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Advanced experimental modelling of hydrodynamic and aerodynamic wakes in floating offshore wind turbines using particle image velocimetry
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Date
2024
Authors
Belvasi, Navid
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Publisher
University College Cork
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Abstract
Accurate evaluation of power generation and aerodynamic performance is essential for optimising energy production in floating wind farms. Understanding the influence of floating substructure motions on turbine aerodynamics is crucial for achieving this, as the two-way coupling between the turbine and substructure impacts aerodynamic performance. Meanwhile, these substructure motions are inherently tied to their hydrodynamics, creating a complex interplay that requires holistic analysis.
A significant knowledge gap lies in understanding the impact of floating offshore wind turbine (FOWT) motions on the wake meandering process. This process is characterised by low-frequency, large-scale instability in the turbine’s wake, causing it to shift in and out of rotor planes. Such shifts lead to energy losses and a reduction in fatigue life for downstream turbines. As FOWT motions occur on a similar scale to wake dynamics and atmospheric turbulence, they are likely to play a role in influencing wake meandering. However, these interactions remain poorly understood.
Additionally, uncertainties persist in estimating such FOWT motions using current potential theory (POT) tools, particularly in low-frequency ranges dominated by nonlinear hydrodynamic forces. While computational fluid dynamics (CFD) methods are employed to calibrate viscous damping coefficients in POT models to improve accuracy, their high computational cost limits practical use. The reliability of simplified CFD models for refining POT viscous damping remains unexplored, representing a clear knowledge gap.
Furthermore, these simplified CFD models require detailed spatial and temporal data on turbulence behaviours in FOWT hydrodynamics for comparisons and cross-validation. Conventional wave basin instruments—such as wave probes, pitot tubes, acoustic and laser doppler velocimeters—are insufficient for capturing the flow velocity characteristics necessary to measure parameters like pressure distributions, Reynolds stresses, turbulent kinetic energy (TKE), and turbulent intensity (TI). This highlights the need to utilise advanced measurement techniques, such as underwater stereoscopic particle image velocimetry (SPIV). While underwater SPIV has shown significant promise in marine engineering wave basin studies, its application in FOWT campaigns remains largely unexplored.
This interdisciplinary PhD research aims to investigate the relationships between FOWT turbine far wake meandering, substructure motions, and hydrodynamics. Its first objective was to study the influence of FOWT motions on turbine far-wake behaviour. The second objective was to analyse the impact of CFD model simplifications on the estimation of viscous damping coefficients to inform POT models. The third objective was to investigate underwater SPIV for 3-dimensional turbulence measurements of FOWT hydrodynamics.
Using SPIV in a wind tunnel, the far-wake meandering of a FOWT model—based on the Floatgen demonstration—was measured at a longitudinal distance of 8.125 times the rotor diameter downstream of the turbine. Recent research in this area has mainly concentrated on the surge motion signature in the wake meandering of an FOWT. This study advances the state of the art by broadening the analysis to 3 degrees of freedom—surge, pitch, and heave—characterised by multi-frequency time series derived from a realistic prototype, as well as under its representative marine atmospheric boundary layer (ABL). The study found that in the presence of ABL, FOWT motions do not affect the turbine far-wake recovery. However, the spectral density analysis showed that FOWT motions frequency could introduce distinct peaks in the far wake frequency spectrum, causing shocks or resonance in downstream turbines if similar FOWT is used.
The PhD research expanded its investigations to SPIV wave basin testing of FOWT to meet Objective 3. Prior to the wave basin experiments, a comprehensive literature review was conducted to examine SPIV applications and challenges in measuring FOWT hydrodynamics. This study was the first to investigate the use of underwater SPIV in addressing critical research gaps in FOWT hydrodynamics, reviewing its limitations, benefits, and disadvantages. Next, the SPIV method was applied in a wave basin to explore the relationship between FOWT motions and hydrodynamic behaviour. The FOWT model was tested under three configurations: fixed, floating, and floating with an operational turbine. Velocity contours, TKE, TI, and Reynolds stress tensors were examined. The analysis then discussed the turbulence characteristics of fluid flow behind these simplified test models.
Using insights from wave basin SPIV tests, simplified CFD models were developed to calibrate viscous damping coefficients in FOWT POT models, aiming to address Objective 2. While existing literature primarily focuses on using fully complex CFD models to analyse FOWT behaviour, the novelty of this study lies in proposing and evaluating a streamlined approach that leverages simplified CFD models to inform the damping coefficients in POT tools for FOWT. The resulting damping coefficients from CFD test configurations—including fixed, floating, and full-system models, as well as decay tests and current flow setups—were analysed and compared. While drag coefficients varied significantly among the test configurations, simulations incorporating environmental conditions—including under fixed, floating, and full system configurations—produced damping values consistent with expected platform behaviour. Finally, a POT model of the FOWT was calibrated using damping coefficients from the CFD models, followed by a sensitivity analysis of response amplitude operators (RAOs). Comparisons of RAOs for surge, heave, and pitch motions revealed that uncorrected POT models tend to overestimate response times, particularly for surge and pitch, by up to double the actual values. However, calibrating the damping coefficients in the POT model using results from simplified CFD models improved precision, reducing RAO errors to within 10–20%.
This PhD research contributes to and advances the development of FOWTs by reviewing the aerodynamic and hydrodynamic wake behaviour. Key contributions summarised as advancing floating wind hydrodynamic testing using underwater SPIV, evaluating simplified CFD models to inform viscous terms in POT models to enhance its motion estimations, and analysing FOWT motions effects on the wake meandering process utilising 3 Degree of freedom motion time series and in the presence of the atmospheric boundary layer.
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Keywords
Floating offshore wind , Stereoscopic particle image velocimetry , Advanced wave basin test methods , Wind tunnel experiment , Computational fluid dynamics , Floating offshore wind turbine
Citation
Belvasi, N. 2024. Advanced experimental modelling of hydrodynamic and aerodynamic wakes in floating offshore wind turbines using particle image velocimetry. PhD Thesis, University College Cork.