Experimental Analysis of the Wake Meandering of a Floating Wind Turbine under Imposed Surge Motion

Abstract

With the development of floating wind farms, the understanding of the far wake becomes of utmost importance because it cannot only be based on the experience acquired on fixed offshore turbine as the range of motion of the floater due to met-ocean conditions is in the same order of magnitude compared to the energetic turbulent scales of the atmosphere and to the characteristic scales of the wake. The objective of this wind-tunnel experiment is to analyse the behaviour of the wake center of a floating turbine subject to imposed surge motion. The experiment is carried out in the LHEEA wind tunnel where a 1/500 scale wind turbine model is immersed in a realistic offshore atmospheric-boundary layer. The model is actuated in surge by a linear motor able to reproduced idealised second order surge motion of a floating platform (mainly due to the waves) with a realistic amplitude and range of frequencies. Stereoscopic particle image velocimetry measurements are performed at two planes normal to the free-stream at distances of 4.6 and 8.1 diameters downstream of the turbine. Instantaneous velocity fields acquired at 14.1 Hz are individually analysed by convolution to find the location of the wake center. Results show that the wake center spreading is largely influenced by the downstream distance and only slightly affected by the surge motion. However, when analysing the wake position time series by a power spectral density, the signature of the surge motion becomes very clear for both downstream distances. These findings tell that the far wake (at least up to 8.1 diameters downstream) of a floating wind turbine has a “memory” of the motion in its frequency content. When extrapolating to a floating wind farm, this result suggests that a turbine inside a floating wind farm will be immersed in a flow including a significant dynamic signature in the range of its own motion.

Far-Wake Meandering of a Wind Turbine Model with Imposed Motions: An Experimental S-PIV Analysis

“…The wind tunnel is 26 m long with a cross-section of 2 m × 2 m. The centrifugal fan of the wind tunnel is powered up by a 45 kW motor, allowing it to reach the maximum wind speed of 10 m/s with the freestream turbulence intensity T i around 1.2%. This facility has been used in the past for atmospheric boundary layer (ABL) studies in the scale-ratio range of 1:200 [19] and 1:500 [14,17,20]. In order to generate the ABL condition, the wind tunnel is equipped with a set of turbulence generators, a fence at the entrance of the test section, as well as perforated metal plates on the floor.…”

“…The post-processing of velocity vectors for the wake centre tracking purpose is carried out with a developed Python code based on the maximum convolution between the field of instantaneous available power and a Gaussian masking function. The method is comprehensively explained in [14].…”

“…instantaneous available power and a Gaussian masking function. The method is comprehensively explained in [14].…”

“…The main aim of this research paper is to determine the influence of the FOWT substructure motion in the far-wake (x > 8.125 D) meandering process and investigate if this influence can be neglectable or should be considered for further wake model development. Most recently, research in this area has focused on the surge motion signature in the wake meandering of FOWTs [14]. The current study advances the state-of-the-art by extending to other motion DoFs, considering the monochromic motions of pitch, as well as 3DoF motion (surge, pitch, heave) with multi-frequency content time series.…”

“…Moreover, the atmos pheric turbulent eddies are larger in offshore fields; therefore, more meandering is ex pected. Importantly, FOWTs substructure motions have the same order of magnitude compared to the energetic turbulence scale of the atmosphere and the characteristics o the wake [14]. As a result, substructure motions due to the environmental loading could potentially influence the wake meandering process.…”