Abstract
Abstract. The sensitivities of idealized large-eddy simulations (LESs) to variations of model configuration and forcing parameters on quantities of interest to wind power applications are examined. Simulated wind speed, turbulent fluxes, spectra and cospectra are assessed in relation to variations in two physical factors, geostrophic wind speed and surface roughness length, and several model configuration choices, including mesh size and grid aspect ratio, turbulence model, and numerical discretization schemes, in three different code bases. Two case studies representing nearly steady neutral and convective atmospheric boundary layer (ABL) flow conditions over nearly flat and homogeneous terrain were used to force and assess idealized LESs, using periodic lateral boundary conditions. Comparison with fast-response velocity measurements at 10 heights within the lowest 100 m indicates that most model configurations performed similarly overall, with differences between observed and predicted wind speed generally smaller than measurement variability. Simulations of convective conditions produced turbulence quantities and spectra that matched the observations well, while those of neutral simulations produced good predictions of stress, but smaller than observed magnitudes of turbulence kinetic energy, likely due to tower wakes influencing the measurements. While sensitivities to model configuration choices and variability in forcing can be considerable, idealized LESs are shown to reliably reproduce quantities of interest to wind energy applications within the lower ABL during quasi-ideal, nearly steady neutral and convective conditions over nearly flat and homogeneous terrain.
Highlights
Accurate characterization and prediction of the microscale wind flow environment plays an important role in many facets of wind power generation, including wind park siting, layout, operations, and the formulation of turbine design standards (e.g., Shaw et al, 2009)
The simulations are assessed against one another, theoretical expectations, and observations taken from two case studies featuring quasi-ideal atmospheric boundary layer (ABL) flow during nearly steady neutral and convective conditions over nearly flat and homogeneous terrain
The same forcing is used for all simulations, with the exception being the geostrophic wind direction, which was set to westerly in the High Gradient applications (HiGrad) simulations, rather than northwesterly in Weather Research and Forecasting (WRF) and Simulator fOr Wind Farm Applications (SOWFA)
Summary
Accurate characterization and prediction of the microscale wind flow environment plays an important role in many facets of wind power generation, including wind park siting, layout, operations, and the formulation of turbine design standards (e.g., Shaw et al, 2009). While wind power generation has grown tremendously over the last few decades, both turbine reliability and plant power generation frequently underperform projections based on existing turbine design standards and site assessments (e.g., Bailey, 2013). A key contributor to these underperformance issues is the disconnect between the data and models used in turbine and plant design and site assessment, and actual characteristics of the atmospheric boundary layer (ABL), and the in situ wind plant operating environment. While remote sensing instruments (e.g., Högström et al, 1988; Barthelmie et al, 2003; Nygaard, 2011; Hirth et al, 2012; Rhodes and Lundquist, 2013; Smalikho et al, 2013; Iungo et al, 2013) provide one pathway to improve site characterization, the absence of fast-response turbulence information and limited sampling volumes provided by many systems, coupled with long deployments required to sample long-term variability, constrain the utility of observations for many applications
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