With the continuous advancement of tidal energy development and utilization, power generation systems are gradually evolving towards floating-type and multi-unit configurations. However, the platform motion has a significant effect on the turbine's hydrodynamic characteristics, presenting challenges to power output control and structural reliability of the power generation systems. Therefore, this paper employs numerical method to simulate the rotational and surging motions of a double-unit vertical-axis tidal turbine (VATT) and examines the impact of surging motion on hydrodynamic loads. In addition, based on the Fourier series expansion, a decomposition model for the VATT hydrodynamic loads under surging motion is developed, and the influence of hydrodynamic loads between the dual units is analyzed. The research results indicate that the hydrodynamic load coefficient fluctuation amplitude, based on rotational frequency, varies periodically with surging motion, and its range increases as the surge amplitude and frequency increase; the average power coefficient of the double-unit VATT decreases (Up to 18 % reduction) as the surge amplitude and frequency, but remains higher (Maximum increase of 6.85 %) than that of the single VATT under the same environment. The research findings provide a reference for the reliability design and operational control strategy optimization of the double-unit VATT power generation systems.

Under actual sea conditions, a floating horizontal-axis tidal power device (FHATPD) will experience a six-degreeof-freedom (6-DOF) wave-induced motion response, resulting in a change in the relative inflow velocity to the horizontal-axis tidal turbine (HATT). If the HATT maintains a constant rotational velocity, its hydrodynamic loads will fluctuate with the 6-DOF motion of the supporting structure. Consequently, to ensure the stable and safe operation of the FHATPD, the rotational speed of the HATT can be controlled. The purpose of rotational speed control is to enable the HATT to operate at its stated power rating. First, the power coefficient is calculated for different tip speed ratios using the established computational fluid dynamics (CFD) method for the surging motion and variable-speed rotation of the HATT. Subsequently, a rotational speed prediction model is established based on a multilayer perceptron (MLP), and the prediction results are verified by a CFD-based method. The training of the MLP model is improved by adjusting the feature parameters, supplementing the training data, and smoothing the mutation curves. Finally, the rotational speed prediction model of the HATT operating at rated power is obtained. These results can be a valuable reference for the FHATPD's operational stability and efficiency.

Scour significantly reduces the lateral load capacity of rigid piles, threatening the stability of offshore wind turbines. During scour, the lower soil around the foundation may be changed from a normally consolidated state to an overconsolidated state due to the removal of the upper soil, altering soil parameters such as friction angle, overconsolidation ratio, relative density, void ratio, and lateral earth pressure coefficient. While these changes have been extensively studied theoretically, they have not been fully incorporated into numerical studies. Ignoring these variations can lead to overly conservative designs. This article focuses on the increased lateral earth pressure coefficient due to scour and its impact on numerical simulations. Using the Boussinesq solution and considering the shading effect of the pile foundation, the vertical effective stress of the remaining soil around the pile was adjusted, resulting in a theoretical formula for the lateral earth pressure coefficient after scour. Based on numerical simulation and model tests, an empirical formula is obtained. Finally, a semiempirical and semitheoretical formula is proposed to calculate the increased lateral earth pressure coefficient postscour. This study provides a reference for determining the appropriate lateral earth pressure coefficient in numerical studies.

A cylinder placed in low Reynolds number flows can cause instabilities, such as unstable fluid induced forces and non-periodic vibrations of structures, leading to increased fatigue loading. It can also generate significant flow-induced noise. Therefore, understanding the vibrational behavior and its acoustic propagation mechanism in this configuration is crucial. This study investigates the aerodynamics and acoustic characteristics of a transversely self-excited oscillating circular cylinder at Ma = 0.2, Re = 150 and m* = 5. To simplify the problem, we model the movement of the cylinder using a mass-spring-damper system and solve the motion trajectory using the Newmark-beta method. The acoustic governing equations formulated within viscous/ acoustic splitting method in terms of a moving mesh are derived and validated by comparison with the direct numerical simulation method results. Key parameters, including amplitude ratio, frequency ratio, lift and drag coefficients, and phase angle between lift and displacement were analyzed over a range from reduced velocity Ur = 2 to Ur = 9. Various vortex-induced vibration phenomena, such as "lock-in," "phase switching," and "beating," are observed. The predicted sound signal exhibits distinct variations across the initial, lower and desynchronization branches:minimal impact of vibration on the acoustic field in the initial branch, a "beating" phenomenon between the initial and lower branches, the acoustic field rotation due to a sudden increase in drag force in the lower branch, and reduced acoustic wave intensity in the desynchronization branch due to vortex shedding suppression.

