Current Incidence Angle Research Articles

Feasibility analysis of deep steep riser based on co-rotational coordinate method

DSR (Deep steep riser) is a new riser structure that reduces the ultra-high-tension load caused by the riser self-weight. In this paper, the mechanical behavior of DSR under different buoyancy module configurations and different ocean currents is studied. The finite element model of DSR is established based on co-rotational coordinate method. The model is solved by arc length method. The accuracy of the numerical method is verified by Abaqus software. Then, the effects of buoyancy module length and buoyancy factor on DSR are analyzed. Finally, the influence of different current incidence angles and velocities on DSR is evaluated. The results show that the DSR model based on the co-rotational coordinate method can effectively simulate the nonlinear behavior of large deformation of DSR. The method is simple, flexible and computationally efficient. This method can quickly improve the efficiency of numerical calculation in static analysis of deepwater riser. And DSR is feasible under certain conditions.

Polarization conversion filter based on electromagnetically induced transparency-like effect

Owing to the large losses in the conversion process of traditional polarization converters, there is an increasing demand for metasurfaces with excellent transmission performance. In this work, an efficient polarization conversion metasurface is proposed based on electromagnetically induced transparency-like (EIT-like) effect in the terahertz band. The multi-level bright mode paths are excited by an asymmetric structure to obtain orthogonal circular polarization conversion windows. The transmission window is generated by the mutual interference of two sets of bright modes with similar resonant frequencies. Then an asymmetric structure is constructed to achieve transmission window shift under TE polarization and TM polarization, thereby realizing dual-frequency polarization conversion. The metamaterial unit structure consists of four open metal resonant rings and four metal resonant strips. The working mechanism is explored by analyzing the surface current distribution, frequency response, and incident angle characteristics. The results show that electromagnetically induced transparency can be achieved under different polarizations. Furthermore, based on the EIT resonance between the two incident polarizations, the conversion from linear polarization to right-hand circular polarization is achieved at 0.692 THz, and the conversion from linear polarization to left-hand circular polarization is realized at 0.782 THz, transmission coefficients are 0.7 and 0.68 respectively. According to the Stokes parameters, the corresponding ellipticity <i>η</i> values are 96% and 98%, respectively. This EIT-based polarization conversion metasurface with low loss and ultra-thin characteristics has great potential applications in compact antennas, derived radar phased arrays, and military detectors.

Short-crested wave-current forces around a concentric multiple-cylinder structure

ABSTRACT The potential flow theory is used to develop a new analytical method to solve the diffraction problem of short-crested incident waves with uniform current acting on a concentric multiple-cylinder system. The influence of uniform current on the hydrodynamic performance of the concentric structure is discussed. The incident angle and speed of the currents have a significant influence on the short-crested wave force and run-up on the concentric structure, i.e. the wave force and wave run-up increase significantly when wave and uniform current directions are the same, while decreasing when in the opposite direction of the wave and uniform current. Additionally, the effects of parameters such as the current incidence angle, current speed, porous-effect parameters, number of perforated walls, and short-crestedness of regular waves on the hydrodynamic performance of the concentric structures are valuated by numerical experiments. It is observed that as the number of permeable walls increases, wave load on the impermeable internal cylinder gradually decreases and the wave surface around is more even. This study is expected to provide theoretical guidance for the design of nearshore architecture.

Extreme Ultraviolet Lighting Using Carbon Nanotube-Based Cold Cathode Electron Beam.

Laser-based plasma studies that apply photons to extreme ultraviolet (EUV) generation are actively being conducted, and studies by direct electron irradiation on Sn for EUV lighting have rarely been attempted. Here, we demonstrate a novel method of EUV generation by irradiating Sn with electrons emitted from a carbon nanotube (CNT)-based cold cathode electron beam (C-beam). Unlike a single laser source, electrons emitted from about 12,700 CNT emitters irradiated the Sn surface to generate EUV and control its intensity. EUV light generated by direct irradiation of electrons was verified using a photodiode equipped with a 150 nm thick Zr filter and patterning of polymethyl methacrylate (PMMA) photoresist. EUV generated with an input power of 6 W is sufficient to react the PMMA with exposure of 30 s. EUV intensity changes according to the anode voltage, current, and electron incident angle. The area reaching the Sn and penetration depth of electrons are easily adjusted. This method could be the cornerstone for advanced lithography for semiconductor fabrication and high-resolution photonics.

