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Upon intense femtosecond photo-excitation, a many-body system can undergo a phase transition through a non-equilibrium route, but understanding these pathways remains an outstanding challenge. Here, we use time-resolved second harmonic generation to investigate a photo-induced phase transition in Ca$_3$Ru$_2$O$_7$ and show that mesoscale inhomogeneity profoundly influences the transition dynamics. We observe a marked slowing down of the characteristic time, $\tau$, that quantifies the transition between two structures. $\tau$ evolves non-monotonically as a function of photo-excitation fluence, rising from below 200~fs to $\sim$1.4~ps, then falling again to below 200~fs. To account for the observed behavior, we perform a bootstrap percolation simulation that demonstrates how local structural interactions govern the transition kinetics. Our work highlights the importance of percolating mesoscale inhomogeneity in the dynamics of photo-induced phase transitions and provides a model that may be useful for understanding such transitions more broadly.

Quantum spin liquids (QSLs) are topologically ordered states of matter that host fractionalized excitations. A particular route towards a QSL is via strongly bond-dependent interactions on the hexagonal lattice. Search for Kitaev materials other than Ru and Ir based compounds is the central topic in condensed matter physics. Using time-domain terahertz spectroscopy (TDTS) we observed a broad magnetic continuum over a wide range of temperature and field in the honeycomb cobalt-based magnet, BaCo$_2$(AsO$_4$)$_2$, which has been proposed to be more ideal versions of a Kitaev QSL. Applying an in-plane magnetic field of $\sim$ 0.5 T suppresses the magnetic order and at higher fields gives rise to a spin-polarized state. Under a 4T magnetic field oriented principally out-of-plane, a broad magnetic continuum was observed that may be consistent with a field induced QSL. Our results indicate BaCo$_2$(AsO$_4$)$_2$ is a promising QSL candidate.

To resolve the severe wear problems of steering engine and fin stabilizer, the nonlinear response characteristics of steering engine and fin stabilizer are studied by analyzing sea trial data of ship Yukun. The nonlinear response characteristics are summarized and applied to a rudder and fin hybrid control system. Decoupling, $$\hbox {H}_\infty$$ and $${\rm H}_2$$ controllers are designed to achieve low control frequency for it. Simulations show that all the three controllers can achieve excellent course-keeping and roll damping performances, and their control frequencies and amplitudes are consistent with those of nautical practice. It can be found that $$\hbox {H}_2$$ controller achieves the best response performance of course-keeping and energy saving performance for rudder engine; $$\hbox {H}_\infty$$ controller achieves the best response performance for roll damping; Decoupling controller has the best energy saving performance for fin stabilizer. When the Beaufort wind force scale is increased from 6 to 8 and model perturbation is involved, $$\hbox {H}_2$$ controller shows the best robustness of control performance and control input. In addition, the designed low-order controllers can reduce hardware cost for autopilot realization in nautical field.

The aim of this study is to propose a novel flexible dry electrode and assembly to a wearable single-lead ECG patch for continuously monitoring atrial fibrillation (AF) in a real-life scenario. Silver ink was screen-printed on a flexible polyurethane substrate to fabricate the multilayered dry electrode. A wearable recording patch with three dry electrodes was developed to collect single-lead ECG signal, particularly to provide a robust anti-artifact capability by employing a driven right leg electrode as reference. The characterization of the dry electrode based patch was quantified by combing the engineering measurement on raw signal quality and the clinical evaluation of automatical AF detection. A total of 50 patients (age 70 ± 11) were enrolled. Each patient wore one or two patches and a standard Holter simultaneously for a continuous 24 hours ECG recording in a real-life scenario. The averaged R wave peak amplitude, signal-to-noise ratio, and signal-to-artifact ratio were 0.75-1.47 mV, 3-6 dB, and 3-7, respectively. Compared with two different cardiologists' interpretations of the ECGs from dry electrodes, an offline detection algorithm automatically interpreted AF with the overall accuracy 93.57% and 85.94% during the stationary and movement states, respectively. The obtained results demonstrated the ability of using a flexible screen-printed dry electrode and assembly to a wearable patch to effectively record ECG signal in patients even disturbed by body motions, which may establish its clinical validity and applicability.

