Simulation of transient gusts on the NREL5 MW wind turbine using CFD

Abstract

Wind turbines operate in unsteady wind conditions that are caused by the terrain and the atmospheric boundary layer which results in statistical fluctuating inflow conditions. Both effects are studied in CFD studies throughout literature, as lately by Castellani et al. Conversely, the specific investigations of single unsteady events as (large) longitudinal transient gusts are rare. Thus, the performed work presents a procedure, implemented in the CFD-solver THETA, to propagate longitudinal transient gusts through a flow field. In a first attempt, the gust is modelled in a cosines shape that strikes the generic NREL 5MW wind turbine at rated operating conditions and lasts about 6 seconds. Thereafter, the capability to simulate the aforementioned wind turbine under an extreme operating gust, following the international standard IEC 61400-13 definition, is presented. The smooth cosines curve is interrupted by the 3/rev characteristic which is caused by respecting the tower during modelling. Moreover, the moderate gust speeds of ±0.5m/s lead to noticeable changes in rotor torque. The flow states on the rotor blade before the gust strike and at maximal gust velocity are compared. An increased blade loading is detectable in the pressure coefficients which are displayed as contour plots. Moreover, the separation region on the blade root increases when the gust strikes the wind turbine.