196,877 research outputs found
Sea level in the Mediterranean Sea: the contribution of temperature and salinity changes
No full textCarbonate chemistry and dissolved oxygen in the Gulf of Papagayo (Culebra Bay), Costa Rica
No full textSalinity, pH, sea water temperature (SWT) and pCO2 was measured at a water-depth of ~ 3 m at the Marina Papagayo (85°39'21.41"W; 10°32'32.89"N) in Costa Rica in April 2009. SWT and the pCO2 was measured by using SUNDANS, which was developed by "Marine Analytics and Data" (MARIANDA, Germany, www.marianda.com) according to the recommendations of the 2002 underway pCO2 system workshop in Miami, Florida. Salinity and the dissolved oxygen concentrations were determined by using WTW probes (Cond3310 and Multi 340i). The pH was measured using an Orion ROSS electrode and an Orion StarTM. The Orion ROSS electrode was calibrated by using NBS standards and re-calibrated to total scale by using the RCM standards (Batch 82: http://andrew.ucsd.edu/co2qc/). Omega_Aragonite (Ωa), DIC, and TA were calculated based on the ƒCO2 and the pH. Rixen et al. 2012 provides more detailed information
Eddy transport of Western Mediterranean Intermediate Water to the Alboran Sea
No full textDuring the second cruise of the EU funded OMEGA project the towed undulating vehicle SeaSoar, was deployed to survey the upper 350 m of the water column in the eastern Alboran Sea and extreme western Algerian basin. With an effective along-track resolution of 4 km, the data sets enabled a detailed description of the different upper ocean water types and the fronts that separate them. The Almeria Oran front forms at the eastern boundary of the Alboran Sea gyre system, in the upper 150–200 m of the water column, and separates waters of predominantly Atlantic origin from those formed in the Western Mediterranean Sea. Below these surface waters, but above the Levantine Intermediate Water, Western Mediterranean Intermediate Waters, believed to be formed to the north of the Balearic Sea, are normally observed in this region. However, to our knowledge, this is the first time a discrete eddy of Western Mediterranean Intermediate Water, a “weddy,” has been described in the extreme western Algerian basin. Repeated surveys of the region allowed us to observe the evolution of the eddy over a period of 40 d. A climatological analysis of historical data in the MEDAR/MEDATLAS database provides evidence for the repeatability of this observation and the significance of the estimated transport
Diagnosis of vertical velocities with the QG omega equation: an examination of the errors due to sampling strategy
No full textVertical motion at the mesoscale plays a key role in ocean circulation, ocean-atmosphere interaction, and hence climate. It is not yet possible to make direct Eulerian measurements of vertical velocities less than 1000 m day?1. However, by assuming quasi-geostrophic (QG) balance, vertical velocities O (10 m day?1) can be diagnosed from the geostrophic velocity field and suitable boundary conditions. Significant errors in the accuracy of this diagnosis arise from the necessary compromise between spatial resolution and synopticity of a hydrographic survey. This problem has been addressed by sampling the output of a numerical ocean model to simulate typical oceanographic surveys of mesoscale fronts. The balance between the number of observations and the synopticity of observations affects the apparent flow and in particular the diagnosed vertical motion. A combination of effects can typically lead to errors of 85% in the estimation of net vertical heat flux. An analytical two-layer model is used to understand components of this error and indicate the key parameters for the design of mesoscale sampling
On the forcing of sea level in the Black Sea
No full textForcing mechanisms for sea level variability in the Black Sea are investigated in the context of an observed increase in the sea level of this basin by 2.5 mm/yr over the last 60 years. Temperature and salinity variations computed from the Mediterranean Data Archeology and Rescue (MEDAR) data set exhibit significant interdecadal variability. However, the corresponding steric height variation does not show a long-term increase and thus cannot account for the observed change in sea level. The impact of surface freshwater flux (P-E) changes is also investigated using two independent data sets. The first data set, which is based on measurements collected in the basin, can explain most of the sea level variability, with only 0.8 mm/yr remaining unexplained. The second data set, output from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, is unable to explain any of the observed trend. Potential contributions from changes in river runoff and surface pressure are quantified but found to be minor terms. By comparing the observed salinity changes with the sea level rise and the P-E variability in the first data set, we infer that the P-E variations are the primary cause for the observed sea level rise, while land movements are likely to partly contribute, too. The relationship of Black Sea temperature and salinity variability with corresponding variability in the connected Aegean Sea has also been explored. A significant correlation is found between the salinity of the upper water of the Aegean Sea and the layer between 50 and 300 m in the Black Sea, indicating that the latter layer is a product of the Mediterranean inflow
Optical monitoring and operational modal analysis of large wind turbines
