At Eldridge, we are in the business of solving the most difficult ventilation problems. To do that, sometimes we will use fluid flow analysis (FFA) software to solve for the flow, pressure and velocity of a complex ventilation system. Some customers will ask if we also use computational fluid dynamics (CFD) software. They are always surprised when we say no since CFD software is now a widely used analytical tool. In the first of a three part series on using FFA, I will answer the question as to why we don’t use CFD software for most applications by first defining what FFA and CFD mean and then discussing the major differences in the two types of software.
FFA and CFD Defined
The definition of FFA is the study of the movement of gases or liquids to determine the flow behavior and parameters. For our purposes, we simplify the FFA definition to mean the study of air flow, at standard density, through a ventilation system to determine the parameters of volume, pressure and velocity. By calculating only those three parameters, we can then determine that we have the correct fan to make the ventilation system work as designed.
CFD is defined as the use of applied mathematics, physics and computational software to visualize how a gas or liquid flows, as well as how the gas or liquid affects objects as it flows past. Simply put, it is a more sophisticated form of FFA that can render 2D or 3D visualizations of fluid flows that can be viewed from different angles.
The foundation of CFD is built on the Navier-Stokes equations, the set of partial differential equations that describe fluid flow. CFD models are built in cells that use the Navier-Stokes equations to solve for velocity, temperature, pressure, and other variables within the cell. Then the Navier-Stokes equations are used to relate the cell results to each other in order to yield a complete description of the fluid flow throughout the system.
Differences Between FFA and CFD Software
We have been using the same software to do our FFAs for over 20 years. What we have found when comparing it to CFD software to model ventilation systems is that the CFD software assumes a steady state air flow. That is a problem because we need to know how the air flow changes when the geometry of the ventilation system changes. Because our FFA software allows the input of an entire fan curve and not just a single data point on the curve, it automatically moves along the fan curve to show how the air flow changes when a component is added or modified in the ventilation system design.
The other big difference in the two types of software is the amount of time and computing power that it takes to create a model. CFD models yield the best results when they are broken into smaller cells. Of course, that takes more time to input the data and requires more memory to solve all of the Navier-Stokes equations.
Conclusion
Despite the drawbacks with CFD software, we found it to be a very useful tool when we are innovating new ventilation system components. However, for most of what we do, simpler is better.
In part two of this series on fluid flow analysis, I’ll discuss how we build models in FFA software.