NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

Turbulence over Complex Terrain: A Wind Energy Perspective

Edward Patton, National Center for Atmospheric Research

NISE project m917

This poject involves the investigation of turbulence interacting with orography, both with and without vegetation. In the recommendations from a recent DOE sponsored workshop on wind resource characterization (Schreck et al, 2008), canopy turbulence and the interaction with orography were noted as critical aspects hindering the wind energy industry and its efforts to meet government mandated goals of 20% US energy coming from renewables before 2020. Vegetation interacts with turbulence in orography in complex ways that are only now beginning to be understood. Recently, it has become known that the pressure drag imposed on the flow by the trees can interact with the orographically induced pressure drag in ways that shift the pressure minima downstream of the hill crest compared to flow over a hill of equivalent roughness but with no resolved canopy. This shift in the pressure minima essentially makes the hill appear more steep to the flow than it otherwise would, and can induce separation on the lee side of terrain that would not be expected with current theories. This turbulence generation mechanism could be extremely damaging for wind turbines on subsequent ridges. Proper characterization of this process is essential for wind turbine deployment strategy and for turbine design capable to withstand these environments. Further investigation is required into the relationship between hill length, canopy height, and canopy density variations before we can develop parameterizations that account for these essential processes codes generating inflow conditions used in turbine load testing and also for predicting the impact of wind farms on downstream climate.

Wind turbines are frequently deployed in regions of undulating topography to take advantage of the expected speed-up of wind as the atmosphere is forced up over the hill. While this reasoning is quite simple for an idealized isolated hill in a non-stratified, non-vegetated environment, vegetation and stratification interacts with turbulence in orography in complex ways that are not yet clearly established. Recently, it has become known that the pressure drag imposed on the flow by the trees can interact with the orographically induced pressure drag in ways that shift the pressure minima downstream of the hill crest compared to flow over a hill of equivalent roughness but with no resolved canopy. This shift in the pressure minima essentially makes the hill appear more steep to the flow than it otherwise would, and can induce separation on the lee side of terrain that would not be expected with current theories. This turbulence generation mechanism could be extremely damaging for wind turbines on subsequent ridges. Proper characterization of this process is essential for wind turbine deployment strategy and for turbine design capable to withstand these environments. Further investigation is required into the relationship between hill length, canopy height, and canopy density variations before we can develop parameterizations that account for these essential processes codes generating inflow conditions used in turbine load testing and also for predicting the impact of wind farms on downstream climate. We intend to evaluate turbulence length scales and how they vary across these parameter space variations which will guide our understanding of the mechanisms generating the turbulence and our ability to introduce the influence of vegetation on turbulence in typical terrain for turbine deployment.