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Scientific Question: Gap Flows

Hypotheses:
The presence of a gap along the ridge leads to local flow distortions, wherein the flow converges, accelerates and jets out of the gap, which interacts with the valley as well as atmosphere aloft. Shear layers at the exit lead to high turbulence levels and enhanced vertical and horizontal mixing.

 

Approach:
A gap in the ridge leads to a variety of phenomena governed by the additional parameters, gap width/height and gap length (streamwise)/height. Determining these phenomena are the balances of forces - inertia, pressure gradient, friction and buoyancy - in the gap (Baines 1987). Numerical simulation of flow at the Perdigão site using the VENTOS® model developed at Porto University (an EU Partner) shows that it is not possible to expect 2D flow near the gap, independent of the cross-wind direction. For winds from the SW, air appears to stall at a low-point (gap) of the downwind ridge even though it is only about 20m lower. Not surprisingly, a region of low winds, high turbulence and recirculation develops in the valley between the ridges, which persist to well above the leeward ridge top (interestingly, this is a location selected for future Vale Cobrão wind turbines!). Flow separation at the gap edges may generate highly unsteady flow structures in time dependent simulations. The simulations do not account for buoyancy effects, which are expected to change the flow character drastically (Mayr et al. 2007). No high-resolution field data are available for gaps, and we will use WindScanner and suitably located ND Triple Lidar to investigate flow structures and turbulence at the entrance/exit areas of the gap and the interactions of valley and gap flows.

 

Implications:
The results will help parameterize subgrid process associated with gaps. High turbulence and unsteadiness are some of the effects expected, and this work will map such processes and will develop parameterizations to describe their effects.