Large-scale atmpospheric structures and their effects on wake dynamics and loads

Description

Beside different inputs, realistic wind fields are crucial to obtain meaningful results from simulations of wind turbine loads and power production. A common practice is to assume turbulent wind fields with an uniform mean wind speed as background, which is reasonable when we analyze the behavior of individual wind turbines or small wind farms. Nevertheless, for wind farms with large spatial extent this assumption becomes less realistic due to changes in the topography of the site and, as it will be shown, the presence of large-scale, organized flow.
The results of wind field characteristics presented by [1] show that one of the most frequent structure for limited negative Richardson number, or moderate unstable conditions with vertical shear, corresponds to longitudinal convective rolls. These structures has been observed in previous experimental studies [2], [3], and appear as the preferred organization under unstable conditions, both on and offshore. The modelling of convective boundary layer by means of linear stability theory of Rayleigh-Bénard convection in a plane Couette flow ([4], [5]) shows convective rolls as the most efficient and preferred mechanism for vertical flux of momentum and heat, results confirmed by Large-Eddy Simulation (LES) [6]. These approaches can generate realistic wind fields, including large-scale structures, but they are complex and computationally costly. This work seeks to formulate a simple approach that includes convective rolls in the Mann-model[7]. We represent such structures as a simple, continuous version of the convective roll discrete Fourier modes. We do so in the autospectra of all wind components, as well as in the co-spectra, which generates characteristic convergent/updraft and divergent/downdraft areas in synthetic wind fields generated using Fourier simulations [8]. Finally, the synthetic turbulence fields generated from the extended Mann-model are used as an input wind field to the aeroelastic wind farm simulation framework, HAWC2Farm. HAWC2Farm is a dynamic wind farm simulation tool recently developed by DTU with the ability to simulate the aeroelastic response of all turbines in a wind farm in a time-stepping manner [9]. Using this tool, we evaluate the impact of large-scale coherent structures in the Dynamic Wake Meandering, power production and wind turbine loads using the TotalControl reference wind farm.

[1] Alcayaga, L., Larsen, G. C., Kelly, M., & Mann, J. (2022). Large-scale coherent turbulence structures in the atmospheric boundary layer over flat terrain, Journal of the Atmospheric Sciences (published online ahead of print 2022). [2]LeMone, M.A. (1973). “The Structure and Dynamics of Horizontal Roll Vortices in the Planetary Boundary Layer”. Journal of the Atmospheric Sciences 30. [3] Weckwerth, T.M. et al. (1997). “Horizontal Convective Rolls: Determining the Environmental Conditions Supporting their Existence and Characteristics”. In: Monthly Weather Review 125. [4] Kuo, H. L. (1963). “Perturbations of Plane Couette Flow in Stratified Fluid and Origin of Cloud Streets”. In: The Physics of Fluids 6.2. [5] Clever, R. M. and F. H. Busse (1992). “Three-dimensional convection in a horizontal fluid layer subjected to a constant shear”. In: Journal of Fluid Mechanics 234. [6] Salesky, S.T., M. Chamecki, and E. Bou-Zeid (2017). “On the Nature of the Transition Between Roll and Cellular Organization in the Convective Boundary Layer”. In: Boundary-Layer Meteorol.

163. [7] Mann, J. (1994). “The spatial structure of surface-layer turbulence”. In: Journal of Fluid Mechanics 273. [8] Mann, J. (1998). “Wind field simulation”. In: Probabilistic Engineering Mechanics 13.4.

[9] Liew, J. et al. (2022). "LES verification of HAWC2Farm aeroelastic wind farm simulations with wake steering and load analysis." In: Journal of Physics: Conference Series. Vol. 2265. No. 2. IOP Publishing

Period	23 May 2023
Held at	University of Strathclyde, United Kingdom
Degree of Recognition	International