No full textIdentification of the dynamic properties and the corresponding structural response of wind turbines is essential for optimizing the energy produced, ensuring safe and reliable operation and increasing the life-time of the system. As the sizes of modern wind turbines increase, their dynamic behaviors get more complicated and it becomes more important to predict the response characteristics of new designs through simulations. Modern computation and simulation tools provide designers with great opportunities to detect and solve most of the possible problems at very early stages and improve their designs. Indeed, several important system properties such as eigenfrequencies and mode shapes, which govern the dynamic response of the turbine, can be estimated very accurately by using structural analysis programs. However, some important dynamic parameters (e.g. damping) cannot be modeled precisely without supplementary information obtained from in-field tests and measurements. Considering the fact that only the models validated by real response measurements are able to represent the complicated dynamic behavior of the structure, various tests have been applied on both parked and rotating turbines for several decades. However, some further improvements are still needed for testing and analyzing the dynamic characteristics of these specific structures in an accurate and efficient way. This thesis aims at making a contribution to this challenging field of experimental and operational modal analyses through several aspects; - Two non-contact optical measurement systems (laser interferometry and photogrammetry) are proposed as alternative turbine monitoring systems. In Chapter 2 and 3, it is demonstrated that optical measurement systems enable the dynamic response of the turbine to be measured with a high precision and spatial resolution both at parked condition and in operation. The pros and cons of the methods and the acquired accuracies are discussed in detail. - In Chapter 3, the vibration data recorded on a 2.5 MW -80 meter diameter- wind turbine by using 3 different measurement systems (laser interferometry, photogrammetry and conventional strain gauges) are analyzed by using modal analysis algorithms based on NExT (Natural Excitation Technique) and LSCE (Least Square Complex Exponential) techniques. Several important turbine parameters (eigenfrequencies and damping ratios) are extracted and compared with the results presented in literature. - In Chapter 4, the main challenges in testing and analyzing the in-operation vibration characteristics of wind turbines are discussed in detail. The factors affecting the accuracies of the estimated modal parameters and the applicability limits of some state of the art system identification tools are examined. In order to investigate specifically the performance of the identification algorithms, numeric response data generated by an analytical model and an aeroelastic simulation tool were used. - In Chapter 5, an alternative method (based on NExT) is proposed for identification of the systems with high modal damping. The introduced technique aims at improving the capabilities of NExT in extracting the highly damped eigenmodes such as the aeroelastic modes of an operating wind turbine. It is demonstrated that the proposed approach enables the eigenfrequencies of the high damping modes to be estimated by using data series which are approximately 30 times shorter than those required for standard NExT algorithm. Results of the analyses show that eigenfrequencies of highly damped modes can be estimated with an average accuracy of 95%. The stability of the proposed method and the possible effects of measurement noise on the estimated modal parameters are also investigated in Chapter 5.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin
Mastering Electro-Mechanical Dynamics of Large Off-Shore Direct-Drive Wind Turbine Generators
No full textThe ever growing population of human beings on earth introduces the challenge of providing affordable, sustainable energy for everyone. Emerging markets, such as China, India or Brazil, quench their thirst for cheap energy by fossil fuels and nuclear power. At the same time researchers from all over the globe warn the public of the advent of a new, civilisation threatening disaster: climate change. Over the last two centuries mankind has gotten used to cheap but polluting energy provided by burning coal, gas and oil. The challenge arises in the form of the transition of our current economy towards a sustainable way of living. Renewable energy sources such as wind, tidal currents, the sun and geothermal heat have seen enourmous growth rates since the early nineties, as they are seen as the best approach to overcome this challenge. Of these renewable energy sources, wind energy is one that has received major attention. In the quest for expanding wind energy capacity, focus has shifted towards the sea in recent years. The potential energy yield is higher off-shore caused by higher average wind speeds. Maintenance and availability are key issues off-shore, due to the more complex logistics. In recent years, the price of on-shore wind energy has decreased to a level that is competitive with prices for energy from some types of fossil fuel. However, the prices for off-shore wind energy remain above the ones of fossil fuels. It is, thus, not surprising that the reduction of off-shore wind energy costs is one of the main innovation drivers within the wind industry. With the advent of off-shore wind energy more and more companies started investigating a new turbine topology called direct-drive wind turbines. This turbine type eliminates the gearbox found in other types of wind turbines, as this might lead to increased availability and lower maintenance costs. In the search for the best design of direct-drive wind turbines, every part of the turbine is investigated, analysed, measured and optimised to improve the functionality of that part. At the heart of