Ventilating and building your PC for CFD calculations

Karim Segond is a freelancer consulting engineer specialized in Computational Flow Dynamics calculations with Simcenter FloEFD. CFD solves the governing equations of the flow using computational power and estimates the flow velocities, temperatures, and pressure. The computing capacity is the biggest limitation to the progress of CFD. He changes the PC every three years, and he prefers to assemble it on my own to select the best components for better performance and less noise.

The Law of Moore is over

Gordon Moore made 50 years ago the observation that the “number of transistors in an integrated circuit doubles about every two years”. his prediction has held since 1975 and has since become known as a "law". I remember that it was a golden rule in the nineties for the electronics industry and PC users. The size of the transistors has already been so miniaturized, that other laws of physis apply; the law of Moore is over.
Besides increasing the CPU frequency, the computing capacity is increased by building CPUs with a higher number of cores. The parallel computing capability of the CFD solver is to decompose the large problem domain into smaller subdomains, which will be run on different cores. The generation of the CAD model and the pre-processing can only run on one core, operations like importing and converting a complex CAD model might take between half an hour and two hours.
The increase in the core numbers means an increase in energy consumption and heat losses. A parallel calculation with Simcenter FloEFD was started on all cores of an intel i9-14900k with 74 GB RAM needed, the power measured at the socket was only 461 Watt or 17 euro cents per hour. We do not have to worry about the electricity bill. The fans should not run at full speed all the time as they are noisy; the BIOS allows to set the speed of the fans as a dependancy from the CPU core temperature.
Anyone who wants to build his high-end PC must be aware of the thermal challenges. The components, casing, fan, and cooler must be carefully selected and assembled, to maintain CPU-Temperatures below 100°C at full CPU speed, when performing calculations with all cores on a hot summer day.

Calculations on PC, workstations or clooud computing

For the complete CFD workflow, I prefer standard CPUs that are also used for gaming, such as Intel i7 and i9 or Ryzen 7 and 9. They offer good computing performance for both single-core and multi-core applications. The depreciation is low because used PCs that are as good as new are in demand.
Workstations based on Intel Xeon are significantly more expensive; they have up to 64 cores (128 threads), but a lower CPU clock frequency. They are mostly used in data centers and accessible via cloud computing.

Gaming PC vs CFD PC

Building your desktop PC is quite a fun little side project, it is also much better value for money as you save the assembly fees. The best period to buy PC components is between single day and black Friday. A gamer might prefer buying a second-hand PC as the 2-3 years old processors are fast enough to run the newest games. Engineers for CFD applications will always buy new PCs with the most expensive components, as they have much more money to spend. An increase of 20-30 % of the PC power means for CFD 20-30 % shorter calculation time.
Instruction manuals for hardware are usually basic, so anyone would look for advice on the internet and YouTube. The instructions on the Thermal Interface Material between the CPU and the cooler were very good. On the other hand, the videos on the ventilation were mostly inadequate. Any PC presented had an expensive and high-losses graphic card for computer games; the graphic cards recommended for CAD and CFD are smaller and have few losses.

Placing the fans

The casing fans should be placed at the front or at the back and push fresh air in the PC. The water coolers should be mounted at the top with their fans which pull air from the PC. The heated air is less dense than the cold air and it will naturally rise and mix with the surrounding air. The manufacturer of water coolers recommends mounting the fans placed below the cooler; however, the manufacturers of the fans recommend having a large distance, especially on the pressure side which is here not ensured. Karim aimed to place the fans above the coolers. The new PC was modelized and ventilation calculations with Simcenter FloEFD confirmed the best design.
The PC was assembled once with the fans above and once with fans below the coolers. The mainboard measures the CPU temperature on the spot, it was slightly lower with the fans above. The volume flow was not measured as this would necessitate a flow measurement device and a bonnet above the casing.