Assessing the top-down wind farm model: Experimental validation and potential improvements

The top-down wind farm model is a low-cost physics-based model of wind farm flow. Assuming the farm is infinite, the top-down model asserts that the turbine thrust force is fully balanced by momentum flux from above the farm. Later works extended the model to finite, irregular layouts. The model can be used to estimate hub-height wind speed and the vertical profiles of velocity above the farm. Though the infinite, regular cases have been validated on simulation data, real-world validation on irregular wind farms is lacking. Furthermore, these extensions face critical limitations in the estimation of the turbine area-averaged. Since the boundary layer above the farm adjusts to the area-averaged thrust force, estimating this quantity accurately is essential. We present a study on the application of the top-down model to wind tunnel model wind farm experiments and real-world farms. We demonstrate that multiple methods of extending the top-down model to irregular layouts fail to appropriately adjust the area-averaged thrust force over the wake-impacted area. This results in poor estimation of the hub-height wind speed. We further propose alternative approaches to formulating the top-down problem and applying the governing equations that better captures the area-averaged thrust forces and results in better performance. Furthermore, though the original top-down derivation assumes fully developed flow conditions, we demonstrate that, through coupling with a wake model, this assumption can be removed. Further models can be added for other effects, such as wake-added turbulence, to refine the top-down formulations. Finally, by considering layout-based geometric parameters of individual turbines and farm averages, we suggest further modifications to top-down predictions to better adjust to layout-induced variabilities in performance and hub-height wind speed.