Abstract
Large eddy simulations of wind farms are often performed with canonical atmospheric conditions, where the background flow is based on precursor simulations with idealized model setups yielding statistically stationary turbulent flows. However, precursor simulations can only handle gradually changing flow conditions, and are not capable of modelling highly transient and statistically non-stationary flows, e.g. frontal passages or large gusts. Such flows frequently occur in nature, and can influence the operation—and potentially design—of wind turbines. It is generally not possible to impose non-stationary features through inlet boundary conditions, if the imposed flow violates the most fundamental assumption of micro-scale flow simulations, namely conservation of mass.This work presents a method for modelling highly transient wind speed ramps by extending and adapting the method of applying body forces to achieve specific flow scenarios, where the wind speed ramps are embedded as constrained turbulent boxes. Several scenarios with significant increases in the streamwise wind speed are simulated. Analyses of the transient wake dynamics, as the wind speed ramps propagate through large wind farms are performed to show how well the momentum is maintained throughout the numerical domain and the influence and operation of turbines during the ramp passages.
Highlights
Large Eddy Simulations (LES) have been used extensively to model large wind farms e.g. [1, 2, 3, 4], and such simulations are generally performed with statistically stationary background flow conditions
Large eddy simulations of wind farms are often performed with canonical atmospheric conditions, where the background flow is based on precursor simulations with idealized model setups yielding statistically stationary turbulent flows
Highly transient flow scenarios are inherently difficult to model in LES, as it violates mass conservation
Summary
Large Eddy Simulations (LES) have been used extensively to model large wind farms e.g. [1, 2, 3, 4], and such simulations are generally performed with statistically stationary background flow conditions. [1, 2, 3, 4], and such simulations are generally performed with statistically stationary background flow conditions. Precursor simulations can generate such statistically stationary turbulent flow scenarios based on simplifying assumptions, e.g. wall-modelling based on Monin–Obukhov similarity theory with the assumptions of homogeneous terrain and constant geostrophic wind. The resulting atmospheric flow exhibits a fully developed balance between the driving geostrophic wind and the shear stress on the ground. Atmospheric flows are typically not in balance as they continuously develop, and scientific focus is increasing on describing the experimental statistics involved with non-canonical scenarios, including extreme wind shear events [5], storms [6], and/or frontal passages, which can give rise to large increases in wind speeds and large directional changes [7]. Transient modelling efforts have predominantly focused on idealized diurnal cycles with only gradual
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