| Representative Turbulence Intensity | |
For a set of 10-minute time steps, each of which contains a value of turbulence intensity, the representative turbulence intensity is equal to the 90th percentile of the turbulence intensity values, meaning the value below which 90% of the values fall. If the turbulence intensity values conform to a normal distribution, the 90% percentile is equal to the mean plus 1.28 standard deviations.
Windographer does assume that the turbulence intensity values are normally distributed, so it calculates the representative turbulence intensity as the mean plus 1.28 standard deviations.
For example, the following table contains 30 time steps of data on mean wind speed, standard deviation of wind speed, and turbulence intensity, which is the ratio of the two:
| Date/Time | Wind Speed (m/s) | Turbulence Intensity | |
|---|---|---|---|
| Mean | Std. Dev. | ||
| 2020-12-22 15:30:00 | 14.5 | 1.2 | 0.0828 |
| 2020-12-22 15:40:00 | 14.2 | 1.3 | 0.0915 |
| 2020-12-22 15:50:00 | 12.6 | 1.2 | 0.0952 |
| 2020-12-22 16:00:00 | 12.3 | 1.0 | 0.0813 |
| 2020-12-22 16:10:00 | 13.3 | 0.8 | 0.0602 |
| 2020-12-22 16:20:00 | 13.0 | 1.0 | 0.0769 |
| 2020-12-22 16:30:00 | 13.4 | 1.0 | 0.0746 |
| 2020-12-22 16:40:00 | 13.7 | 1.0 | 0.0730 |
| 2020-12-22 16:50:00 | 12.8 | 1.0 | 0.0781 |
| 2020-12-22 17:00:00 | 13.7 | 1.0 | 0.0730 |
| 2020-12-22 17:10:00 | 14.2 | 0.9 | 0.0634 |
| 2020-12-22 17:20:00 | 13.9 | 0.7 | 0.0504 |
| 2020-12-22 17:30:00 | 13.8 | 0.9 | 0.0652 |
| 2020-12-22 17:40:00 | 13.6 | 1.1 | 0.0809 |
| 2020-12-22 17:50:00 | 13.1 | 1.0 | 0.0763 |
| 2020-12-22 18:00:00 | 12.2 | 1.1 | 0.0902 |
| 2020-12-22 18:10:00 | 12.5 | 1.0 | 0.0800 |
| 2020-12-22 18:20:00 | 12.4 | 0.8 | 0.0645 |
| 2020-12-22 18:30:00 | 12.7 | 0.8 | 0.0630 |
| 2020-12-22 18:40:00 | 12.3 | 1.1 | 0.0894 |
| 2020-12-22 18:50:00 | 12.1 | 0.7 | 0.0579 |
| 2020-12-22 19:00:00 | 12.7 | 0.7 | 0.0551 |
| 2020-12-22 19:10:00 | 11.4 | 1.0 | 0.0877 |
| 2020-12-22 19:20:00 | 12.1 | 1.0 | 0.0826 |
| 2020-12-22 19:30:00 | 11.6 | 0.8 | 0.0690 |
| 2020-12-22 19:40:00 | 11.4 | 0.9 | 0.0789 |
| 2020-12-22 19:50:00 | 10.7 | 0.7 | 0.0654 |
| 2020-12-22 20:00:00 | 11.0 | 0.8 | 0.0727 |
| 2020-12-22 20:10:00 | 10.8 | 0.8 | 0.0741 |
| 2020-12-22 20:20:00 | 11.7 | 0.7 | 0.0598 |
Those 30 turbulence intensity values (highlighted in the table) have a mean of 0.0738 and a standard deviation of 0.0113. Windographer would therefore calculate the representative turbulence intensity over that period as 0.0738 + 1.28 * 0.0113 = 0.0883.
The 3rd edition of IEC standard 61400-1 plots the representative turbulence intensity versus speed for turbulence classes A, B, and C. You can plot these curves, with your own measured representative TI for comparison, in the Turbulence window.
See also
Characteristic turbulence intensity