Posters

Abstract

In the presence of stratification, large flow perturbations can trigger gravity waves. These gravity waves can also be induced by large offshore wind farms if the atmospheric boundary layer exhibits stratified layers, such as the inversion and the free atmosphere in a conventionally neutral boundary layer (CNBL) [1]. The wind farm-induced gravity waves alter the pressure gradient across the rows of the wind farm. These large-scale changes decelerate and accelerate the flow at the entrance and the exit of the farm, respectively, which impact the performance of the wind turbines. According to one study, the associated annual energy loss in the Belgian-Dutch wind farm cluster is in the order of 4 – 6% [2]. Although subsequent studies explored various properties of these gravity waves, the sensitivity to rotor tip clearance (which is the distance between the lowest point of the turbine blade tip and the surface) and the turbulence intensity in the inflow is yet to be investigated. The sensitivity is being analysed using four large eddy simulations. One of them acts as the reference for both parameters. The reference simulation consists of a 10 X 10 array of IEA 22MW reference wind turbines, which have a rotor diameter of 284 m and a hub height of 170 m. The non-dimensional turbine spacing in the wind farm is 5D (where D is the rotor diameter of the wind turbine). The reference simulation retains the rotor tip clearance of 28 m described in the turbine documentation. Two more simulations with a clearance of 20 m and 36 m are being simulated to investigate the effect of this parameter. The fourth simulation sets the turbulence intensity at hub height to 8%, instead of the 4% in the reference simulation. The inflow in all cases is CNBL with an inversion height of 500 m, an inversion thickness of 100 m, an inversion strength of 2 K, a free atmospheric lapse rate of 3 K/km, and a hub height velocity of 10 m/s. The simulations are being carried out using the open-source incompressible finite-volume solver TOSCA [3]. TOSCA employs the Boussinesq approximation for buoyancy and accounts for Coriolis effects. Using the simulation outputs, the sensitivity of the velocity, pressure, and temperature fields, turbine power production, and the characteristics of the gravity waves (eg, amplitude) to the two parameters will be examined and presented in the workshop. References [1] Allaerts D., and Meyer J., (2017), Boundary-layer development and gravity waves in conventionally neutral wind farms, Journal of Fluid Mechanics, vol. 814, pp. 95-130 [2] Allaerts D., Broucke S. V., van Lipzig N., and Meyer J., (2018), Annual impact of wind-farm gravity waves on the Belgian–Dutch offshore wind-farm cluster, Journal of Physics: Conference Series, vol. 1037, pp. 072006 [3] Stipa, S., Ajay, A., Allaerts, D., and Brinkerhoff, J., (2024), TOSCA – an open-source, finite-volume, large-eddy simulation (LES) environment for wind farm flows, Wind Energy Science, vol. 9(2), pp. 297–320