The RaptorNL model is a computational flow model that models turbulent flow over topography and is used for wind resource assessment in steep and complex terrain.  The main advantage of RaptorNL is that, unlike linear models, it does not lose accuracy in modeling flow over steep terrain.  The model is typically used within the framework of the European Wind Atlas methodology to provide wind resource estimates based on tall tower data.

Canopy Capability

As many areas are affected by the presence of medium to tall forest canopies, the RaptorNL model includes a distributed drag formulation to account for the affect of canopies on the wind flow.  These affects can be particularly important in assessing vertical shear over and near forest cover and edges.

Model Formulation

The RaptorNL model employs a number of leading edge numerical techniques to give a high level of accuracy and efficiency.  The model uses a combination of finite volume and finite difference methods on a staggered grid.  The grid is terrain following with logarithmic stretching in the vertical direction to resolve strong wind gradients near the surface.  RaptorNL employs a fractional step method to solve the incompressible pressure and momentum equations.  The resulting sparse linear systems are solved using a Fourier space preconditioner within a Krylov subspace iterative solver.  A new compressible immersed boundary version of the model is currently under development.

Meso/Regional-scale Adjustment

The standard output from RaptorNL is in the form of a Wind Resource Grid for use in industry standard tools for optimising turbine layouts and calculating energy yields, within the European Wind Atlas methodology.  To make the methodology more broadly applicable, meso/regional-scale adjustment can also be incorporated into RaptorNL output to account for variation in the background wind climatology, as occurs with large wind farm sites or in topographically complex regions.