WIND FLOW MODELING - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 176

diverse characteristics and applications to take such a position. Instead, we provide
an overview of the different modeling approaches that are available, including their
strengths and weaknesses, and establish some general guidelines applicable to all
methods—and most important, the appropriate use of measurements to manage and
limit errors.
13.1 TYPES OF WIND FLOW MODELS
Spatial modeling approaches can be conveniently classed in four general categories:
conceptual, experimental, statistical, and numerical.
13.1.1 Conceptual Models
Conceptual models are theories describing how the wind resource is likely to vary
across the terrain. They are usually based on a combination of practical experience
and a theoretical understanding of boundary layer meteorology.
A very simple conceptual model might state that the wind resource at one location
(a turbine) is the same as that measured at a different location (a met mast). This could
be quite a good model in relatively flat terrain or along a fairly uniform ridgeline,
for example. Where the terrain and land cover vary substantially, a more nuanced
picture is usually required. This might include theories concerning the influence of
elevation on the mean wind speed, the relationship between upwind and downwind
slope and topographic acceleration, channeling through a mountain gap, and the impact
of trees and other vegetation. These concepts or theories are then turned into practical
recommendations for the placement of wind turbines, accompanied by estimates of
the wind resource they are likely to experience.
As wind projects become larger and are built in ever more varied wind climates,
it becomes more and more difficult to implement a purely conceptual approach in a
rigorous or repeatable way. Nevertheless, a good conceptual understanding of the wind
resource is an invaluable asset in all spatial modeling. Most important, it provides a
check on the reasonableness of other methods. A good conceptual understanding is
better than a bad numerical model or a good numerical model that is wrongly applied.
13.1.2 Experimental Models
Experimental in this context refers to creating a sculpted scale model of a wind project
area (such as that shown in Figure 13-1) and testing it in a wind tunnel. (This is also
known as physical modeling , a term we avoid because of possible confusion with
numerical wind flow models, which are based on physical principles.) The conditions
in the wind tunnel, such as the speed and turbulence, must be matched to the scale of
the model to replicate real conditions as closely as possible. While the wind tunnel
is running, the wind speeds are measured at various points on the scale model using
tiny anemometers (usually hot-wire anemometers). The results form a picture of how