Terrain classification and ground airflow

Terrain classification and ground airflow

Topographical features can have such a large effect on wind turbine energy output that the economics of the entire project depend entirely on the appropriate selection of the site.

In the previous article, two methods for modeling vertical wind profiles (logarithmic wind profile¬†and power exponential wind profile) were described. But these methods were developed for flat and homogeneous terrain. Any irregularities in the Earth’s surface can be expected to affect the flow of wind, so these prediction tools need to be applied in combination. This article provides a qualitative discussion of the terrain effects of several of the more important terrains of general interest.

1. Terrain classification

The most basic terrain classification divides terrain into flat terrain and uneven terrain. Many authors define uneven terrain as complex terrain (ie, as an area where topographic effects have a large effect on flow through the considered ground area). Flat terrain has only a small amount of irregularities, such as trees, shielding belts, etc. (see Wegley et al., 1980), and uneven terrain has large-scale rise and fall, such as hills, ridges, valleys, and canyons. In order to quantify flat terrain, the following conditions must be observed.

Note that some of these rules take into account the geometry of the wind turbine:

The height difference between the wind farm site and the surrounding terrain cannot be greater than about 60m anywhere within the 11.5km diameter around the wind farm site.

There should be no hills with a size ratio (height to width) greater than 1/50 within 4km of the upwind and downwind directions of the site.

The height difference between the lower end of the wind rotor and the lowest terrain must be greater than three times the maximum height difference of the terrain within 4km upstream.

According to the research of Hiester and Pennell (1981), uneven terrain or complex terrain has various characteristics, and they are usually classified as follows: (1) isolated rise or fall of the ground; (2) mountainous terrain. Air flow in mountains is very complex because the ground rises and falls randomly. Therefore, the airflow in such areas is divided into two categories: small-scale and large-scale. The difference between the two is drawn in comparison with the Earth’s boundary layer, which is assumed to be about 1 km thick. Thus, if the height of a mountain is small (about 10%) relative to the thickness of the Earth’s boundary layer, it has a small-scale topographic feature. It should be pointed out here that information on wind direction should be taken into account when performing terrain classification. For example, if an isolated mountain (200 m high and 1000 m wide) is located 1 km south of the proposed site, the site may be marked with uneven terrain. But if the wind blows in this direction only 5% of the time, and the average wind speed is low, say 2m/s, then the terrain can be considered flat.

2. Airflow on flat ground with obstacles

Airflow over flat terrain with man-made or natural obstacles has been extensively studied. Man-made obstacles are defined as buildings, silos, etc. Natural obstacles include rows of trees, shielding tapes, etc. The usual approach for man-made obstacles is to treat the obstacle as a rectangular block and consider the flow to be two-dimensional. The momentum wake produced by this flow pattern is shown in Figure 1. The free shear layer starts to separate from the leading edge of the obstacle and attaches downstream, forming a boundary layer between the inner recirculation zone (vortex) and the outer flow zone.

Terrain classification and ground airflow
Figure 1 – Schematic diagram of a momentum wake (Rohatgi and Nelson. 1994)

The results of the analysis to quantify the impact of man-made obstructions are shown in Figure 2, which presents the variation in available power and turbulence intensity in the wake of a pitched-roof building. Note that the results in the figure are only applicable to a building at a height h from the ground, and the power loss is already small downstream from the building at a distance of 15h.

Terrain classification and ground airflow
Figure 2 – Wind speed, power, turbulence effects downstream of buildings (Wegley et al., 1980)

3. Airflow on flat ground with varying ground roughness

The ground conditions of most natural terrains are not the same, but vary widely along the ground. This affects the local wind profile.

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