To predict the evolution of wave spectrum in the real ocean, a machine learning framework is developed by training a long short-term memory (LSTM) neural network on a physics-based third-generation wave model (Simulating WAve Nearshore (SWAN)). Considering the realistic ocean waves are usually mixtures of windsea and swells, the wave spectrum is partitioned using a watershed algorithm, such that the windsea and swells are analyzed and predicted separately. Four parameters are selected to capture the wave spectrum of each system, including the significant wave height Hs, peaked wave period Tp, mean propagation direction θm, and directional spreading width σθ. The results demonstrate that the LSTM neural network can achieve accurate prediction of wave condition, the mean absolute error percentage (MAEPs) of 1-h prediction is less than 5.9%, 3.3%, 3.5%, and 3.3% for Hs, Tp, θm, and σθ, respectively, and accurate prediction of wave spectra is achieved. Even for the 10-h prediction, satisfactory results are obtained, e.g., the MAEP of Hs is less than 15.5%. The effects of output size (i.e., prediction duration), input data size (i.e., number of delays), as well as different combinations of input features on predictions of wave conditions are examined.

Vortex Induced Motion (VIM) of multi-column floating platforms, such as Tension Leg Platform (TLP) and semi-submersible (SEMI), in current is well-acknowledged. Substantial VIM response of the multi-column floating platform may cause fatigue failure of mooring and riser systems, which can affect the normal operation of the platform. The present paper focuses on the numerical investigation on VIM of a TLP with circular columns using Computational Fluid Dynamics (CFD). Sensitivity analyses (e.g., mesh size, the number of prism layers and time-step size) for the VIM responses of the TLP are conducted. The effects of the current heading and mooring stiffness on the VIM are investigated. The three degrees of freedom VIM responses (in-line, transverse and yaw responses) and corresponding amplitude spectra are computed and analyzed. Motion trajectories are plotted to understand the VIM behaviors. Regarding the effect of the current heading, the largest transverse response is examined at 15° current heading and the corresponding maximum nominal amplitude is around 0.43. The difference of the maximum nominal amplitudes between the cases at 0° and 15° current headings is less than 5%. For 15°, 30° and 45° current headings, the nominal transverse amplitudes decrease as the current heading increases in the lock-in range. For the four studied current headings, the maximum width of the lock-in range is found at 0° current heading and narrows as the current incidence increases. The largest yaw response is observed at 0° current heading and the maximum nominal amplitude is around 9.1°. Regarding the effect of the mooring stiffness, the lock-in ranges and the maximum nominal amplitudes of the transverse motions have little difference for the four mooring stiffnesses. The maximum nominal transverse and yaw responses are around 0.25 and 5.1°, respectively, which occur when the mooring stiffness reaches the maximum. The flow pattern analyses indicate that the flow interference between the upstream and downstream columns may have significant effects on the VIM responses and a stronger interference at the present spacing ratio may lead to a larger VIM response. The contours of the vertical vorticity in the horizontal plane show that the mean positions of the flow separation points are always on highest or lowest (in the transverse direction perpendicular to the current heading) points of the columns, which is the reason that the VIM trajectories for the TLP with circular columns are always along the direction perpendicular to the current heading.

