Lateral acceleration zones in complex terrain

Evaluating wind energy potential in complex terrain typically requires a detailed analysis of the prevailing low-level flow over and around obstacles, and frequently this has to be broken down into seasonal and even climate-scale regimes or cycles where possible, in order to accurately assess long-term wind speeds at hub-height. For a low-level flow analysis, typical upstream stability values in areas of significant barriers are such that most of the low-level flow does not ascend significant barriers; rather, it tends to be blocked into a direction parallel to the barrier. If the barrier is generally three-dimensional, ie. of finite length, this flow will move around the obstacle.

The work of Justus (1985) and later summarized by Whiteman (2000) has documented that the flow frequently accelerates on the lateral sides of the barrier in this situation. As Whiteman points out, “When stable air flows around an isolated peak or on the edges of an elongated mountain range, the highest wind speeds are on the hillsides where the flow is parallel to the peak's contour lines (ie. as this flow progresses around the obstacle, its elevation remains constant).”

When one considers the complex terrain of the US west, the implications of these lateral acceleration zones loom large in the context of wind energy. For placement of wind turbines in locations that exhibit climatologically high wind speeds in the lowest ~150 m of the atmosphere, a fixed site with wind maxima forced by a stationary (terrain) feature is advantageous. In evaluating these regions, existing site long-term wind measurements are typically sparse due to the lack of human population, the general placement of aviation facilities away from strong mountain-induced winds, and the complicated logistics and maintenance associated with placing measurement towers on steeply-sloped terrain that may be located on federal land.

Here are some primary questions that need to be answered in order to accurately evaluate wind potential in or near lateral acceleration zones: 1. Upstream stability: is the Froude number in the blocked or partially-blocked regime such that the flow does not typically ascend the barrier? How often does this occur? How can one extend the upstream stability approach to three dimensions (eg. the 3-D nature of the barrier)? 2. Is the undisturbed flow upstream strong enough in the first place (from a climatological standpoint) to put the associated accelerated wind speed into the economically-viable category for hub-height wind speeds? 3. How much does the flow accelerate relative to the upstream inflow and how can the horizontal extent of the acceleration zone be estimated? How does this relate to potential turbine array design, and is the acceleration associated with unacceptable levels of turbulence or eddies/rotors? 4. What is the vertical extent of the acceleration zone, ie. what is the magnitude of acceleration at hub-height?

This manuscript will take a look at past studies of this phenomenon and attempt to place these and some ongoing measurements in the context of the questions listed above. In addition, other relevant issues and wind features/effects such as gap flow, observed tree flagging, and the potentially complicated logistics of placing wind turbines in acceleration zones will be discussed.