Investigation on vortex-induced motions of four-square columns in a square configuration considering galloping under different current velocity and incident angle

The study of vortex-induced motions (VIM) is practical significance for offshore oil and gas development. The VIM of multi-column structures, which has not been completely understood, is more complex compared to that of an isolated column structures. The VIM of four rigidly coupled square columns in a square configuration was investigated numerically using Reynolds-averaged Navier–Stokes (RANS) equations with the SST k–ω turbulence model for incident angles of α = 0°, 15°, 30°, and 45°. The time histories of the displacement, the motion amplitudes, the frequency characteristics, the hydrodynamic forces, the motion trajectories, and the vortex flow patterns were analyzed and discussed. The results indicate that the motion behavior of the four-square columns structure is more likely to be a combined motion of VIM and galloping. More specifically, the critical reduced velocities for occurrence of sway and surge galloping are both approximately 9.23, while yaw galloping may occur at a lower reduced velocity. It is found that the complex motion frequency components cause irregular motion trajectories and vortex flow patterns in the pre-lock-in and post-lock-in cases; but unitary motion frequency components lead to regular motions in the lock-in cases. Additionally, there is a strong coupling relationship between the sway and yaw motion, which is independent of the incident angle.

PASSIVE CONTROL OF VORTEX-INDUCED MOTIONS ON DEEP-DRAFT SEMISUBMERSIBLE PLATFORMS

The deep-draft semisubmersible (DDSS) platform is well known due to its favourable vertical motion routine. However, DDSS platforms face a serious challenge in vortex-induced motion (VIM) due to the changing forces of the column. A computational fluid dynamic (CFD) simulation using AcuSolve is conducted to investigate the effect of a twisted-square column with different twisted angles, 0°, 22.5°, 45.0°, 67.5° and 90.0°, on the VIM responses and performance at current incidence angles of 0° and 45°.Platform models at a scale of 1:70 are studied to determine that the alteration of vortex shedding due the separation line at the twisted column expressively influences the mean and fluctuation of hydrodynamic forces and motion. In particular interest is the drag and lift coefficients, surge, sway amplitudes, as well as the yaw motion of the DDSS platform. The simulation was conducted at a velocity of 0.16 m/s.

Research on seriation design of Dynamic Kill Drilling (DKD) mixer

Abstract Automatic slurry mixing device, one of the core equipment of dynamic kill drilling (DKD) system, is the key to determine whether the drilling fluid system can provide precise density for bottom layer in time and effectively, and also the key equipment to control the operation risk of ultra-deep water drilling in the face of narrow pressure window. Based on the incompressible viscous flow characteristics of non-Newtonian fluid mixed with seawater and combined with the actual drilling fluid field situation of offshore drilling platform, a kind of tubular DKD mixer with dumbbell shaped nozzle as jet element is designed in this paper, which is verified by theoretical calculation and numerical simulation. Finally, “Y type reverse reflux (60°) + 90° cross jet element” was determined as the final structural design scheme. At the same time, the influences of inlet flow rate, length of eddy current section and incident Angle on mixing effect of mixer are studied, and a seriation design scheme of dynamic kill drilling (DKD) mixer is proposed, and a seriation design chart of product is completed.

