wind energy resource evaluation in a site of central italy ... - WindSim

6.6 Turbulence Intensity Vertical ProfilesIn this paragraph vertical profiles of turbulence intensity (T.I.) are reported andanalyzed. The figures 6.5 and 6.6 show the comparison between the measuredturbulence intensity values and the results of CFD simulation.The experimental T.I. values have been calculated by the time-history of wind dataas a ratio between the standard deviation of wind velocity and the mean windvelocity evaluated for the instantaneous measurements taken in 10 minutes6.3.3) . .=whereσ is the standard deviation of wind velocity [m/s]U is the mean velocity [m/s]By organizing experimental data for wind direction it is possible to group theseangles in sectors (in the specific case one sector every 30° for a total of 12 sectors)and to consider the wind velocity and the associated T.I. value. Then, it is possible tocreate twelve scatter graph, where T.I. is a function of U. In general, these graphsshow a cloud of points that can be approximate by an hyperbolic function. Datareaching an infinite value of T.I. for low wind velocity are present while the expectedtrend goes toward zero when wind velocity grows. The common practice eliminatesthe high values of turbulent intensity associated to low wind speed, that areprobably affected by thermal effects, when an interpolation of the cloud of points isrequired. According to this practice rule the TI was evaluated rejecting the valuesconnected to low mean wind velocity. The threshold value was chosen following theconclusions of the previous paragraph 6.5, where a threshold range was indicated inU30 = 3÷4 m/s. In these particular analysis the value of U=4 m/s has been used tofilter experimental data.The CFD simulations did not take into account thermal effects, thereafter underestimating the turbulence intensity data. A procedure of rejecting these data is thenneeded to reduce the thermal effects in the experimental data used for comparison.101