Quantum spin liquids (QSLs) are topologically ordered states of matter that host fractionalized excitations. A particular route towards a QSL is via strongly bond-dependent interactions on the hexagonal lattice. A number of Ru- and Ir-based candidate Kitaev QSL materials have been pursued, but all have appreciable non-Kitaev interactions. Using time-domain terahertz spectroscopy, we observed a broad magnetic continuum over a wide range of temperatures and fields in the honeycomb cobalt-based magnet BaCo

A phenomenological model for predicting the vortex-induced motion (VIM) of a single-column platform with nonlinear stiffness has been proposed. The VIM model is based on the couple of the Duffing-van der Pol oscillators and the motion equations with non-linear terms. The model with liner stiffness is presented for comparison and their results are compared with the experiments in order to calibrate the model. The computed results show that the predicted VIM amplitudes and periods of oscillation are in qualitative agreements with the experimental data. Compared with the results with linear stiffness, it is found that the application of non-linear stiffness causes the significant reductions in the in-line and transverse motion amplitudes. Under the non-linear stiffness constraint, the lock-in behavior is still identified at 8

The wave crest height qualification checks are required during the wave calibration before the model test in wave basin. However, the reliable criteria of nonlinear wave crest probability distribution in 3-h duration (full-scale) has not been well established yet. We investigate wave crest-height statistics of long-crested nonlinear wave fields using high-order spectral (HOS) method, which can take the effects of both second-order bound waves and third-order free waves into account. The energy dissipation effects due to wave breaking were included by employing an eddy viscosity model. Sensitivity analyses to the wave breaking onset criterion have been performed. Validation is provided by comparing the obtained numerical results with the available calibration test data. Based on extensive and direct numerical simulations, semi-empirical single realization distributions for wave calibration have been developed through 3-parameter Weibull fitting and systematic regression analyses. Particular attention has been paid to the tail of upper bound of wave crest distributions. The effects of wave steepness and water depth on the maximum wave crest height in 3-h duration have been examined. It is found that with the increase of wave steepness, the extreme wave crest height increases until it reaches a critical value. In addition, for the scale water depth kph

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 ( R e ) 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 × 1 0 4 ≤ R e ≤ 5 . 7 × 1 0 5 span from the subcritical to supercritical flow regimes. The experimental results show that at R e 2 . 2 × 1 0 5 , the mean streamwise force coefficient C ¯ x increases as R e grows for 0° current incidence, while it reduces slightly for 15°, 22 . 5 ∘ , 30° and 45° current incidences. At R e ≥ 2 . 2 × 1 0 5 , C ¯ x remains roughly unvaried with R e for each current incidence angle. The fluctuating streamwise force coefficient C x ′ drops gradually as R e increases from 9 . 5 × 1 0 4 to 2 . 2 × 1 0 5 , and then they rise slightly for all the current incidences. The fluctuating transverse force coefficients ( C y ′ ) in the supercritical regime ( 2 . 2 × 1 0 5 R e 5 . 7 × 1 0 5 ) are less than those in the subcritical regime ( 9 × 1 0 4 R e 1 . 5 × 1 0 5 ), which is independent of θ . The fluctuating yaw moment coefficients ( C M z ′ ) show a declining trend with the increase of R e for 0°, 15°, 22 . 5 ∘ and 30° current incidences. In the supercritical regime, the measured C y ′ and C M z ′ 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 C y ′ and C M z ′ grow as H ∕ D (where D is the column diameter) increases from 1.0 to 1.57.

