Turbulent flows are messy—swirling, chaotic, and full of eddies that vary from millimeters to meters. Placing probes too close clogs the flow; too far, and you miss critical details. Here’s how to calculate the perfect spacing.

Start with the eddy size:
Turbulence has “energy scales”—large eddies (10–100cm) carry most energy; small eddies (0.1–1cm) dissipate it. Measure your flow’s integral length scale (L) using a hot-wire anemometer—it tells you the average size of large eddies.

Rule of thumb:
Space probes 0.5L to 1L apart. For example:

· If L = 20cm (typical in automotive wind tunnels), space probes 10–20cm apart.

· For smaller eddies (e.g., L = 5cm in a small turbine test), use 2.5–5cm spacing.

This captures both large-scale patterns and small-scale fluctuations.

Avoid gridlock:
Probes themselves create turbulence. Keep the total cross-sectional area of all probes <5% of the wind tunnel’s test section area. For a 1m² section, that’s 0.05m² total probe area.

Adjust for speed:
Faster flows (supersonic) stretch eddies. Multiply spacing by the Mach number (e.g., 2x for Mach 2 flow) to account for this.

Test first:
Run a trial with temporary tape markers (no probes) to map flow patterns. Then place probes where turbulence is strongest (e.g., behind a model’s wing).

Stuck measuring L? Borrow a hot-wire anemometer for a day—most universities rent them for <$100.