Winter One Airflow Details - A Technical, Nerdy Post.
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Hi everyone! ?
In this post, I will be detailing the Airflow Considerations, and Design, for Winter one. Until now, I've mainly been showing you CFD images, but hadn't really talked about what I was attempting to do, and what the point of all this work was. Now I'm ready to reveal some of that. Buckle in, though, it's going to be a long read 
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I. Why bother?
The promise of Small Form Factor systems has always been “you can cram desktop components into a small space, and it’s a better experience than those large, space-inefficient towers”
But the biggest drawback to SFF systems has always been thermals or noise. Most SFF cases handle mid tier components just fine, but most people will struggle to cool high end components crammed into a sub-10L chassis. The truth is there are few cases that can adequately cool top tier parts, like an OC’d 9900K + OC’d 2080Ti. And even the ones that *can* adequately cool these parts struggle to do so while remaining quiet.
At the end of the day, you cannot cheat physics… High end components in a Tiny Box = lots of heat. The only way to remove that heat? Airflow. And yes, this applies even if you liquid cool. Radiators are only as effective as the amount of air you can push through them to pull heat from the coolant.
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II. What Good Airflow in an SFFPC case look like?
Here are the results from the final airflow simulations for Winter One. There are 4 configurations that I’ll be showing. For a detailed explanation, please read onward
Bottom >> Top with solid side panels:
Bottom >> Top with Vented side panels:
All-exhaust with Vented Side Panels:
All-intake with Vented Side Panels: (NOT RECOMMENDED):
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III. Now let's talk about the Airflow Enhancements in Winter One:
1. Reducing Flow Restriction across panels
When designing a perforated sheet / panel, most people treat the percent open area as the “gospel” of panel design. While % OA is important, it is not everything. The Hole Size in all the panels for Winter One was chosen based on simulations that looked at ∆P across the plate, and edge perimeter vs Circle Area, while also balancing Percent Open Area.
Smaller holes are inherently more restrictive, even if you have a lot of them, compared to larger holes… this is because the flow of particles at any boundary becomes stagnant, and creates drag for nearby particles. So, the greater your Perimeter / Area ratio, the more flow restriction there will be.
The lower the ∆P across the plate, the easier it is for air to cross from one side of the plate to the other, preventing stoppage of flow. This is especially important when it comes to passive cooling, where natural convection leads to incredibly low airflow velocity, and too much flow restriction can trap the heat within the case.
2. Foot Height — ensuring the case can breathe
With a hole diameter chosen, I had to make sure that when Winter One was placed on a surface, The case was able to intake or exhaust air satisfactorily. Far too many small form factor cases use the smallest possible feet to avoid adding volume. However, this drastically harms cooling, whether the case is set up for intake or exhaust at the bottom.
Winter One's volume is 14.4L without protrusions, and 15.4L with protrusions.
After some testing, I found that a foot-height of 2cm ensured excellent airflow into / out of the case. While this worked very well for our design, it’s dependent on the hole geometry of your panels, as well as some other factors that are discussed in #6, so I would encourage other case makers to at least do some rudimentary testing, before settling on a foot height. For more restrictive panels, this value can be smaller, as your panel is restricting flow more than the availability of air through the gap created by your case feet.
3. Linear Airflow Path / minimizing 90º Turns
For every 90º Turn made by flowing air, you lose about ⅓ of the pressure (and therefore, velocity). So, if there is a 90º turn being made, it’s important to make sure that it’s happening for very good reasons… In the case of Winter One, the only recommended airflow configuration where 90º turns occur is the all-exhaust configuration, where the 90º turn allows each radiator to receive room-temperature ambient air.
In all other airflow configurations, a linear path is preserved, whether solid or perforated side panels are used, the bottom >> top airflow is maintained. Perforated Side Panels are better for helping thicker components, especially those pressed up against the panel itself, to breathe better, especially if intake fans are too close to the wall. Therefore, it is recommended, especially with Triple Slot GPUs, or taller CPU air coolers (above 55mm).
4. Turbulent vs laminar flow, and Optimizing for Human Perception of Acoustics.
One of the interesting things we found is that the transition from laminar to turbulent flow can create a single acoustically distinct whine. This has a significant effect on the perceived acoustics of the case. I tuned End Plate thickness, and distance from the plate to the fan blades, in order to create 2 smaller transitions to turbulent flow, before air is accelerated by the fans.
This spreads one acoustic peak into 3 separate peaks, creating a more pleasant noise profile.
5. Eliminating Internal Surfaces, and Boundary Layer Drag.
Barriers that flow “runs up against” are typically bad… Especially in situations where a 90º Turn is being combined with a barrier. Removing the central spine in Winter One led to a 25% increase in airflow velocity throughout the case regardless of airflow configuration.
Another area where we took great care to remove internal surfaces or barriers is In creating the frameless design of Winter One. In addition to opening up more internal volume for building in, the elimination of protrusions allows air to flow cleanly through the enclosure.