An Innovative Semi-submersible platform Optimization Program (ISOP) has been developed to solve the multi-objective optimization problem for semi-submersible platforms (SEMI). Three types of SEMIs, including semi-submersible floating production unit (SEMI FPU), heave and vortex induced motion (VIM) suppressed semi-submersible (HVS) and semi-submersible floating drilling unit (SEMI FDU) are selected for case studies. The hydrodynamic performances of three types of semi-submersible platforms are analyzed by using panel method and Morison's equation. In order to improve the computing efficiency, the hydrodynamic performances for different hull forms during optimization process are estimated by the surrogate models, which are built by artificial neural network prediction method and Inverse Multi-Quadric (IMQ) radial basis function (RBF). The accuracy of surrogate models is ensured by performing leave-one-out cross validation (LOOCV). The most probable maximum (MPM) heave motion and total weight, representing the safety and economy, respectively, are chosen as the two objectives for optimization. The transverse metacentric height, the MPM surge motion, and the most probable minimum (MPMin) airgap are selected as constraints. Based on surrogate models, multi-objective particle swarm optimization (MOPSO) is employed to search for the Pareto-optimal solutions. A Computational Fluid Dynamics (CFD) tool is adopted to validate the proposed model for the prediction of the motion responses. By comparing the obtained Pareto-optimal solutions with the initial design using simple panel method plus Morison's equation, it is confirmed that the MPM heave motions for SEMI FPU, HVS and SEMI FDU can be suppressed by up to 12.68%, 11.92%, and 14.96%, respectively, and the total weights can be reduced by up to 12.16%, 13.00%, and 24.91%, respectively. Through the detailed analyses of optimization results, the most efficient design strategies for semi-submersible platforms are discussed and proposed.

Considering the massive simulations required by the full long-term analysis, the environmental contour method is commonly used to predict the long-term extreme responses of an offshore renewable system during life time. Nevertheless, the standard environmental contour method is not applicable to the wind energy device due to the non-monotonic aerodynamic behaviour of the wind turbine. This study presents the development of a modified environmental counter method and its application to the extreme responses of a hybrid offshore renewable system. The modified method considers the variability of the responses by checking multiple contour surfaces so that the non-monotonic aerodynamic behaviour of the wind turbine is considered. The hybrid system integrates a floating wind turbine, a wave energy converter and two tidal turbines. Simulation results prove that the modified method has a better accuracy.

In this paper theoretical models are proposed for computing the natural frequencies and modal shapes of two-dimensional asymmetric and symmetric moonpools in the finite water depth. The boundary value problem is solved by using a domain decomposition approach. On the outer vertical boundary bounded by the beam of the two bodies, linearized velocity potential is assumed to be nil. Eigenvalue problem is formulated by matching the velocity potential and fluid flux on the common boundaries to obtain the natural frequencies and modal shapes of the free surface elevation. In the symmetric moonpool cases, so-called single mode approximations (SMA) have been derived and can be adopted for rapid estimation of the natural frequencies for both piston and sloshing modes. The present results have been extensively compared with the solutions using the two-dimensional infinite water depth model developed by Molin [ 1 ], the numerical solutions and experimental data by Faltinsen et al. [ 2 ]. It is found that the solutions have been improved from the infinite water depth model. It is demonstrated that the proposed models can well predict the resonance frequencies and modal shapes for the two-dimensional asymmetric and symmetric moonpools.

Early risk identification of an unexpected sudden cardiac death (SCD) in a person who is suffering malignant ventricular arrhythmias is highly significant for timely intervention and increasing the survival rate. For this purpose, we have presented an automated strategy for prediction of SCD with a high-level accuracy by using measurable arrhythmic markers in this paper. The set of arrhythmic parameters includes three repolarization interval ratios, such as TpTe/QT, JTp/JTe, and TpTe/JTp and two conduction-repolarization markers, such as TpTe/QRS and TpTe/(QT×QRS). Each of them is calculated directly from the detected QRS complex waves and T-wave of electrocradiogram (ECG) signals. Then, all calculated markers are used for the automatical classification of normal and SCD risk groups by employing machine learning classifiers, such as k-nearest neighbor (KNN), decision tree (DT), Naive Bayes (NB), support vector machine (SVM), and random forest (RF). The effectiveness and usefulness of the proposed method is evaluated using a database of measured ECG data acquired from 28 SCD and 18 normal patients. For the automated strategy, the set of five arrhythmic risk markers can predict SCD in less than one second with an average accuracy of 98.91% (KNN), 98.70% (SVM), 98.99% (DT), 97.46% (NB), and 99.49% (RF) for 30 minutes before the occurrence of SCD. Moreover, a practical and straightforward SCD index (SCDI) through a judicious integration of these markers is also proposed by using the Student's t-test. The obtained SCDIs are 1.2058 ± 0.0795 and 1.7619 ± 0.1902 for normal and SCD patients, respectively, which provide a sufficient discrimination degree with a p-value of 6.5061e-35. The present results show that both the automated classifier and the integrated SCDI can predict the SCD up to 30 minutes earlier, and that these predictions could be more practical and efficient if applied in portable smart devices with real-time requirements in hospital settings or at home.

