Ventilating and building your PC for CFD calculations

Karim Segond is a freelance consulting engineer specializing in Computational Fluid Dynamics calculations with Simcenter FloEFD. CFD solves the governing equations of the flow using computational power and estimates flow velocities, temperatures, and pressure. Computing capacity is the biggest limitation to the progress of CFD. He replaces the PC every three years, and he prefers to assemble it himself to select the best components for better performance and less noise.

The Law of Moore is over

Gordon Moore made the observation 50 years ago 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 physics apply; Moore's Law is over.
Besides increasing the CPU frequency, computing power is increased by building CPUs with a higher number of cores. The parallel computing capability of the CFD solver allows decomposing the large problem domain into smaller subdomains, which are run on different cores. The generation of the CAD model and 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 number of cores 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 requiring 74 GB RAM; the power measured at the socket was only 461 Watts, 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 setting the speed of the fans dependent on the CPU core temperature.
Anyone who wants to build their 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 PCs, Workstations, or Cloud 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. 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 own 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 Singles' Day and Black Friday. A gamer might prefer buying a second-hand PC as 2-3-year-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 usually have a higher budget. An increase of 20-30% in PC power translates to 20-30% shorter calculation times for CFD.
Instruction manuals for hardware are usually basic, so people often look for advice on the internet and YouTube. The instructions regarding the Thermal Interface Material between the CPU and the cooler were very good. On the other hand, the videos about ventilation were mostly inadequate. Every PC presented had an expensive and high-power-consumption graphics card for computer games; the graphics cards recommended for CAD and CFD are smaller and have lower power consumption.

Fan Placement

The case fans should be placed at the front or at the back and push fresh air into the PC. The radiators for water cooling should be mounted at the top with their fans pulling air from the PC. Heated air is less dense than cold air and will naturally rise and mix with the surrounding air. The manufacturer of water coolers recommends mounting the fans below the radiator; however, fan manufacturers recommend having a large distance, especially on the pressure side, which is not ensured here. Karim aimed to place the fans above the radiators. The new PC was modeled, 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 radiators. The motherboard measures the CPU temperature on-die; it was slightly lower with the fans above. The volume flow was not measured as this would necessitate a flow measurement device and a hood above the case.