Steps towards a self calibrating, low reflection numerical wave maker using narx neural networks

Numerical wave-makers are important for accurate simulations of marine two-phase flows. One important aspect of the suitability is the ability to recreate a given surface elevation time trace at a given location in the tank. This paper presents first applications of NARX neural network techniques for the calibration of wave tanks. Preliminary results indicate good results can be achieved even for highly non-linear waves

Contribution to the CCP-WSI Blind Test Series 2: CFD-based numerical wave tank experiments employing an impulse source wave maker

During the development and optimisation of wave energy converters, numerical wave tanks are useful tools, providing detailed insight into the hydrodynamic performance of devices. Specifically, computational fluid dynamics based numerical wave tanks can deliver highfidelity, high resolution, results for a wide range of test conditions. However, CFD-based numerical wave tanks come at significant computational cost and require more man-hours during model setup, compared to lower-fidelity, frequency domain based models. The computational costs can only be significantly decreased by improving the numerical solvers, or increased expenditure on computational power. The required man-hours for the model setup, however, can be reduced by streamlining the setup of CFDbased numerical wave tanks. To this end, the formulation of best-practice guidelines and benchmark test cases can expedite this streamlining. A step towards such bestpractice guidelines, and, furthermore, towards an increased confidence in CFD-based numerical wave tanks, are blind tests. This paper presents the CFD-based numerical wave tank used for the authors’ contribution to the Collaborative Computational Project in Wave Structure Interaction Blind Test Series 2. In the employed numerical wave tanks, a recently developed, self-calibrating, impulse source wave maker is implemented for the wave generation [1]. In addition to the required data for the submission to the blind test, the paper contains preliminary studies on the necessity of turbulence modelling, spatial and temporal convergence studies, as well as results for the numerical wave maker calibration

Contribution to the CCP-WSI Blind Test Series 3: Analysis of scaling effects of moored point-absorber wave energy converters in a CFD-based numerical wave tank

Computational fluid dynamics (CFD) based numerical wave tanks (NWTs) can provide valuable insight into the hydrodynamic performance of wave energy converters (WECs). Being able to capture hydrodynamic non-linearities, CFD-based NWTs (CNWTs) allow the analysis of WECs over a wide range of test conditions, such as sea states, power take-off control settings and model scale. The capabilities of a CNWT are exploited in this paper, which aims to analyse of the scaling effects of two moored point-absorber type WECs, exposed to focussed waves. To this end, three different scales are considered: 1:1, 1:10 and 1:10PWT. The latter, 1:10PWT scale, refers to the typical scale used in physical wave tanks (PWTs), complying with Froude scaling, but violating Reynolds scaling. In the 1:10 scale model, fluid viscosity is scaled, in line with the geometric properties, thereby achieving both Froude and Reynolds similitude. From the results, average differences between the three considered scales of around 5% have been observed, and the overall greatest sensitivity to scale effects can be found in the surge and pitch degree of freedom

How stellar activity affects the size estimates of extrasolar planets

Lightcurves have long been used to study stellar activity and have more recently become a major tool in the field of exoplanet research. We discuss the various ways in which stellar activity can influence transit lightcurves, and study the effects using the outstanding photometric data of the CoRoT-2 exoplanet system. We report a relation between the `global' lightcurve and the transit profiles which turn out to be shallower during high spot coverage on the stellar surface. Furthermore, our analysis reveals a color dependence of the transit lightcurve compatible with a wavelength-dependent limb darkening law as observed on the Sun. Taking into account activity-related effects, we re-determine the orbit inclination and planetary radius and find the planet to be approximately 3% larger than reported previously. Our findings also show that exoplanet research cannot generally ignore the effects of stellar activity.Comment: submitted to A&A (to be accepted soon

A comparison of tidal turbine characteristics obtained from field and laboratory testing

Experimental testing of physical turbines, often at a smaller scale, is an essential tool for engineers to investigate fundamental design parameters such as power output and efficiency. Despite issues with scaling and blockage which are caused by limitations in size and flow velocity of the test facilities, experimental tank testing in laboratory environments is often perceived as offering more control and thus trustworthier results than field testing. This paper presents field tests of a tidal turbine, performed using a self-propelled barge in real tidal flow and still water conditions, that are compared to a towing tank test. Factors influencing the performance characteristics, such as the choice of velocity sensor, vessel handling and data processing techniques are investigated in this paper. Direct comparison with test results of the exact same turbine obtained in an experimental test facility further confirms that field testing with robust data analysis capabilities is a viable, time and cost efficient alternative to characterise tidal turbines

Hot Jupiters and the evolution of stellar angular momentum

Giant planets orbiting main-sequence stars closer than 0.1 AU are called hot Jupiters. They interact with their stars affecting their angular momentum. Recent observations provide suggestive evidence of excess angular momentum in stars with hot Jupiters in comparison to stars with distant and less massive planets. This has been attributed to tidal interaction, but needs to be investigated in more detail considering also other possible explanations because in several cases the tidal synchronization time scales are much longer than the ages of the stars. We select stars harbouring transiting hot Jupiters to study their rotation and find that those with an effective temperature greater than 6000 K and a rotation period shorter than 10 days are synchronized with the orbital motion of their planets or have a rotation period approximately twice that of the planetary orbital period. Stars with an effective temperature lower than 6000 K and a rotation period longer than 10 days show a general trend toward synchronization with increasing effective temperature or decreasing orbital period. We propose a model for the angular momentum evolution of stars with hot Jupiters to interpret these observations. It is based on the hypothesis that a close-in giant planet affects the coronal field of its host star leading to a topology with predominantly closed field lines. Our model can be tested observationally and has relevant consequences for the relationship between stellar rotation and close-in giant planets as well as for the application of gyrochronology to estimate the age of planet-hosting stars.Comment: 18 pages, 4 tables, 8 figures, accepted by Astronomy and Astrophysic

A consistent analysis of three years of ground- and space-based photometry of TrES-2

The G0V dwarf TrES-2A, which is transited by a hot Jupiter, is one of the main short-cadence targets of the Kepler telescope and, therefore, among the photometrically best-studied planetary systems known today. Given the near-grazing geometry of the planetary orbit, TrES-2 offers an outstanding opportunity to search for changes in its orbital geometry. Our study focuses on the secular change in orbital inclination reported in previous studies. We present a joint analysis of the first four quarters of Kepler photometry together with the publicly available ground-based data obtained since the discovery of TrES-2b in 2006. We use a common approach based on the latest information regarding the visual companion of TrES-2A and stellar limb darkening to further refine the orbital parameters. We find that the Kepler observations rule out a secular inclination change of previously claimed order as well as variations of the transit timing, however, they also show slight indication for further variability in the inclination which remains marginally significant