Modeling flow over roughness changes and applications to wind energy for sites on the Great Lakes (Invited)

The wind energy resource offshore is not well understood in the Great Lakes area. This is unfortunate as potential offshore wind farm sites are attracting great interest. In an effort to improve our knowledge of the offshore resource, data are now being collected in specific locations. Preferred Great Lakes wind farm sites are in relatively shallow water reasonably close to shore. Airflow over these sites will often be within a transition zone as air flows from a rough land surface to a smoother lake surface. There will also often be changes in the thermal stratification of the air column within this Internal Boundary Layer (IBL). For long fetches and heights of order 100m we can formulate and solve the Reynolds averaged Navier-Stokes equations. We make boundary-layer approximations for pressure and assume that along-wind diffusion is minimal. Effects of gentle terrain can be added separately. Results suggest that relatively long fetches of order 100 km are required before winds at 100m fully adjust to the smoother lake surface. Over Lakes Erie and Ontario winds are frequently from the 180-270 degree quadrant and this suggests that the wind resource should be better in the northern (Canadian) half of these lakes. We illustrate this with energy density plots for locations near lakes Erie and Ontario.