Research on the Water Ridge and Slamming Characteristics of a Semisubmersible Platform under Towing Conditions

During the towing of semisubmersible platforms, waves impact and superpose in front of the platform to form a ridge shaped “water ridge”, which protrudes near the platform and produces a large slamming pressure. The water ridges occur frequently in the towing conditions of semisubmersible platforms. The wave–slamming on the braces and columns of platform is aggravated due to the water ridges, particularly in rough sea conditions. The effect of water ridges is usually ignored in slamming pressure analysis, which is used to check the structural strengths of the braces and columns. In this paper, the characteristics of the water ridge at the braces of a semisubmersible platform are studied by experimental tests and numerical simulations. In addition, the sensitivity of the water ridge to the wave height and period is studied. The numerical simulations are conducted by a Computational Fluid Dynamics (CFD) method, and their accuracy is validated based on experimental tests. The characteristics of the water ridge and slamming pressure on the braces and columns are studied in different wave conditions based on the validated numerical model. It is found that the wave extrusion is the main reason of water ridge. The wave–slamming pressure caused by the water ridge has an approximately linear increase with the wave height and is sensitive to the wave period. With the increase of the wave period, the wave–slamming pressure on the brace and column of the platform increases first and then decreases. The maximum wave–slamming pressure is found when the wave period is 10 s and the slamming pressure reduces rapidly with an increase of wave period.

Coupled CFD-FEM Simulation of Steel Box Bridge Exposed to Fire

Increasing fire-induced bridge failures are demanding more precise behavior prediction for the bridges subjected to fires. However, current numerical methods are limited to temperature curves prescribed for building structures, which can misestimate the fire impact significantly. This paper developed a framework coupling the computational dynamics (CFD) method and finite element method (FEM) to predict the performance of fire-exposed bridges. The fire combustion was simulated in CFD software, Fire Dynamic Simulator, to calculate the thermal boundary required by the thermomechanical simulation. Then, the adiabatic surface temperatures and heat transfer coefficient were applied to the FEM model of the entire bridge girder. A sequential coupled thermomechanical FEM simulation was then carried out to evaluate the performance of the fire-exposed bridge, thermally and structurally. The methodology was then validated through a real fire experiment on a steel beam. The fire performance of a simply supported steel box bridge was simulated using the proposed coupled CFD-FEM methodology. Numerical results show that the presented method was able to replicate the inhomogeneous thermomechanical response of box bridges exposed to real fires. The girder failed due to the buckling of a central diaphragm after the ignition of the investigated tanker fire in no more than 10 min. The framework presented in this study is programmatic and friendly to researchers and can be applied for the estimation of bridges in different fire conditions.

Study on Lubrication Characteristics of Novel Forced Wave Generator of Harmonic Drive without Flexible Bearing

In contrast to the conventional forced wave generator which consists of cam and flexible bearing in harmonic drive, the novel forced wave generator retains cam but cancels flexible bearing. In this article, the lubrication characteristics of the novel forced wave generator in harmonic drive is studied. First, an elliptical sliding bearing (ESB) model of simplified structure between the novel forced wave generator and the flex spline is established. Further, the computational fluid dynamics (CFD) method is employed to study the effect of some factors on the lubrication characteristics of the ESB model including elliptical gap ratio, width, and rotational speed. According to the analysis, the elliptical gap ratio has a great impact and its optimal value is 3, which is used in the design of the novel forced wave generator. Last, the practical design of the novel forced wave generator in harmonic drive is given, which can provide a basis for design and optimization of a forced wave generator without flexible bearing of the harmonic drive.

Computational Fluid Dynamics Support for Fontan Planning in Minutes, Not Hours: The Next Step in Clinical Pre-Interventional Simulations

Computational fluid dynamics (CFD) modeling may aid in planning of invasive interventions in Fontan patients. Clinical application of current CFD techniques is however limited by complexity and long computation times. Therefore, we validated a “lean” CFD method to magnetic resonance imaging (MRI) and an “established” CFD method, ultimately aiming to reduce complexity to enable predictive CFD during ongoing interventions. Fifteen Fontan patients underwent MRI for CFD modeling. The differences between lean and established approach, in hepatic and total flow percentage to the left pulmonary artery (%LPA), power loss and relative wall shear stress area were 1.5 ± 4.0%, -0.17 ± 1.1%, -0.055 ± 0.092 mW and 1.1 ± 1.4%. Compared with MRI, the lean and established method showed a bias in %LPA of -1.9 ± 3.4% and -1.8 ± 3.1%. Computation time was for the lean and established approach 3.0 ± 2.0 min and 7.0 ± 3.4 h, respectively. We conclude that the proposed lean method provides fast and reliable results for future CFD support during interventions. Graphical abstract

Study on the Resonant Behaviors of a Bottom-Hinged Oscillating Wave Surge Converter

