Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 69

The one‐dimensional actuator disc model and associated vortex cylinder representation of the flow field is the simplest model of a HAWT that can provide useful results. This model may be developed in several ways to be more representative of the details of the flow.

Radial variation across the actuator disc may be considered as in BEM theory, Section 3.5 below.

Also, recognition may be given to the fact that the turbine has a finite number of blades, usually a small number such as two or three, each of which may be treated individually as a momentum sink actuator. In the simplest version taking average values, the forces on each blade are assumed to be radially constant. The lift and drag forces calculated from the flow angles at the blades with the relevant aerofoil section characteristics (as in Section 3.5.2) are converted into rotating axial and tangential momentum sinks projected onto a larger field grid computation. This is the basis of the simplest actuator line model (see Section 3.6).

With the development of large wind farms, particularly offshore, it has become important to simulate the flow through the whole wind farm to calculate the effect of multiple wakes interacting with each other and with the incident atmospheric boundary layer (ABL) and impinging on downstream rotors. Wake interactions have a very significant effect on power generated by turbines downstream of others (see, e.g. Argyle et al. 2018) and on the buffeting of downstream rotors. The usual method of carrying out these computations is to embed actuator models of the turbines within much larger numerical grid representations of the flow through and surrounding the whole wind farm. This outer large‐scale flow is solved numerically on the grid by conventional, and now well‐established, CFD Reynolds averaged Navier–Stokes (RANS) or higher fidelity but much more computationally expensive large eddy simulation (LES) computer codes. The actuator model embedded in the grid to represent the action of each turbine may be at the simplest level of an actuator disc model, in which the thrust force on the rotor disc is inserted as a momentum sink, i.e. a step change in momentum in the streamwise direction across grid cells that are intersected by the rotor disc. However, it is usually found desirable to go to a higher level of representation including swirl and embed an actuator line model for each turbine blade in the grid. The rotating actuator lines are now the momentum sinks of both axial and azimuthal forces including the radial variations, which are projected at each timestep onto the adjacent grid points (see, for example, Soerensen and Shen 2002).