The pyrochlore magnet ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform time-domain THz spectroscopy on high-quality ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ crystals and study the low-energy excitations as a function of [111] magnetic field with high energy resolution. The low-energy crystal-field excitations change their energies anomalously under magnetic field. Despite several sharp field-dependent changes, we show that the material's spectrum can be described not by phase transitions but by field-dependent hybridization between the low-energy crystal-field levels. We highlight the strong coupling between spin and lattice degrees of freedom in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ as evidenced by the magnetic-field tunable crystal-field environment. Calculations based on single ion physics with field-induced symmetry reduction of the crystal-field environment can reproduce our data.

In this paper, we apply a CFD computer code to study the hydrodynamic behavior of a stand-alone cylinder and a dual coaxial-cylinder system (DCCS) via free-decay motion tests. The geometric proportions of a stand-alone cylinder and the inner and outer cylinder of the DCCS are chosen to be the same as those in [1] and [2], respectively, as ocean wave-energy converter (WEC) devices. Overset mesh based on the commercial code ‘Star-CCM+ 11’ is used to simulate the free-decay motion of the two systems. Five parameters chosen for the CFD implementation are: turbulence model, initial displacement, time step, number of prism layers and mesh size. Results obtained from using different values of these parameters are compared so as to confirm the validity of choices made. The hydrodynamic performance of the stand-alone cylinder and outer cylinder in the DCCS are compared with the experimental results to assess and validate the CFD models. In addition, the heave hydrodynamic coefficients, namely, the added mass and total damping, and ‘resonance’ frequency of the stand-alone cylinder and those of the inner cylinder of the DCCS, with the outer cylinder being fixed, are obtained by using the CFD procedure. The hydrodynamic coefficients of another stand-alone cylinder with the same dimensions as the inner cylinder of the dual-coaxial cylinder are also obtained by simulations. The vorticity-contour plots for the stand-alone cylinder and the outer cylinder in the DCCS in free-decay motion are presented and analyzed. Finally, the results of the three cases are compared to examine the effect of the outer cylinder on the heave hydrodynamic coefficients of the inner cylinder.

This study was to assess the feasibility of using non-standardized single-lead electrocardiogram (ECG) monitoring to automatically detect atrial fibrillation (AF) with special emphasis on the combination of deep learning based algorithm and modified patch-based ECG lead. Fifty-five consecutive patients were monitored for AF in around 24 hours by patch-based ECG devices along with a standard 12-lead Holter. Catering to potential positional variability of patch lead, four typical positions on the upper-left chest were proposed. For each patch lead, the performance of automated algorithms with four different convolutional neural networks (CNN) was evaluated for AF detection against blinded annotations of two clinicians. A total of 349,388 10-second segments of AF and 161,084 segments of sinus rhythm were detected successfully. Good agreement between patch-based single-lead and standard 12-lead recordings was obtained at the position MP1 that corresponds to modified lead II, and a promising performance of the automated algorithm with an R-R intervals based CNN model was achieved on this lead in terms of accuracy (93.1%), sensitivity (93.1%), and specificity (93.4%). The present results suggest that the optimized patch-based ECG lead along by deep learning based algorithms may offer the possibility of providing an accurate, easy, and inexpensive clinical tool for mass screening of AF.