This paper studied the resonant behaviors of a bottom-hinged oscillating wave surge converter (OWSC) as well as the relationship of resonance with the response and capture width ratio (CWR). The time-domain dynamic equation of an OWSC in shallow water based on the boundary element method (BEM) was solved by a Python code, considering the corrected wave surface and the nonlinearities of restoring moment, drag, and friction. The unknown factors, such as wave surface corrected factor and drag coefficient, were effectively calibrated with computational fluid dynamics (CFD) method. An intermediate initial angle in free decay is appropriate for use to determine the natural period. Under regular waves, the resonance occurs near the natural period for the uniform wave amplitude, rather than the uniform wave torque amplitude, and can disappear due to the amplification of Power Take-Off (PTO) friction. Under unit-amplitude regular waves, the period of maximum CWR is relatively close to the period of maximum velocity, but far from the resonant period. Under irregular waves, no stable resonance is observed because the maximum equivalent pitch angle appears at different peak periods of wave spectra with the variation in PTO damping. When the period of a regular wave or the peak period of an irregular wave is close to the natural period, a phase hysteresis of velocity relative to wave torque always occurs.

Six-DOF CFD Simulations of Underwater Vehicle Operating Underwater Turning Maneuvers

Maneuverability, which is closely related to operational performance and safety, is one of the important hydrodynamic properties of an underwater vehicle (UV), and its accurate prediction is essential for preliminary design. The purpose of this study is to analyze the turning ability of a UV while rising or submerging; the computational fluid dynamics (CFD) method was used to numerically predict the six-DOF self-propelled maneuvers of submarine model BB2, including steady turning maneuvers and space spiral maneuvers. In this study, the overset mesh method was used to deal with multi-body motion, the body force method was used to describe the thrust distribution of the propeller at the model scale, and the numerical prediction also included the dynamic deflection of the control planes, where the command was issued by the autopilot. Then, this study used the published model test results of the tank to verify the effectiveness of the CFD prediction of steady turning maneuvers, and the prediction of space spiral maneuvers was carried out on this basis. The numerical results show that the turning motion has a great influence on the depth and pitch attitude of the submarine, and a “stern heavier” phenomenon occurs to a submarine after steering. The underwater turning of a submarine can not only reduce the speed to brake but also limit the dangerous depth. The conclusion is of certain reference significance for submarine emergency maneuvers.

Effect of Dissipation on the Moonpool-Javelin Wave Energy Converter

In this work, the hydrodynamic performance of a novel wave energy converter (WEC) configuration which combines a moonpool platform and a javelin floating buoy, called the moonpool–javelin wave energy converter (MJWEC), was studied by semianalytical, computational fluid dynamics (CFD), and experimental methods. The viscous term is added to the potential flow solver to obtain the hydrodynamic coefficients. The wave force, the added mass, the radiation damping, the wave capture, and the energy efficiency of the configuration were assessed, in the frequency and time domains, by a semianalytical method. The CFD method results and the semianalytical results were compared for the time domain by introducing nonlinear power take-off (PTO) damping; additionally, the viscous dissipation coefficients under potential flow could be confirmed. Finally, a 1:10 scale model was physically tested to validate the numerical model and further prove the feasibility of the proposed system.

Three-Dimensional CFD Simulations of Start-Up Processes of a Pump-Turbine Considering Governor Regulation

The pumped-storage power station is an efficient stability regulator of the power grid. However, due to the instability of the pump-turbine in the S-shaped characteristic region, rotational speed fluctuation is easy to occur in the speed no-load condition, making synchronization with and connection to the grid difficult. To investigate the key factors of these difficult grid connections, the start-up processes of a practical pump-turbine under the lowest head condition were simulated by using the three-dimensional CFD method, in which the governor regulating equations with different regulating parameters were integrated successfully. The results show that the working points oscillate with the fluctuations of rotational speed, discharge, and torque, and different regulating parameters have a significant influence on the dynamic histories. In addition, the internal flow patterns, especially the backflows at the runner inlet, keep apparent values at the middle span (0.5 span) but have regular transitions near the shroud side (0.7–0.8 span). The faster the guide vanes adjust, the faster the backflows change, and the larger the macro parameters fluctuate. Overall, the instability of the start-up is the result of the periodical evolutions of backflows at the runner inlet, because the trend and period of the radial velocities at different inlet span locations are consistent with those of the discharge.

Analysis of propulsion performance of wave-propelled mechanism based on fluid-rigid body coupled model

Wave-propelled mechanisms are applied to propel unmanned marine vehicles such as Wave Glider and wave-powered boats, which can convert wave energy directly into propulsion. In this paper, a fluid-rigid body coupled dynamic model is utilized to investigate the propulsion performance of the wave-propelled mechanism. Firstly, the coupled dynamic model of the wave-propelled mechanism is developed based on relative motion principle by combining rigid body dynamics model and CFD method. Then, the motion responses of wave-propelled mechanism are calculated. The relationship between the propulsion force, heave and pitch motion of hydrofoil are analyzed by using phase diagrams and the actual operation conditions of propulsion mechanism are obtained. Besides, the effects of restoring spring stiffness and wave heights on the propulsion performance are also investigated, and the vortex evolution is illustrated at different moments of movement and different restoring stiffness. These works can be helpful for the design and optimization of different kinds of wave-propelled vehicles.