# What Is Computational Fluid Dynamics?

Written by: Dave Martin
1/24/2022

Computational fluid dynamics is simulation and analysis performed in computer-aided design (CAD) software to calculate the flow of liquids or gases in or around a product.

It is a multiphysics solution since it involves the interaction of multiple phenomena including fluid dynamics, thermodynamics, and conservation of momentum. Like finite element analysis (FEA), the fluid volume is broken up into smaller elements that are composed into a matrix. CFD has many uses beyond product development and aerodynamics, such as weather forecasting and visual effects.

In product development, CFD enables us to design products and systems that meet requirements for fluid flow and heat transfer. Let’s take a look at how this works.

## Capabilities

By using CFD software, you can calculate and display fluid quantities such as:

• Velocity, the speed and direction of particles inside or outside the model.
• Temperature.
• Pressure.
• Vortices, which provide an indication of the spinning motion of the fluid at the points throughout the domain.

These results can be calculated and displayed (1) at specific locations in a model; (2) for the maximum or minimum value on a surface or a component; or (3) throughout the fluid volume. When displayed in the fluid, the results can be depicted as color contours, particles, a direction field, or streamlines. To further facilitate understanding of the behavior and to accelerate calculations, the results can be displayed at a specific cut plane.

## General Process

CFD can be executed by performing the following steps:

1. Start with a model. Prior to entering the CFD simulation environment, create the 3D CAD part or assembly to be analyzed. The geometry can be native to the CAD software or imported.

2. Define the fluid domain. The liquid or gas in the simulation can be either internal, like water flowing through a piping system, or external, like air flowing over the external surfaces of a vehicle. Define the volume region and apply material properties to the fluid, including density, viscosity, coefficient of thermal expansion, specific heat capacity, and thermal conductivity.

3. Establish boundary conditions. These represent the movement of fluids at the inlet and outlet of the analysis model. The fluid motion can be defined by flow velocity, inlet and outlet pressure, and mass flow. For internal flow, additional boundary conditions include swirl inlet (velocity with both normal and radial components), rotating wall to simulate moving components, and gravity.
Include thermal conditions. Prescribed temperatures at geometry in the model can also be used as a boundary condition. Thermal loads in the system can be defined as heat flow, heat flux, convection, and convection radiation.

4. Perform the analysis. The CFD study can be run as either transient to see the effects on flow and temperature as a function of time, or steady state to see the results at equilibrium.

5. Evaluate the results. As mentioned above, a variety of quantities can be displayed graphically in the model to provide an understanding of the system behavior.

6. Optimize the system. In real time, the CFD analysis updates with changes to your model, providing instant feedback for improving your model for its operating environment.

CFD used to be the domain of specialists with years of training and experience in the field. However, advances in recent years have enabled designers and engineers to perform their own CFD simulations without the aid of experts.

In addition, simulations that used to take hours to calculate can now be performed in minutes or seconds. Real-time simulation results act as a design assistant to help engineers make the right choices as they build their products, rather than waiting for a dedicated specialist to become available.

If you design vehicles or products with enclosures where air or fluid flow is important, you should consider adding CFD to the suite of tools that help your designers and engineers perform their best.

## Simulation-Driven Design

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