We Went to NASA To Solve a Computer Mystery

TL;DR

LMG tested PC fan intake clearance at NASA Langley using PIV and acoustic arrays, finding 15mm is the minimum safe distance from any obstruction.

Key Points

• 1.Tufting revealed reversed airflow at extreme restriction. Using a Noctua NFA12X25 with glowing string tufts and a high-speed Kronos 4K camera at 1,000fps, NASA showed fans begin losing efficiency at 1.5–2cm clearance and actually reverse airflow when nearly touching a panel.
• 2.Particle Image Velocimetry (PIV) quantified the dead zone. At 15mm clearance, the stagnant hub zone grows dramatically and airflow curls outward instead of moving straight through — reducing streamwise momentum needed to push air through heatsinks or radiators.
• 3.Adding a radiator makes restriction catastrophic. With a water-cooling radiator creating back pressure, a completely obstructed fan produced near-zero flow; even at 15mm gap, only the outer 50% of fan blades contributed usable airflow and speed dropped roughly in half.
• 4.Noise increases — not decreases — when fans are obstructed. Testing in NASA Langley's anechoic chamber (as quiet as 18dB) with a 40-microphone spiral phase array showed a broad-spectrum noise increase at 15mm restriction, caused by the turbulent unsteady flow from the stalled hub zone.
• 5.The practical takeaway: 15mm minimum, 20mm with obstructions. Keep intake fans more than 15mm from any surface for acceptable performance; 20mm or more is recommended when a heatsink or radiator adds back pressure, and more clearance always improves acoustic performance.
• 6.NASA Langley is the OG aerospace research facility. Founded in 1917 as part of NACA before NASA existed, it employs 3,500 people and currently supports the Artemis moon program; its hypersonic test complex hosted all the fan airflow and acoustic experiments in this video.