The Science of Making Torque from Wind (TORQUE 2018)

Authors:
Antonia Englberger, Andreas Dörnbrack

Abstract:
The wake characteristics of a hill-top wind turbine are investigated by means of large-eddy simulations of a stably stratified atmospheric boundary-layer flow. The wind turbine with a hub height of 100 m is placed on the top of a 3 D hill with heights up to 50 m. Synchronized turbulent inflow data of a stable ABL simulation drive the wind-turbine simulations. They are imported from a precursor simulation of a full diurnal cycle of the ABL. The wake characteristics are compared with simulation results of the same flow field passing through the same wind turbine standing on a homogeneous surface. Overall, the velocity deficit in the near wake of the hill-top wind turbine is reduced compared to the homogeneous surface case. As a consequence, its wake recovers more rapidly. Both effects are caused by stronger ambient turbulence which is produced by the enhanced shear of the flow over the hill. Furthermore, the wake of the hill-top wind turbine bends downward in the lee roughly following the mountain shape.

Authors:
Jacob Berg, Niels Troldborg, Robert Menke, Edward G. Patton, Peter P. Sullivan, Jakob Mann, Niels N. Sørensen

Abstract:
We present Large-Eddy Simulation results of flow over the double ridge complex site at Perdigao in Portugal. The focus is a comparison between two codes with different discretization technique. The wind speed predicted from a finite volume code and a pseudo spectral code is very alike and the resolved turbulent kinetic energy is in agreement in the lee side of the ridges but not on the upslope side.

Authors:
Pascal Weihing

Abstract:
The objective of this study, which was part of the project AssiSt, is the numerical investigation of wind turbine power, loads and wake development in the extreme wind conditions found on a complex terrain site in northern Spain. The turbine is located near the edge of a characteristic cliff. Realistic atmospheric inflow conditions are prescribed by a coupling of the meteorological code PALM and the CFD solver FLOWer. High turbulence levels and significant inclination angles are found upstream of the turbine which result in strong load variations over one revolution and complex wake terrain interactions.

Authors:
Jakob Mann

Abstract:
Pairs of synchronously scanning Doppler lidars measure the average wind speed of flows crossing the parallel ridges at Perdigão, Portugal. The variations along the ridges are compared to neutral LES calculations making a good match at the upstream ridge but a significantly worse prediction at the downstream ridge. One reason could be an insufficient representation of the terrain. Another unknown is the influence of the atmospheric stability on the flow which is clearly seen by the scanning lidars.

Authors:
Matias Avila

Abstract:
In this work the annual wind power of a wind farm sitted over complex terrain is predicted using annual wind measurements in one mast,  and running a RANS-CFD model. The wind turbines are modelled  as uniformly loaded actuator discs. The used RANS model is the Realizable model of Shi et al modified to account for Coriolis forces, and with modified model coefficients to predict an appropriate TKE level for wind applications in terms of the shear stress over the surface. The proposed model is validated against wind farm measurements together with other well known models. Finally the methodology to predict annual energy production of a wind farm with 167 wind turbines is detailed and validated against wind power measurements obtaining a difference 3.5%.

Authors:
Paolo Luzzatto-Fegiz

Abstract:
In this conference presentation, we build on the turbulent entrainment hypothesis to develop models for flow past wind turbines and general canopies. The entrainment hypothesis consists of a concise turbulence model, which has been widely used in other areas of geophysical fluid mechanics. Without having to linearize the governing equations, or assuming a wake radius that grows linearly with distance downstream, one can obtain a model for the wake of a single wind turbine. For an axisymmetric wake, this recovers a result first due to Morton (J. Fluid Mech. 10:101), which however does not appear to have been employed or tested before. We next consider flow adjustment at the front of a canopy comprising an array of obstacles. The resulting theories agree with laboratory measurements, without the need for introducing parameters that are specific to flow past turbines and canopies; instead, all coefficients that characterize the turbulence can be directly related to a classical entrainment parameter. Since these models do not assume a specific dependence on distance downstream, they also enable extensions for examining flows over complex terrain.