Computational Fluid Dynamics (CFD) uses computer calculations to solve thermal and fluid problems. These results can then be processed and visualized for further insight and understanding. Unlike traditional testing, these simulations do not require physical construction or data acquisition hardware. Another advantage is that alternate scenarios can be investigated relatively easily.
Photo credit: NASA
About fifty years ago, CFD use was limited to university research and specialized programs, such as NASA. Today, with the advent of increasing computer power and further development of algorithms, CFD can be reasonably utilized in a broad range of engineering applications.
Brief History of CFD:
While it may seem new, the basic methods of CFD were known at the time of Sir Isaac Newton (c. 1700s). Since that time and prior to the 1960s, fluid flow problems were solved almost exclusively using analytical (aka hand calculation) and experimental methods. Analytical methods generally have the limitation of being simplified so that a solution is feasible. Experimental methods can require extensive time and cost to setup plus data is typically limited to the number of sensors one has. As computer capability evolved, solutions that did not have these limitations and that could take into account more real-world complications were possible. In the progressing decades, refinement of CFD algorithms, graphical representation and computing power lead to adoption by the aerospace, automotive and other non-aerospace industries. Decades ago, these analyses could take days to run with the computer power available at the time.
Today, 3D CAD software, such as SolidWorks®, can produce mathematical representations of complex geometries and assemblies. These models can then be seamlessly imported into CFD software, such as SolidWorks® Flow Simulation. Then the software essentially breaks down the fluid and solid geometry into smaller volumes so that these complex geometries can be resolved. Based upon known inputs and physics, the software calculates the end result for the entire volume.
CFD in Forensics:
Cases involving leaking fuel gas (such as propane or natural gas) are relatively common in forensic engineering. The image below shows results from one of those cases. In this scenario, propane leaks from the top of one of the cylinders. Weather data and scene photographs from the time of the incident showed that there was a wind out of the west. The computer simulation results shows the direction that the propane travels as it mixes with the surrounding air in that crosswind. It also estimates the local concentration of propane; red areas indicate higher concentrations of propane while blue areas indicate lower concentrations of propane. Note that downstream, the concentration of propane tends towards the ground due to buoyancy effects. These contour plots are similar to weather maps, which show temperature gradients throughout an area. The image below shows one slice of the results, but the data from this simulation is available in all three dimensions.
After the first or baseline simulation is completed, additional scenarios can be run relatively easily. In the example shown here, that could be wind speed, wind direction, leak rate, cylinder configuration, level of snow on the ground, etc.
Fuel gas migration is just one of the many applications that CFD is capable of modeling. Simulation of heat transfer, pipe fluid flow, humidity, cavitation estimation are also feasible among others.
Right Sized Analysis; From the Simple to the Complex:
One does not need to jump directly to a full intricately detailed model that takes several weeks to setup and run. Often times, a simplified model yields an accurate and sufficient result much like the hand calculations that are still a cornerstone of engineering education today. CFD is a flexible and powerful tool that can be sized for the application. Decades ago, CFD was only feasible for organizations like NASA using supercomputers. Today, CFD is more readily available to solve a variety of engineering problems efficiently and with the added benefit of visualization.