SOSeas Web App: An assessment web-based decision support tool to predict dynamic risk of drowning on beaches using deep neural networks

ABSTRACT People still drown on beaches in unacceptable numbers due to the lack of knowledge about the risks taking place in them. The proposed methodology forecasts electronic bathing flags in beaches by integrating the benefits of metocean operational systems, machine learning and web-based decision support technologies into a 24/7 risk assessment service that could be easily implemented at any beach worldwide with low costs of maintenance. Firstly, a crosscutting analysis between metocean conditions, beach characteristics and flag records was performed. Secondly, an expert system, based on Deep Learning, was developed to obtain electronic bathing flags as an indicator of the dynamic risk of drowning on beaches. The input variables of the Deep Neural Network were significant wave height, mean wave period, wind velocity, marine current velocity, incidence angle, and beach modal state. Finally, the application of the method to the Santa Catarina’s beaches (Brazil) conveniently reproduced the status flag of beaches.

On Extreme Waves in Directional Seas with Presence of Oblique Current

This paper will address two questions: i) How the fully nonlinear wave-current interactions modify the extreme wave statistics, spectrum characteristics and average shape of extreme waves in directional seas subject to current with different incident angles; ii) Whether the NewWave model is adequate to describe the average shape of nonlinear extreme waves in directional seas with presence of opposing and oblique current. This study employs fully nonlinear numerical simulations, and the results demonstrate that current can enhance the wave crest exceedance probability at distribution tail and kurtosis, broaden the spectra, and cause severe vertical and horizontal asymmetry of extreme wave profiles depending on the incident angle and initial wave steepness. The assessment on the NewWave models reveals that they fail to predict the reduction of the crest width with increasing current incident angle and significantly underestimate the asymmetry parameters for large steepness waves.

Close encounters with the Death Star: Interactions between collapsed bodies and the Solar System

Aims. We aim to investigate the consequences of a fast massive stellar remnant – a black hole (BH) or a neutron star (NS) – encountering a planetary system. Methods. We modelled a close encounter between the actual Solar System (SS) and a 2 M⊙ NS and a 10 M⊙ BH, using a few-body symplectic integrator. We used a range of impact parameters, orbital phases at the start of the simulation derived from the current SS orbital parameters, encounter velocities, and incidence angles relative to the plane of the SS. Results. We give the distribution of possible outcomes, such as when the SS remains bound, when it suffers a partial or complete disruption, and in which cases the intruder is able to capture one or more planets, yielding planetary systems around a BH or a NS. We also show examples of the long-term stability of the captured planetary systems.

Toward Operational Wave‐Current Interactions Over the Agulhas Current System

AbstractA computationally inexpensive operational wave forecasting system taking wave‐current interactions over the fast‐flowing Agulhas Current into account is explored. Spectral wave model hindcasts at 1/16 of a geographical degree are employed using the Simulating Waves in the Nearshore (SWAN) wave model. Unidirectional wave‐current coupling, with currents affecting waves, is performed using both Operational Mercator 1/12 degree (ORCA12) and GLOBCURRENT 1/4 degree currents. Comparisons between the coupled and uncoupled simulations indicate that wave amplification or lowering is highly dependent on the current magnitude and wave incident angle. In opposing current scenarios, the significant wave height can increase by 20–40% and by 60% in extreme cases. Model validation is performed using Jason‐3 satellite altimetry measurements and in situ directional wave buoy measurements. The nearshore wave directional dispersion is found to be significantly influenced by the core of the Agulhas Current. This is especially prominent in current‐following cases with wave‐current direction differences of up to 20°. The findings are all corroborated by simplistic, idealized models.

Force Measurements of the Flow Around Arrays of Three and Four Columns With Different Geometry Sections, Spacing Ratios, and Incidence Angles

