The paper presents incompressible Navier-Stokes simulations of the dynamics of a floating wave energy converter (WEC) coupled to a high-order finite element solver for cable dynamics. The coupled model has very few limiting assumptions and is capable of capturing the effects of breaking waves, green water loads on the WEC as well as non-linear mooring forces and snap loads, all of which are crucial for correct estimates of the extreme loads acting on the system in violent seas. The cable dynamics model has been developed as a stand-alone library that can be coupled to any body motion solver. In this study the open-source CFD package OpenFOAM has been employed. Preliminary test cases using incident regular Stoke's 5th order waves are presented, both for wave heights corresponding to operational conditions of the WEC as for a more severe condition in survival mode. It is illustrated that the coupled model is able to capture the complicated force propagation in the mooring cables.
Various sources of harmonic problems in large wind power plants (WPPs) and optimized harmonic mitigation methods are presented in this paper. The harmonic problems such as sources of harmonic emission and amplification as well as harmonic stability are identified. Also modern preventive and remedial harmonic mitigation methods in terms of passive and active filtering are described. It is shown that WPP components such as long HVAC cables and park transformers can introduce significant low-frequency resonances which can affect wind turbine control system operation and overall WPP stability as well as amplification of harmonic distortion. It is underlined that there is a potential in terms of active filtering in modern grid-side converters in e.g. wind turbines, STATCOMs or HVDC stations utilized in modern large WPPs. It is also emphasized that the grid-side converter controller should be characterized by sufficient harmonic/noise rejection and adjusted depending on WPPs to which it is connected.
This study concerns nitrogen based emissions from a hydrogen enriched ammonia fueled SI engine. These emissions deserve special attention as their formation may differ from conventional HC combustion due to the nitrogen content in the fuel. A range of experiments are conducted with a single cylinder 0.612 l CFR engine with a compression ratio varying from 7 to 15 using a fuel composition of 80 vol% NH3 and 20 vol% H2. Wet exhaust samples are analysed with an FT-IR. Emission measurements reveal that nitric oxide stem from other reaction paths than the dissociation of molecular nitrogen. This causes the NO emissions to peak around 35% rather than 10% excess air, as is typical in HC fueled SI-engines. However the magnitude of NO emissions are comparable to that of measurements conducted with gasoline due to lower flame temperatures. Nitrogen dioxide levels are higher when comparing with gasoline, but has a relatively low share of the total NOx emissions (3–4%). Nitrous oxide is a product of NH2 reacting with NO2 and NH reacting with NO. The magnitude is largely affected by ignition timing due to the temperature development during expansion and the amount of excess air, as increased oxygen availability stimulates the formation of the NH2 radical and the levels of NO2 are higher. Under ideal operating conditions (MBT ignition timing) N2O levels are very low. The dominating contributors to unburned ammonia are chamber crevices as the magnitude of these emissions is greatly affected by the compression ratio. However, levels are lower than required in order to eliminate all NOx emissions with a SCR catalyst.
We present computations of cavitating flow over a NACA0015 hydrofoil. The simulations are performed by a finite volume compressible Euler model with dynamic mesh adaptation. The adaptive mesh refinement (AMR) is driven by a generic, simple and efficient error estimator based on the jump in value between cell faces for a given variable. It is shown that AMR based on vapour fraction provide unsatisfactory results both for (quasi-) steady and unsteady cavitation, as the major flow features are not captured. Instead, adaptivity driven by the Q-value proved successful even for resolving the cavity interface.
For the EU's Common Fisheries Policy, solving the discard problem is a central issue. Through a study of the institutional set-up and initiatives to solve the discard problem in Denmark, the Faroe Islands, Iceland and Norway, the article identifies the discard problem as related to natural and other material conditions as well as cultural conditions. Therefore, solving the discard problem requires not only technical and regulatory instruments, but also arenas and structures that allow and support cultural change processes.
An adaptive spectral/hp discontinuous Galerkin method for the two-dimensional shallow water equations is presented. The model uses an orthogonal modal basis of arbitrary polynomial order p defined on unstructured, possibly non-conforming, triangular elements for the spatial discretization. Based on a simple error indicator constructed by the solutions of approximation order p and p-1, we allow both for the mesh size, h, and polynomial approximation order to dynamically change during the simulation. For the h-type refinement, the parent element is subdivided into four similar sibling elements. The time-stepping is performed using a third-order Runge-Kutta scheme. The performance of the hp-adaptivity is illustrated for several test cases. It is found that for the case of smooth flows, p-adaptivity is more efficient than h-adaptivity with respect to degrees of freedom and computational time.
This paper explores the application of modular multi-level converters (MMC) as a means for harnessing the power from off-shore wind power plants. The MMC consists of a large number of simple voltage sourced converter (VSC) submodules
that can be easily assembled into a converter for high-voltage and high power. The paper shows that the MMC converter has a fast response and low harmonic content in comparison with a two-level VSC option. The paper discusses the modeling approach used, including a solution to the modeling challenge imposed by the very large number of switching devices in the MMC.
Several large offshore wind power plants (WPP) are planned in the seas around Europe. VSC-HVDC is a suitable means of integrating such large and distant offshore WPP which need long submarine cable transmission to the onshore grid. Recent trend is to use large wind turbine generators with full scale converters to achieve an optimal operation over a wide speed range. The offshore grid then becomes very much different from the conventional power system grid, in the sense that it is connected to power electronic converters only. A model of the wind power plant with VSC-HVDC connection is developed in PSCAD for time-domain dynamic simulation. This paper presents the modeling and simulation of such a system. A single line to ground fault has been simulated and fault currents for the grounded and ungrounded offshore grid system are obtained through simulation and then compared.
The depth-integrated shallow water equations are frequently used for simulating geophysical flows, such as storm-surges, tsunamis and river flooding. In this paper a parallel shallow water solver using an unstructured high-order discontinuous Galerkin method is presented. The spatial discretization of the model is based on the Nektar++ spectral/hp library and the model is numerically shown to exhibit the expected exponential convergence. The parallelism of the model has been achieved within the Cactus Framework. The model has so far been executed successfully on up to 128 cores and it is shown that both weak and strong scaling are largely independent of the spatial order of the scheme. Results are also presented for the wave flume interaction with five upright cylinders.