Together, these efforts eliminated almost all regions of stagnant air behind the GPU, motherboard, and power supply. This especially confers some benefits to NVME SSD cooling in instances where the drives are mounted on the backside of the motherboard.
6. Backflow Barriers on the End-plates
When fan speeds are pushed higher, we found that air had a tendency to loop around, and enter from the edge of the end plates, so the geometry of the inside of the end plate was altered, and the fan / radiator plate was widened, in order to provide a physical barrier to limit the backflow of air. This has an especially pronounced effect when fans are operated at higher speeds. At the same time, this does not narrow the intake or exhaust "cones" of each fan, which would negatively impact flow rates.
7. Utilizing Exterior Eddies to Separate intake and outlet Flow, a difficult problem with great benefits.
Notice the swirling currents near the top and bottom corners of the case? in the all-exhaust configuration, the This is not an accident. The feet height and the size, distance, and even spacing of the holes on the side panel, the top and bottom plates, and more, were ALL carefully controlled to purposely create large eddies *outside* the case.
Why go through the effort? Eddies are circular regions of “locally” stagnant airflow. If we design our case to precisely place them where they need to go,
these eddies separate the intake and exhaust flows, preventing recirculation of air in the case!!! This is one of the secrets to Winter One's ability to cool so well.
These are present in every supported airflow configuration in Winter One (
scroll up, and you'll see them!). Getting this to work with the variety of ways one can set airflow in Winter One was incredibly difficult. They are formed anywhere an intake and exhaust come too close to one another, and we’d normally see recirculating flow. Whether the case is laying horizontally, the "ends" are up in the air, or against a table surface, or facing the edge of the table... a lot of iterative problem solving across a point-field of variable combinations enabled me to find some optimal setups. I'm not going to speak too much about how I went about figuring this out, or exactly what variables are involved, but I will say, there's a significant amount of permutation involved. This was the single most difficult airflow problem to solve for Winter one. It took about 1500 hours of engineering and simulations. Around 25GB of CFD data sitting on my computer (more than half of ALL the CFD data!!!) is related to this problem alone.
8. Why Positive Pressure is BAD for SFFPCs.
This brings us to the 4th CFD image I highlighted earlier. This is the “All Intake” configuration, tested for Winter One. This is NOT a recommended or supported cooling configuration, and calls into question the common wisdom of operating SFFPC cases with positive pressure configurations. (obviously exceptions exist). Some of the wisdom commonly seen in PC building forums DOES NOT translate to the smaller size cases of the Small Form Factor world. Positive Pressure airflow configurations are one example of such pieces of wisdom.
In small form factor cases,
fan intake and exhaust flows are often far too close to one another to run positive pressure airflow configurations. The issue with positive pressure configurations is that the highest airflow velocity is at the intake, and the
lowest airflow velocity exists at the exhaust. If this exhaust comes too close to an intake, there is a significant risk of re-circulation, especially as the low exhaust velocities do not meet the conditions necessary to create the Polar Vortex Barriers that we rely on to keep airflow performance of Winter One in peak condition.
The All-Intake flow data is included so we can see what happens when these barriers breakdown — OVER 60% of the warm air leaving Winter One is too slow to escape the intake of the fans, and is sucked back in!!! Mind you, in this simulation, the fans are only operating at 1000 rpm. This problem worsens, the higher the fan speed gets. Furthermore, the “bad” (unintentional) kind of eddies are formed within the case, further trapping hot air. Both of these issues lead me to suggest that no one should use an all-intake airflow configurations in Winter One.
I'm not saying positive pressure is bad -- It has its uses for keeping dust out of the system!!! So, If you’d like to keep some positive pressure in the case to combat dust, consider running the intake fans at about +150 rpm, compared to the exhaust fans,
creating more balanced flow conditions with a push/pull setup, through the case. The enemy is not
necessarily positive pressure --
the enemy is low exhaust velocities. So please be mindful of those
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IV. Concluding Statements, Mini Update.
When I set out to design Winter One, my goal was to create a cooling and airflow focused SFF case. I hope that the little peek I gave you behind the scenes of Winter One's "CFD Driven Airflow Design" have given you a fair idea of what such a statement
actually means
. If even a few people understand and appreciate how much thought and effort have gone into crafting the flow of air both inside and outside the case, it will be worth it. After a year of work, I’m pretty happy with the results.
Thanks again for all the enthusiasm and support you’ve shown me / this project. I literally could not have done it without you… ❤
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And finally a couple of updates on Winter One:
1. Beta applications are still open, so if you want to be one of the lucky 3 that get to build early in Winter One, please
fill out the form HERE. The application will close by Friday August 21, 2020!
2. Website Version 2.0 is coming in the next couple of days, with more information about hardware compatibility, build instructions, and more.
3. I’m just waiting on 1 more set of parts for the Winter One Prototype, before I can make a full build video for you all to enjoy