Abstract An experimental campaign for the flow around a stationary array of three and four columns with low aspect ratio, H/L = 1.5, piercing the water free surface, was carried out in a towing tank. These numbers of columns correspond to typical multi-column offshore systems, such as semi-submersibles (SS), tension leg platforms (TLPs), and floating offshore wind turbines (FOWTs). Three parameters were investigated: the spacing ratio between column centers (from two up to four characteristic lengths), current incidence angles, and column section geometries (circular, square, and diamond). The Reynolds number of the experiments was 100,000. Forces were measured in each column using a three degrees-of-freedom load cell, and results of lift and drag forces were presented for each column separately and the whole system. The results of mean and standard deviation of forces were assessed using a statistical uncertainty analysis procedure for finite length measurements’ signals. This methodology not only assesses the quality of the experimental data but also facilitates validation of numerical tools. The objectives of the current work were therefore manifold: to better understand the influence of the relative position, shape, and incidence angle on multi-column offshore structures; to create a reliable database for computational fluid dynamics (CFD) validation; and to prepare the path to flow-induced motions (FIMs) experimental and numerical work of free-moving multi-column offshore systems.

Experimental studies of the unsteady hydrodynamic loads on a tension-leg platform at high Reynolds numbers

This paper presents experimental studies on the unsteady hydrodynamic loads (forces and moments) on a Tension-Leg Platform (TLP) due to the action of a steady current at high Reynolds numbers (Re) in a high-speed cavitation water tunnel. Five current incidence angles (θ) ranging from 0° to 45° are selected for the present tests. The achieved Reynolds numbers of 9.5×104≤Re≤5.7×105 span from the subcritical to supercritical flow regimes. The experimental results show that at Re<2.2×105, the mean streamwise force coefficient C¯x increases as Re grows for 0° current incidence, while it reduces slightly for 15°, 22.5∘, 30° and 45° current incidences. At Re≥2.2×105, C¯x remains roughly unvaried with Re for each current incidence angle. The fluctuating streamwise force coefficient Cx′ drops gradually as Re increases from 9.5×104 to 2.2×105, and then they rise slightly for all the current incidences. The fluctuating transverse force coefficients (Cy′) in the supercritical regime (2.2×105<Re<5.7×105) are less than those in the subcritical regime (9×104<Re<1.5×105), which is independent of θ. The fluctuating yaw moment coefficients (CMz′) show a declining trend with the increase of Re for 0°, 15°, 22.5∘ and 30° current incidences. In the supercritical regime, the measured Cy′ and CMz′ for 22.5∘ current incidence are larger, comparing with those for the other current incidence angles. In addition, the correlations between forces and yaw moment indicate that the fluctuating transverse force is the main contribution to the fluctuating yaw moment. Furthermore, the effect of the immersed column height (H) on the forces and yaw moment is examined. It is found that, in the supercritical regime, generally Cy′ and CMz′ grow as H∕D (where D is the column diameter) increases from 1.0 to 1.57.

Short-Crested Wave-Current Forces on Composite Bucket Foundation for an Offshore Wind Turbine

An analytical solution for the diffraction of short-crested incident wave with uniform current on a composite bucket foundation is derived. The influences of the uniform current on wave frequency, wave run-up, wave force, and inertia and drag coefficients on the composite bucket foundation are investigated. The numerical results indicate that the current incident angle and current velocity have significant effects on the short-crested wave run-up, wave force, and inertia and drag coefficients on the composite bucket foundation. For a fixed wave number, the wave frequency, wave run-up, wave forces, and inertia and drag coefficients obviously increase with the increase of current velocity when the relative angle between the current velocity and wave propagation direction is smaller than 90°, whereas they obviously decrease when the relative angle is larger than 90°. It also can be found that the effect of wave-current interaction on the short-crested wave increases with the increase of the total wave number and the decrease of the water depth. The short-crested wave forces will be significantly increased when the current incident angle parallels to the direction of the wave propagating. Therefore, the short-crested wave-current load should be carefully considered in the design of the composite bucket foundation for an offshore wind turbine.

Improving compatibility between thermoelectric components through current refraction**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. NS2016008).

