Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 75

Actuator line theory combines the 2‐D blade sectional characteristics used in BEM theory with, usually, a CFD grid calculation of the whole flow field external to the rotor blades including the wake. It is particularly useful for calculating the aerodynamic loads and flow field quantities where wind turbine rotors operate within a larger complex flow field such as a wind farm or a non‐simple ABL topography.

In this method, the outer flow is computed on a finite volume or element grid by some method of numerical simulation (usually viscous and turbulent) of the unsteady flow equations, such as unsteady Reynolds averaged Navier–Stokes (URANS) or LES (see discussion in Chapter 4). A number of open‐source or commercial codes are available to do this with varying degrees of fidelity and cost. Because the computations are carried out over a sequence of timesteps and are spatially 3‐D, this always requires significant computing resource. The discretisation scale should be appropriate to resolve the major structures of the ABL and its turbulence and the rotor (diameters) and the turbulent structures in their wakes. But it does not resolve the flows on the length scales of the blade chords and their boundary layers and hence is orders of magnitude faster than a complete simulation of all scales in the flow field.

Instead of resolving the sectional blade flows, these are replaced, as in BEM theory, by aerofoil characteristics from look‐up tables (or possibly a fast panel method such as XFOIL; Drela 1989). The rotor blades are tracked through the outer flow grid and the velocity field, which has been computed on that grid, is interpolated onto the designated rotor blade sections. The resulting sectional blade forces obtained by interpolating from the blade characteristic look‐up tables are projected back onto the outer grid as a series of momentum sinks for the components of force in the three coordinate directions. These sinks then form part of the grid flow field calculation at the next timestep. This coupling between the inner and outer flow calculations may be either loose going from timestep to timestep as indicated or may be a strong coupling in which the flow is converged within each timestep by iteration or by solving the whole in a single very large matrix. Transfer of force and large‐scale velocities between the inner and outer flow fields is well established, but methods of determining the effective turbulence input from the smaller‐scale structures in the rotor blade flows as sources for the larger‐scale outer flow are not, and further work is required here. Good references for this method are Mikkelsen (2003) and Troldborg et al. (2006).