Terrain matters because obstacles slow the wind and fill the air behind them with turbulence, which damages a turbine even when the average speed looks good. An obstacle’s disturbed wake extends roughly 10 times its height downwind, so a 25 m tree line affects the air for about 250 m behind it, and a rotor that turned at 250 RPM all year would bend each blade 131 million times, with every gust adding to the fatigue.
Part of Advanced Wind Turbine
Terrain effects are the ways hills, trees and buildings change the speed and the steadiness of the wind a rotor sees.
Slow Air And Rough Air Are Different Problems
Ground friction slows the wind near the surface, which a taller tower fixes (Site Selection & Tower Height). Turbulence is a second problem: rapid, irregular changes in speed and direction. The Tower Height page makes the point that turbulent wind is not just slower but chaotic, causing fatigue stress on blades and bearings and shortening a turbine’s life. The carrier agrees: obstacles within 150 m create turbulence that damages turbines.
How Far An Obstacle’s Wake Reaches
The Tower Height page gives the wake as roughly 10 times the obstacle’s height downwind. Applying it:
| Obstacle height | Wake length (10 × height) |
|---|---|
| 5 m (hedge, low building) | 50 m |
| 10 m | 100 m |
| 15 m | 150 m |
| 25 m (tall tree line) | 250 m |
The carrier’s 150 m rule and this 10-times rule agree only for a 15 m obstacle. For anything taller, the wake runs past 150 m, so use the longer distance: a turbine 200 m downwind of a 25 m tree line is still inside its wake although the carrier’s 150 m circle would exclude it. The longer-distance version is my combination of the two figures, not a stated corpus rule. There are two ways out. Raise the hub to clear the obstacle by the carrier’s 10 meters (25 + 10 = 35 m, above the carrier’s 30 m guyed-pipe limit), or site the turbine outside the wake, upwind of the obstacle or more than 250 m past it.
The wake matters only when the obstacle is upwind. A tree line downwind of the turbine does nothing, so measure the clearance along the prevailing wind direction and also check the directions that carry strong winds.
Landforms Decide Whether Wind Is Fast, Steady Or Rough
The carrier calls hilltops, ridgelines and coastal bluffs ideal, and observing trees: permanently bent trees mark consistently strong wind. The rest of this table is qualitative and recalled, not from the corpus.
| Terrain | What the wind does | Verdict |
|---|---|---|
| Windward top of a smooth hill | accelerates up and over the crest | good |
| Lee side of a hill or steep ridge | flow separates, rough and gusty | avoid |
| Valley floor | funnelled along the valley axis, sheltered across it | direction-dependent, check with streamers |
| Open plain or coast | smooth and steady | good, low shear exponent |
| Forest edge | strong wake behind the trees, large shear | poor close to the edge |
| Town or village | turbulent in the roughness layer above rooftops | poor unless the hub clears it by a wide margin |
Method: Measure Turbulence With What You Have
Step 1: Fit the anemometer at the intended hub height (or, on a temporary mast, as high as you can).
Step 2: Read the speed every 10 seconds for 10 minutes, which gives 60 readings.
Step 3: Compute the mean and the standard deviation. Turbulence intensity = standard deviation ÷ mean.
Step 4: Compare candidate sites. For illustration: a site with a mean of 5.0 m/s and a standard deviation of 1.0 m/s has intensity 1.0 ÷ 5.0 = 0.20; a site behind trees with the same mean and a deviation of 2.0 m/s has 0.40, twice as rough. These numbers are invented to show the method, and the corpus gives no limit. Choose the site with the lower figure when the means are close.
Step 5: Watch the streamers. The carrier hangs streamers at tower height to see wind patterns. A streamer that whips between directions in a steady wind marks rough air.
Why Fatigue Is Counted In Revolutions
A blade lying horizontal sags under its own weight and, a half-turn later, hangs from its root, so every revolution reverses the bending load. A rotor turning at an average of 250 RPM turns 250 × 60 × 24 × 365 = 131 million times a year if it never stops. Gusts add loading cycles of their own on top. That is why roughness shortens the life of wooden blades and bearings far more than the average speed would suggest (Carving Wooden Blades, Blade Balancing).
Common Mistakes
- Choosing by average speed alone — a fast but turbulent site wears the machine out sooner.
- Measuring clearance in one direction — wake distance depends on where the strong winds come from.
- Treating the 150 m circle as a limit for tall obstacles.
What’s Next
- Learn next: Blade Design & Aerodynamics for the rotor that must live in this air
- Combine with: Tilt-Up Design for Maintenance, since rough sites need more inspection