In this study new extensions of the theoretical models developed by Molin (2001) and Molin et al. (2018) are proposed to solve the resonance problem for three-dimensional circular and rectangular moonpools with recesses. Eigenfunction expansions are adopted to describe the velocity potentials in all the subdomains. Then, the velocity potentials and normal velocities are matched at the common boundaries such that the eigenvalue problems are formulated and solved, yielding the natural frequencies and associated modal shapes of the free surface. Both the models for infinite and finite water depths are derived for circular moonpools with recesses. Applications are also made for the three-dimensional rectangular moonpools with recesses. In addition, frozen-mode approximation is derived, which yields simple formulas for prediction of the natural frequencies for piston-mode resonances. The proposed models in this study are validated by comparing the obtained results with the experimental data and the results using other numerical models. Moreover, for the first sloshing mode in rectangular moonpool, simple approximation model is proposed based on the assumption that the half of the wavelength of a standing wave is the same as the moonpool length. The validity of the model is examined by comparing the solutions with the results by the diffraction–radiation code WAMIT.

OBJECTIVES This study evaluated the stiffness of tissue surrounding thyroid nodules using shear wave elastography (SWE). METHODS A total of 128 thyroid nodules in 108 patients were examined with conventional ultrasound imaging and SWE. The maximum Young modulus value was measured to evaluate the stiffness of the thyroid nodules (E) and the 2-mm perinodular region (Eshell ). The number of thyroid fibrocytes was evaluated by Masson trichrome staining and image-processing software. The arrangement of the fibrous structure was also classified. RESULTS The mean age ± SD of the 108 patients was 33.12 ± 13.34 years (range, 18-80 years). Thirty-nine thyroid nodules were classified as benign and 89 as malignant. Eshell was significantly higher for malignant nodules (95.0 ± 21.9 kPa) compared with benign nodules (48.1 ± 17.0 kPa; P

Due to the existence of the side walls in a towing tank, the measured hydrodynamic forces would present some discrepancies compared to the open sea results. This phenomenon is referred to as the side wall effect. The objective of the present study is to investigate the side wall effects on ship model testing in a towing tank. The method used in the present study involves a 3D panel method based on the Rankine type Green function. Both the steady and unsteady problems were investigated numerically. The numerical results were validated against ship model test results. After the validations, a large scale computations were performed to investigate the parameters which could determine the side wall effects. Two diagrams of side wall effects (one in calm water and the other one in waves), were obtained which showed whether the side wall effect was less than the permissible error to be included in the measured values.

This Letter proposes two methods for the Gaussian-tailed zero-memory non-linearity (GZMNL) design in symmetric α -stable noise. The optimal GZMNL is designed by maximising the efficacy via a derivative-free method. The efficient GZMNL is designed by polynomial fitting with the derived coefficients. Simulated results demonstrate that the GZMNL designs are nearly as optimal as the locally optimal detector, with an advantage of closed-form formulas.

Anti-adhesion barriers such as films and hydrogels used to wrap repaired tendons are important for preventing the formation of adhesion tissue after tendon surgery. However, sliding of the tendon can compress the adjacent hydrogel barrier and cause it to rupture, which may then lead to unexpected inflammation. Here, a self-healing and deformable hyaluronic acid (HA) hydrogel was constructed as a peritendinous anti-adhesion barrier. Matrix metalloproteinase-2 (MMP-2)-degradable gelatin-methacryloyl (GelMA) microspheres (MSs) encapsulated with Smad3-siRNA nanoparticles were entrapped within the HA hydrogel to inhibit fibroblast proliferation and prevent peritendinous adhesion. Silencing effect of Smad3-siRNA nanoparticles was around 75% towards targeted gene. The mean adhesion scores of composite barrier group were 1.67±0.51 and 2.17±0.75 by macroscopic and histological evaluation, respectively. Furthermore, GelMA MSs were responsively degraded by upregulation of MMP-2, achieving on-demand release of siRNA nanoparticles to inhibit the formation of adhesion tissue. The proposed self-healing hydrogel anti-adhesion barrier with MMP-2-responsive drug release behavior is highly effective for decreasing inflammation and inhibiting tendon adhesion. Therefore, this research provides a new strategy for the development of safe and effective anti-adhesion barriers. This article is protected by copyright. All rights reserved.

