Which way does the NH-D15S sit on top of the CPU?

Solution
You can actually mount it either way, but generally vertical (|| in your example) is the way to go. The RAM cutouts on the cooler are designed for vertical mounting and you'll see the entire cooler is offset slightly too. Noctua call that "Asymmetrical layout". When you mount it vertically you'll find it's offset towards the top of your case. That makes the cooler less likely to infringe on the first PCIe slot.

Noctua have extensive support and compatibility lists if you need any further advice: https://noctua.at/en/products/cpu-cooler-retail/nh-d15s/video
You can actually mount it either way, but generally vertical (|| in your example) is the way to go. The RAM cutouts on the cooler are designed for vertical mounting and you'll see the entire cooler is offset slightly too. Noctua call that "Asymmetrical layout". When you mount it vertically you'll find it's offset towards the top of your case. That makes the cooler less likely to infringe on the first PCIe slot.

Noctua have extensive support and compatibility lists if you need any further advice: https://noctua.at/en/products/cpu-cooler-retail/nh-d15s/video
 
Solution