- freely available
Energies 2015, 8(7), 6468-6496; https://doi.org/10.3390/en8076468
2. Numerical Methods
2.1. The Actuator Line Method
2.2. The ABLPisoSolver in OpenFOAM (OpenFOAM-LES)
2.3. Computational Grids and Simulation Methodology
2.4. FieldView XDB Workflow
3. Results and Discussion
3.1. Atmospheric Boundary Layer Precursor Simulations (OpenFOAM-LES)
|Precursor Simulation||Stability||Ug (m/s)||Ih (%)||u* (m/s)||w* (m/s)||zi (m)||−zi/L||t* (min)|
|Turbine-Turbine Interaction (z0 = 0.001 m)||NBL||9.1||4.9||0.27||0||755||0||46|
|5-Turbine Wind Farm (z0 = 0.2 m)||MCBL||10.4||8.9||0.53||1.00||765||2.70||13|
3.2. A Turbine-Turbine Interaction Problem (ALM OpenFOAM-LES)
3.2.1. Turbine Power
3.2.2. Wake Velocity Deficit
3.2.3. Wake Turbulence
3.3. A Wind Farm Consisting of Five Wind Turbines (ALM OpenFOAM-LES)
3.3.1. Turbine Power
3.3.2. Dynamic Surface Clipping for Flux Analysis
3.3.3. Integrated Quantities along “Dynamic Surface Clips” of Wake Surface Cutting Planes
- The ABL stability state defines the shear in the atmospheric boundary layer (ABL) and has a profound effect on the wake recovery and power production of wind turbine arrays.
- Array efficiency increases in a moderately-convective boundary layer (MCBL) compared to a neutral boundary layer (NBL), thus leading to a faster recovery of the wake momentum deficit. This is supported by R13 distributions along vertical lines.
- Higher turbulence levels in MCBL compared to NBL conditions also lead to higher fatigue loads, in particular for the downstream turbine in a turbine-turbine interaction problem, as supported by higher standard deviation of turbine power.
- The peak TKE occurs at the upper edge of the wake and on the sides of the wake at hub height due to the presence of a developing shear layer and tip vortices.
- The shear due to the wake is far more pronounced than the shear due to ABL.
- The TKE distribution is broadened due to wake rotation and ensuing turbulent mixing.
- For wind farms arranged in staggered arrays, the downstream array is affected by the wake of the upstream array, even for perfectly aligned wind conditions (zero yaw). This leads to a small power loss in the (downstream) staggered array; however, the standard deviation in power, and most probably fatigue loads, are reduced at a higher rate compared to the power loss. This may be of interest for future array planning.
- The observed fact from many wind farms that power production has leveled out by the third turbine in an array has been confirmed in the computations of this work in the example of a 5-turbine wind farm.
- The use of the “dynamic surface clipping” method in FieldView allowed to separate integrated fluxes of mass, momentum, power density, and TKE in wake surface cutting planes above and below hub height. It was discovered that the lower portion of a wake surface cutting plane lags in its recovery process by about one turbine spacing.
Conflicts of Interest
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