The paper presents the study on hydro-elastic responses of a floating raft with a mooring system in both regular and irregular waves. The floating raft consists of a number of buoys, plates and a mooring system, which are modeled by Morison type elements with proper coefficients, segmented line elements with stiffness and several mooring cables, respectively. The added mass coefficients of the buoys are computed by using the panel code WAMIT. Damping coefficients are obtained by CFD simulations of the forced surge/sway motions of a buoy with a free surface. Due to the nonlinear performance of the mooring system, the periods of maximum responses of the structure decrease as the wave height increases. For regular waves, the most of dangerous wave conditions are found in the wave periods 6 ∼ 10 s with the incident wave angles 0 ∘ ∼ 2 0 ∘ or 7 0 ∘ ∼ 9 0 ∘ , while the floating raft is relatively safe for the incident wave angle being around 45°. For irregular waves, the impacts of randomness are analyzed by using different wave seeds. Most of the results are not sensitive to the randomness of the phase angles, except for the maximum mooring tensions which show noticeable fluctuations ( ∼ 15 % ). By comparing the results of the long-crested wave and short-crested wave, it is found that the predictions of the mooring tension and the responses of the floating raft are more conservative in long-crested wave than those in short-crested wave. Besides, the impacts of different frequency ranges on the time histories of the responses are discussed. The maximum structural responses are compared with the most probable maximum (MPM) values obtained by assuming Rayleigh distribution for the responses. The most vulnerable locations of the structure are analyzed based on the maximum dynamic loads on the plates of the floating raft. When the zero-crossing period exceeds 5 s, the relatively large tensions on the plates occur in the north and south sides of the floating raft. The relatively large shear force, bending moment, normal stress, torque and shear stresses on the plates always occur in the four corners of the floating raft.

In this paper, the theoretical model proposed in Zhang et al. (2019) is extended to compute the natural frequencies and modal shapes for two-dimensional moonpools with one or two recesses in finite water depth. In the framework of linear potential flow theory, the boundary value problem is solved by using a domain decomposition method, assuming the velocity potential at the outer boundaries to be nil. In particular, a new and efficient approximation method, double-mode approximations (DMA), is derived to estimate the natural frequencies and modal shapes for both piston-mode and sloshing-mode resonances. The present results using the derived DMA formulas are compared with the experimental results by Ravinthrakumar et al. (2019) and the solutions using the frozen-mode approximation (FMA). The comparisons show satisfactory agreement with the experimental data. In particular, it is shown that, in contrast to FMA, DMA can well predict the non-flat modal shape of the free surface at piston-mode resonance. Moreover, extensive parametric studies are performed to examine the effects of the moonpool geometry on natural frequencies and free-surface modal shapes.

This paper presents numerical studies on vortex-induced motions (VIM) of a multi-column floating oil and gas exploration platform. Numerical computations are performed using an improved delayed detached eddy simulation (IDDES) together with a moving grid approach. The transverse (sway) and yaw motion responses, motion trajectories, motion frequencies and power spectral density of motions are computed and analyzed systematically. After extensive comparisons with experiments, it is confirmed that the present numerical solutions using IDDES agree well with the experimental results and are better than those via delayed detached eddy simulation (DDES). The differences of transverse motion responses between computational results and experiments are less than 10% in the lock-in region. The numerical simulations reveal that the transverse VIM responses occur in a range of reduced velocities from 7.0 to 14.0 at H / D = 1.44 ( H and D are the column height and width, respectively). The largest nominal transverse amplitude, around 35 % of the column width, occurs for 22 . 5 ∘ current incidence. It is found that the VIM responses mainly perform along the platform diagonals for 1 5 ∘ , 22 . 5 ∘ and 4 5 ∘ current incidences. The transverse and yaw motion frequencies for 1 5 ∘ , 22 . 5 ∘ and 4 5 ∘ current incidences are higher than those for 0 ∘ current incidence. The energy levels of the yaw motion responses for 1 5 ∘ , 22 . 5 ∘ and 4 5 ∘ current incidences are about 10% of that for 0 ∘ current incidence. Moreover, parametric studies have been performed to examine the effect of submerged column height on VIM. It demonstrates that VIM in transverse direction grows significantly when submerged column height H / D is greater than 1.0. To be more specific, compared to the case with H / D = 1.44, VIM in transverse direction at H / D = 3.0 increase by around 120%.

