Wind direction error in the Lillgrund offshore wind farm

The reduction of wake effects and maximisation of wind farm power output are always of interest to wind researchers [2][3]. However, one of the research challenges is to distinguish sensor errors from wake losses and losses due to poor yaw alignment. SCADA data from Lillgrund, the Swedish offshore wind farm, shows significant differences between the wind direction as measured at the met mast and as indicated by the nacelle directions measured at the individual wind turbines. Various possibilities might explain this data, the most likely being poor yaw control at particular wind turbines and/or sensor error. Both of these make power performance assessment and wake analysis problematic. In this paper, animations presented have been proved useful in the initial identification of turbines with potential problems. These anomalous turbines are then subject to a range of analyses designed to distinguish the different potential issue

Hydrodynamics, control and numerical modelling of absorbing wavemakers

This research investigates the effects that geometry and control have on the absorption characteristics of active wavemakers and looks at the feasibility of modelling these wavemakers in commercial computational fluid dynamic software. This thesis presents the hydrodynamic coefficients for four different types of wavemakers. The absorption characteristics of these wavemakers are analysed using different combinations of control impedance coefficients. The effect of combining both geometry and control is then investigated. Results, quantifying the absorption characteristics are then presented. It is shown that the amount of absorption for a given paddle differs greatly depending on the choice of control coefficients used to implement complex conjugate control. Increased absorption can be achieved over a broader bandwidth of frequencies when the geometry of the wavemaker is optimised for one specific frequency and the control impedance is optimised for an alternate frequency. In conjunction to this theoretical study, a numerical investigation is conducted in order to verify and validate two commercial computational fluid dynamic codes' suitability to model the previously discussed absorbing wavemakers. ANSYS CFX and FLOW3D are used to model a physical wavemaker. Both are rigorously verified for discretisation errors and CFX is validated against linear wavemaker theory. Results show good agreement and prediction of the free surface close to the oscillating wavemaker, but problems with wave height attenuation and excessive run times were encountered

Turbulence characteristics in offshore wind farms from LES simulations of Lillgrund wind farm

AbstractThe effect of wind turbine wakes in large offshore wind energy arrays can be a substantial factor in affecting the performance of turbines inside the array. Turbulent mixing plays a key role in the wake recovery, having a significant effect on the length over which the wake is strong enough to affect the performance of other turbines significantly. We highlight how turbulence affects wind turbine wakes using results from LES simulations of Lillgrund offshore wind farm in the context of SCADA data selected to mirror the wind conditions simulated. The analysis here concentrated on temporal spectra of wind velocities measured by the turbine's nacelle anemometer and calculated at the turbine locations in the computational model. The effect of the wind turbine rotor on the downstream flow is quantified by analysing the change in spectral features of turbines within the wind farm compared to turbines at the side of the farm exposed to the wind

Simulations of an Offshore Wind Farm Using Large-Eddy Simulation and a Torque-Controlled Actuator Disc Model

We present here a computational fluid dynamics (CFD) simulation of Lillgrund offshore wind farm, which is located in the Oresund Strait between Sweden and Denmark. The simulation combines a dynamic representation of wind turbines embedded within a Large-Eddy Simulation CFD solver, and uses hr-adaptive meshing to increase or decrease mesh resolution where required. This allows the resolution of both large scale flow structures around the wind farm, and local flow conditions at individual turbines; consequently, the response of each turbine to local conditions can be modelled, as well as the resulting evolution of the turbine wakes. This paper provides a detailed description of the turbine model which simulates interactions between the wind, turbine rotors, and turbine generators by calculating the forces on the rotor, the body forces on the air, and instantaneous power output. This model was used to investigate a selection of key wind speeds and directions, investigating cases where a row of turbines would be aligned with the wind or at specific angles to the wind. Results shown include presentations of the spin-up of turbines, the observation of eddies moving through the turbine array, meandering turbine wakes, and an extensive wind farm wake several kilometres in length. The key measurement available for cross-validation with operational wind farm data is the power output from the individual turbines, where the effect of unsteady turbine wakes on the performance of downstream turbines was a point of interest. The results from simulations were compared to performance measurements from the real wind farm to provide a firm quantitative validation of this methodology. Having achieved good agreement between the model and actual wind farm measurements, the potential of the methodology to provide a tool for further investigations of engineering and atmospheric science problems is outlined.Comment: 48 pages, 36 figure

A prospective evaluation of thiamine and magnesium status in relation to clinicopathological characteristics and 1-year mortality in patients with Alcohol Withdrawal Syndrome

Background: Alcohol withdrawal syndrome (AWS) is routinely treated with B-vitamins. However, the relationship between thiamine status and outcome is rarely examined. The aim of the present study was to examine the relationship between thiamine and magnesium status in patients with AWS. Methods: Patients (n = 127) presenting to the Emergency Department with AWS were recruited to a prospective observational study. Blood samples were drawn to measure whole blood thiamine diphosphate (TDP) and serum magnesium concentrations. Routine biochemistry and haematology assays were also conducted. The Glasgow Modified Alcohol Withdrawal Score (GMAWS) measured severity of AWS. Seizure history and current medications were also recorded. Results: The majority of patients (99%) had whole blood TDP concentration within/above the reference interval (275–675 ng/gHb) and had been prescribed thiamine (70%). In contrast, the majority of patients (60%) had low serum magnesium concentrations (< 0.75 mmol/L) and had not been prescribed magnesium (93%). The majority of patients (66%) had plasma lactate concentrations above 2.0 mmol/L. At 1 year, 13 patients with AWS had died giving a mortality rate of 11%. Male gender (p < 0.05), BMI < 20 kg/m2 (p < 0.01), GMAWS max ≥ 4 (p < 0.05), elevated plasma lactate (p < 0.01), low albumin (p < 0.05) and elevated serum CRP (p < 0.05) were associated with greater 1-year mortality. Also, low serum magnesium at time of recruitment to study and low serum magnesium at next admission were associated with higher 1-year mortality rates, (84% and 100% respectively; both p < 0.05). Conclusion: The prevalence of low circulating thiamine concentrations were rare and it was regularly prescribed in patients with AWS. In contrast, low serum magnesium concentrations were common and not prescribed. Low serum magnesium was associated more severe AWS and increased 1-year mortality

IEA-Task 31 WAKEBENCH: Towards a protocol for wind farm flow model evaluation. Part 1: Flow-over-terrain models

The IEA Task 31 Wakebench is setting up a framework for the evaluation of wind farm flow models operating at microscale level. The framework consists on a model evaluation protocol integrated on a web-based portal for model benchmarking (www.windbench.net). This paper provides an overview of the building-block validation approach applied to flow-over-terrain models, including best practices for the benchmarking and data processing procedures for the analysis and qualification of validation datasets from wind resource assessment campaigns. A hierarchy of test cases has been proposed for flow-over-terrain model evaluation, from Monin-Obukhov similarity theory for verification of surface-layer properties, to the Leipzig profile for the near-neutral atmospheric boundary layer, to flow over isolated hills (Askervein and Bolund) to flow over mountaneous complex terrain (Alaiz). A summary of results from the first benchmarks are used to illustrate the model evaluation protocol applied to flow-over-terrain modeling in neutral conditions