the turbine, where the mechanical is transformed into electrical energy, is the generator. Also this component needs to be optimised with respect to weight and efficiency. This thesis aims to find the structural design that optimally utilises the mass of the generator structure to minimise deformation. This is done for the dynamic loads encountered in the generator. Special focus is given to the interaction between the structural dynamics and the magnetic field. This is important as the interaction between these two physical domains can lead to unexpected dynamic behaviour of the system. In Part I of this thesis, the modelling techniques that accurately include the interaction between the structural part of the turbine and the magnetic field in the generator are introduced. These techniques can, for the first time, predict the modal parameter changes, including damping changes, due to the interaction by forming a monolithic eigenvalue problem of the coupled system. The model neglects certain nonlinear influences on the dynamics, such as hysteresis and saturation. Its ability to predict changes of the modal parameters is validated by vibration measurements of a magneto-mechanical coupled system. Furthermore, this part develops new methods to handle huge magneto-mechanical coupled models that emerge when magnetic fields and structural dynamics of a direct-drive wind turbine are modelled. The bottleneck is the memory requirements of the monolithic formulation that makes it necessary to solve for all degrees of freedom simultaneously. Part II applies the techniques developed in Part I to the generator of the XD-115, a 5 MW direct-drive wind turbine and conducts the first two-way coupled analysis of such a generator type. The detailed dynamic analysis of the generator gives new insights in the dynamic behaviour of the generator. Furthermore, the eigenfrequencies, modes and possible causes for excitation are identified. An experimental validation of the XD-115 models was conducted using in-situ experimental and operation modal analyses. Various techniques are compared for the challenging task of exciting the rotor structure. In the second part of Part II, the loads identified during the dynamic analysis are used as load case for a structural optimisation. Topology and shape optimisation were used to identify the optimal mass distribution for the rotor structure that minimises the deformation in the air gap. This way, the weight of the structure could be reduced significantly without compromising the static and dynamic performance of the generator structure. During the optimisation the suitability and potential of topology optimisation for direct-drive wind turbines was evaluated. Although the introduced methodology can be applied to any electric machine, the implications for direct-drive wind turbine generators are most significant, as for these machines the ratio between produced torque and weight is especially high. Important influences on and encountered challenges for improving the design are collected to improve future turbine designs.PMEMechanical, Maritime and Materials Engineerin
Diagnosis of vertical velocities with the QG Omega equation: a relocation method to obtain pseudo-synoptic data sets
No full textpeer reviewedThe quantification of vertical motion and vertical fluxes is essential in our ability to predict and tolerate climate change. However, diagnostic estimations might be affected by the errors arising from the necessary compromise between spatio-temporal resolution and cost of hydrographic surveys. Observations of a numerical ocean model have been made in order to test the accuracy of different sampling strategies and their possible a-posteriori corrections. A simple first-order correction method, computing a pseudo-synoptic data set from a non-synoptic data set and involving a geostrophic relocation of the stations is shown to correct significantly the synopticity error in hydrographic data, derived QG vertical motion and vertical temperature fluxes. Sensitivity analyses also show that the lack of synopticity is more critical than other factors, including the sampling resolution, the level of no-motion and the analysis. (C) 2001 Elsevier Science Ltd. All rights reserved
Non-synoptic versus pseudo-synoptic data sets: an assimilation experiment
No full textSeveral first-order correction methods are implemented to compute pseudo-synoptic data sets from non-synoptic raw data sets. These include a geostrophic relocation method, a linear and a quadratic interpolation method, and a method using spatio-temporal correlation functions. The relocation method involves analyses and geostrophic velocity computations to allow the relocation of stations in time and space to a particular analysis time. Interpolation methods involve several almost identical and consecutive surveys interpolated in time. Temporal weighting methods are based upon a spatio-temporal function modifying the weight on data with respect to the time at which they have been sampled. These techniques are tested on the OMEGA data set and are validated by simple nudging assimilation into a 3D primitive equation model (PE). It is shown that, under certain hypothesis, these methods are able to correct the lack of synopticity in hydrographic data sets, and improve the diagnosis of vertical velocities computed from the Omega equation.These methods are of particular interest for the scientific community. They might be used together with diagnostic models. They might provide suitable pseudo-synoptic fields required by 3D PE models as initial conditions, boundary conditions or assimilation data sets. They may also be useful in the design of mesoscale samplings
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