Internal waves generated by submerged bodies such as towed spheres, towed cylinders or self-propelled models in stratified fluids are of great interests in oceanic hydrodynamics. Several classifications of internal waves were proposed based on experimental observations, including the Lee waves, the random waves and the gravitational collapsed waves. A more general classification is proposed in recent studies, consisting only two types. The first type is the body-generated internal wave which is stationary to the towed sphere and the second is the wake-generated internal wave which is non-stationary to the towed sphere. According to Robey’s work, experiments in this paper were performed for the time-space characteristics of internal waves generated by a horizontally towed revolution slender body with the aspect ratio λ =7.7, located at five different submerged depths below the strong pycnocline of a stratified fluid. The Reynolds number range is from 2500 Re = UD / v Fr = U / N max D Fr c=3.64 which is independent of submerged depths of the revolution slender body. For Fr Fr c, the internal waves are dominated by body-forced effect, and their correlation velocities are equal to the towing speeds of the revolution slender body, as well as the degree of anti-symmetry for such internal waves becomes larger with the increasing Fr . Moreover, the dimensionless maximum peak-peak amplitudes for such internal waves dominated by body-forced effect firstly increase with the growth of Fr until Fr reaches a certain value of approximately 1.33 regardless of submerged depths, and then begin to decrease. For Fr > Fr c, the internal waves are dominated by wake-forced effect, and their correlation velocities are noticeably lower than the towing speeds of the revolution slender body, as well as the Froude numbers with respect to the correlation velocities for such internal waves vary within the range from 0.7 to 1.4. Results further show that the degree of anti-symmetry for such wake-generated internal waves fluctuates around a constant smaller than 0.5, indicating that the symmetric component is slight stronger than the anti-symmetric component in the wake-generated waves, as well as the opening angles of the wave patterns in the symmetric and anti-symmetric components remain at relatively constant value of approximately 20.6 ° regardless of submerged depths and towed speeds of the revolution slender body. Moreover, the dimensionless maximum peak-peak amplitudes for such internal waves dominated by wake-forced effect linearly increase with Fr for different submerged depths, the slopes of such linear relations decrease with submerged depths and follow an exponential law. In particular, the present work demonstrates that it is unreasonable to assume that the wake- generated internal waves for an axis-symmetric revolution body are symmetric about the longitudinal section in center plane of the body.

A series of directly numerical simulations of potential Euler equation have been performed using high-order spectral (HOS) method, to investigate the nonlinear wave statistics and the probability of rogue wave occurrence in crossing sea states. Several typical crossing sea states in deep water with different wave steepness are chosen for the computations. The ensemble statistical properties for those crossing waves are measured, including the temporal evolution of directional and omnidirectional wave spectra, exceedance probability of wave crest amplitude, as well as the kurtosis and skewness of free surface elevations. Particular attention is paid to the correlation between kurtosis and rogue wave occurrence. Our numerical results suggest that the global wave steepness plays a significant role in the statistical properties of crossing seas. Results also show the dependence of rogue wave occurrence probability on the kurtosis of free surface elevations.