Temporal characteristics of wind
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Atmospheric motion varies both in time (from seconds to months) and in space (from centimeters to thousands of kilometers). The previous article discussed the spatial characteristics of wind, which generally depend on height from the ground, global and local geographic conditions.
Change of time
According to common practice, changes in wind speed over time can be divided into the following categories:
· Year-to-year change;
· Changes within the year;
· Diurnal variation;
· Short-term changes (gusts and turbulence).
Each type of wind speed variation and wind direction characteristics due to location are discussed in turn below.
The inter-annual variation in wind speed occurs on a time scale greater than one year. It has a great influence on the long-term output of the wind turbine. The ability to assess the inter-annual variation in wind speed at a given site is as important as the ability to assess the long-term average wind speed at a given site. Meteorologists generally agree that 30 years of data, and at least 5 years of data, are required to determine long-term climate or weather data in order to obtain reliable average annual wind speeds for a given site. However, short-term data logging can also be useful. Aspliden et al. (1986) proposed an empirical method based on statistics, one year of recorded data is generally sufficient to predict the long-term seasonal average wind speed, and the reliability is 90% when the prediction accuracy is within 10%.
Researchers are still looking for more reliable models to predict long-term average wind speeds. The complexity of the interplay between meteorological and geographic factors that influence wind speed changes makes this task very difficult.
Changes within the year
The monthly average wind speed or the seasonal average wind speed varies greatly from region to region, which is the same in most regions of the world. For example, in the eastern third of the United States, maximum wind speeds occur in winter or early spring. In the Great Plains, the northern central states, the Texas coast, the western basins and valleys, and the central and southern coasts of California, spring maximums are seen. The winter winds are greatest in the mountainous regions of the United States, with the exception of some areas in the lower Southwest, where the maximum occurs in the spring. The largest winds in the Oregon, Washington, and California wind corridors occurred in the spring and summer.
Figure 1 shows the significance of annual wind speed variation and its effect on available wind energy (error bars represent standard deviation).
Diurnal variation (time of day) In tropical and temperate regions, large changes in wind can also occur within a day or on a daily time scale. This form of wind speed variation results from differential heating of the Earth’s surface in the solar radiation cycle. The typical diurnal variation is that wind speeds increase during the day and are lowest between midnight and sun rise. For relatively flat areas in temperate regions, the diurnal variation in solar radiation is responsible for the diurnal variation in wind speed. Generally speaking, the largest diurnal variation occurs in spring and summer, and the smallest diurnal variation occurs in winter. In addition, the diurnal variation of wind speed varies with location and altitude. For example, at a certain height above the ground, the diurnal patterns of mountains and ridges can be quite different. This change can be explained by the transfer or mixing of momentum between the upper air and the lower air.
As shown in Figure 2, the diurnal pattern can vary greatly from year to year, even in areas with strong winds. While general characteristics of the daily cycle can be established from a single year of data, more detailed characteristics, such as the magnitude of the diurnal fluctuations and the timing of maximum wind speeds, cannot be accurately determined.
Concerns with short-term changes in wind speed include turbulence and gusts. Figure 3 is a record of the output of an anemometer (described later) showing the short-term wind speed variation patterns that exist under normal conditions.
Short-term changes usually refer to changes in time intervals of 10 minutes or less. The 10-minute average is generally determined by sampling values with a frequency of about once per second. Wind speed variations from less than 1 second to 10 minutes are generally acceptable. It’s just that random properties need to be considered when representing turbulent flow. In wind energy applications, the turbulent pulsation of the flow needs to be quantified, for example, in wind turbine design, in terms of maximum load, fatigue prediction, structural excitation, control, system operation, and power quality.
Turbulence can be thought of as random wind speed fluctuations imposed on the average wind speed. These pulsations occur in all three directions: longitudinal (along the direction of the wind), transverse (perpendicular to the direction of the mean wind speed), and vertically.
Gusts are independent phenomena that occur in turbulent wind fields. One way to characterize a gust is to determine its magnitude, time of rise, change in maximum gust, and time of fall. The wind turbine structural loads generated by gusts are affected by the above four factors.