Wind turbulence and wake models

The wind conditions are divided into deterministic and stochastic wind. The deterministic part of the wind includes mean wind velocity, sudden acceleration, linear trend, special gust events, and special shears. The user may prescribe wind shear and wind direction changes as a function of height. The stochastic wind, usually referred to as turbulence, is generated outside the HAWC2 code. HAWC2 is able to read two different formats of turbulence data. One is in Cartesian coordinates (e.g. Mann turbulence that can be generated by a standalone DLL or by WAsP Engineering) and the other is in polar coordinates (the Veers model used by FLEX5). Tower shadow effects are also a part of the wind module as it changes the wind conditions locally near the tower. For upwind turbines a potential flow method is used whereas a jet-model produces much better results for downwind turbines.

Wake effects can be treated using the effective turbulence intensity method, suggested in the optional Annex D of the IEC 61400-1 Ed. 3 standard, or a more accurate method called the Dynamic Wake Meander (DWM). The DWM model is basically a calculation of a cascade of velocity deficits emitted from the upwind turbine, which is then transported downstream by the wind. During this transport, a meandering process of the wake position occurs caused by the large scale turbulence components of the surrounding turbulence field. The DWM method has successfully been verified against detailed CFD simulations and measurements on turbines operating in wake conditions and equipped with pitot tubes, LIDARs, and sensors for load and power measurements. As a further supplement to built-in wind models, HAWC2 can work with wind and turbulence fields produced by external programs such as WAsP Engineering. This enables site-specific turbine simulations. Turbulence simulations in WAsP Engineering are also done by the Mann method, although in this case using a target spectrum modified by terrain topography and upwind changes in surface roughness. With WAsP Engineering, it is possible to export wind shear, veer and flow inclination in the file format accepted by HAWC2.