It is well known that components with dissimilar compatibility factors cannot be combined by segmentation into an efficient thermoelectric generator, since each component needs a unique optimal current density. Based on the complex variable method, the thermal-electric field within a bi-layered thermoelectric composite has been analyzed, and the field distributions have been obtained in closed-form. Our analysis shows that current refraction occurs at the interface, both the refraction angle and current density vary with the incidence angle. Further analysis proves that the current densities in two components can be adjusted independently by adjusting the incidence current density and incidence angle, thus the optimal current density can be matched in both components, and the conversion efficiency can be significantly increased. These results point to a new route for high efficiency thermoelectric composites.

Study on Vortex-Induced Motions of A New Type of Deep Draft Multi-Columns FDPSO

A numerical simulation and an experimental study on vortex-induced motion (VIM) of a new type of deep draft multi-columns floating drilling production, storage and offloading (FDPSO) are presented in this paper. The main dimension, the special variable cross-section column and the cabin arrangement of the octagonal pontoon are introduced based on the result. The numerical simulation is adapted to study the effects of current incidence angles and reduced velocities on this platform’s sway motion response. The 300 m water depth equivalent truncated mooring system is adopted for the model tests. The model tests are carried out to check the reliability of numerical simulation. The results consist of surge, sway and yaw motions, as well as motion trajectories. The maximum sway amplitudes for different types of offshore platform is also studied. The main results show that the peak frequencies of sway motion under different current incidence angles and reduced velocities vary around the natural frequency. The analysis result of flow field indicates that the change of distribution of vortex in vertical presents significant influences on the VIM of platform. The trend of sway amplitude ratio curve of this new type FDPSO differs from the other types of platform. Under 45° current incidence angle, the sway amplitude of this new type of FDPSO is much smaller than those of other types of offshore platform at 4.4 ≤ Vr ≤ 8.9. The typical ‘8’ shape trajectory does not appear in the platform’s motion trajectories.

Experimental study on vortex-induced motions of a semi-submersible platform with four square columns, Part I: Effects of current incidence angle and hull appendages

An experimental study on Vortex-Induced Motion (VIM) of the semi-submersible platform concept with four square columns is presented. Model tests were carried out to check the influence of different headings and hull appendages (riser supports located at the pontoons; fairleads and the mooring stretches located vertically at the external column faces; and hard pipes located vertically at the internal column faces). The results comprise in-line, transverse and yaw motions, as well as combined motions in the XY plane, drag and lift forces and spectral analysis. The main results showed that VIM in the transverse direction occurred in a range of reduced velocity 4.0 up to 14.0 with amplitude peaks around reduced velocities around 7.0 and 8.0. The largest transverse amplitudes obtained were around 40% of the column width for 30° and 45° incidences. Another important result observed was a considerable yaw motion oscillation, in which a synchronization region could be identified as a resonance phenomenon. The largest yaw motions were verified for the 0° incidence and the maxima amplitudes around 4.5°. The hull appendages located at columns had the greatest influence on the VIM response of the semi-submersible.

Origins of performance in fiber-based organic photovoltaics

Using ray tracing and optical path iteration, we present a mathematical model for light transmission, absorption, and loss in fiber-based organic photovoltaics. Simulations based on this model give an optimum incident angle, position, and an optimum aspect ratio in terms of other parameters of the fiber photocell. Further, the predicted relationship between current generation and incident angle correlates well with experimental data for a given fiber length. Finally, we show that experimental results on current generation versus fiber diameter are well reproduced by our simulation for fiber diameters of the order of the wavelength of incident light.

The hydrodynamic forces acting on a cylinder array oscillating in waves and current

The problem of the interaction of multiple cylinders oscillating in waves and slow current is considered. The interaction is represented by waves emitted from adjacent cylinders towards the cylinder under consideration. Wave drift forces and moment in the horizontal plane are calculated by the far-field method based on the conservation of momentum or angular momentum. A semianalytical formula for the calculation of the wave drift damping is then deduced. The conservation of the integrals in these formulae is proved. Special treatments to improve the accuracy of results are discussed. Comparisons between calculated results and experimental measurements are made, showing satisfactory agreement. Effects of various combinations of current direction and incident wave angle on the wave drift damping and damping moment are also examined.