-dependent density

• Turbulence modeling (k-ε, k-ω, SST, etc.)

Buoyancy is included using the Boussinesq approximation, which is suitable for HVAC temperature ranges.

3. Why buoyantSimpleFoam Is Ideal for HVAC Simulations

Indoor environments involve many buoyancy-driven effects:

• Warm air rising from people, equipment, and heating systems

• Cold supply air sinking due to higher density

• Stratification layers forming near ceilings

• Drafts and recirculation zones

• Temperature gradients affecting PMV/PPD

buoyantSimpleFoam is built specifically for this physics. It resolves:

• Airflow from diffusers

• Thermal plumes

• Natural convection near windows or heaters

• Mixed convection in large rooms

• Steady comfort conditions

4. Solver Workflow

Internally, buoyantSimpleFoam performs the following loop:

1. Predict velocity field

2. Solve pressure correction

3. Solve temperature and energy equations

4. Update turbulence fields

5. Include buoyancy corrections

6. Repeat until convergence

This iterative method continues until the solution stops changing significantly.

Use of buoyantSimpleFoam in tensorHVAC-Pro

tensorHVAC-Pro uses buoyantSimpleFoam as its core engine for most HVAC room simulations because:

• It is fast, thanks to its steady-state SIMPLE algorithm

• It accurately captures comfort-critical physics

• It handles ventilation + natural convection simultaneously

• It is robust for common HVAC temperature ranges

• It integrates well with comfort metrics like PMV, PPD, DR, and air age

The solver is fully automated within tensorHVAC-Pro—users do not need to edit dictionaries or manage OpenFOAM settings manually. All configurations, boundary conditions, turbulence models, and thermophysical properties are applied automatically based on the geometry and user inputs.