In the actual operation of a floating vertical axis tidal turbine (VATT), the VATT undergoes wave-induced motion with the floating carrier, resulting in a constantly changing relative inflow velocity of the VATT. With the VATT rotating at a fixed speed, the tip speed ratio would vary over time, leading to a lower average energy utilization rate. Therefore, a variable speed control model based on surge velocity is proposed, and a CFD numerical method is presented for a VATT rotating at variable speed under surge motion. The proposed variable speed control model is effective in improving the average energy utilization rate, e.g., by 36.09 % at a surge period of 2.9 s and a surge amplitude of 0.1 m, as compared to fixed speed rotation. Based on this, a rapid forecast method for hydrodynamic loads of the VATT during variable speed rotation and surge motion is established, considering the variation of the damping coefficient during fixed speed rotation and surge motion. Compared with the CFD results, the proposed method can quickly and effectively forecast the VATT's hydrodynamic loads. The findings can provide a reference for the speed control of floating VATT in actual operation and the rapid prediction of the VATT's hydrodynamic load.

During the hydraulic performance experiment, significant vibration and noise were observed in the mixed-flow pump operating in the hump region. Cavitation occurrence in the impeller flow channels was confirmed through the transparent chamber. To analyze cavitation flow structure evolution in the mixed-flow pump, this paper integrates numerical and experimental approaches, capturing cavitation flow structures under the valley condition through high-speed photography technology. During the various stages of cavitation development, the cavitation forms are mostly vortex cavitation, cloud cavitation, and perpendicular vortex cavitation. Impeller rotation induces downstream transport of shedding cloud cavitation shedding structures. Flow blockage occurs when cavitation vortexes obstruct specific passages, accelerating cavitation growth that culminates in head reduction through energy dissipation mechanisms. Vortex evolution analysis revealed enhanced density of small-scale vortex structures with stronger localized core intensity in the impeller and diffuser. Despite larger individual vortex scales, reduced core intensity persists throughout the full flow domain. Concurrently, velocity profile characteristics across flow rates and blade sections (spanwise from tip to root) indicate heightened predisposition to flow separation, recirculation zones, and low-velocity regions during off-design operation. This study provides scientific guidance for enhancing anti-cavitation performance in the hump region.

Utilizing computational fluid dynamics (CFD), this study analyzes the relative pitching motion amplitude and conversion efficiency of the parallelogram raft wave energy converter (R-WEC) under wave current conditions, examining the effects of power take-off (PTO) parameters, wave parameters, and flow velocity on R-WEC hydrodynamic performance. The research includes an analysis of a single point mooring system to determine optimal mooring conditions. Through comparative analysis of energy conversion efficiency across 10 single mooring modes and nine double-mooring modes, the study evaluates their impact on the R-WEC. Findings demonstrate that flow velocity adversely affects wave energy capture. Energy conversion efficiency exhibits an initial increase followed by a decrease as damping coefficient or wave frequency coefficient increases. An optimal anchor chain unit mass coefficient exists that maximizes R-WEC energy conversion efficiency. The dual mooring system demonstrates marginally enhanced energy conversion efficiency compared with single mooring, with specific impacts on R-wave energy converters (WECs) documented. These findings provide valuable reference data for R-WEC design optimization and operational strategies to enhance conversion efficiency.

The jet in crossflow describes a unique flow characteristic, which involves the injection of a fluid jet into a turbulent boundary layer. This flow structure is significant for applications in vibration reduction, noise suppression, and cooling and heat transfer. In this study, a numerical model for jet-in-crossflow was established using Large Eddy Simulation, and the accuracy of simulation results was validated by comparison with experimental data. On the basis of maintaining the same jet exit flow rate, the study investigated the differences in fundamental flow characteristics, wall pressure fluctuations, and flow noise among jet-in-crossflow cases with circular, square, and elliptical orifices. The distribution of sound sources for the three cases was identified using vortex sound theory. The study shows that the elliptical orifice case did not develop a jet blockage effect similar to the other cases. All three orifice cases generated counter-rotating vortex pairs, although the vortex core positions varied. For the circular and square orifices, the root mean square pressure along the lower edge of the orifice exhibited a symmetric distribution, while the elliptical orifice case showed no clear symmetry. In the downstream region, spanning 2–10 orifice diameters, a stable dominant frequency was observed for all three cases, which is attributed to vortex transport. The elliptical orifice demonstrated significant noise reduction performance, with a noise reduction of 3–8 dB, mainly concentrated in the downstream region up to 10 orifice diameters. Using Proper Orthogonal Decomposition and Dynamic Mode Decomposition, the relationship between vortex transport and wall pressure fluctuations for the three cases was explained. The analysis revealed the fundamental reason behind the noise reduction capabilities of the elliptical orifice and the vortex structures responsible for generating the dominant frequency.

The finite volume method demonstrates superior performance in computational fluid dynamics due to its high mesh adaptability, strict conservation properties, excellent computational efficiency, robust stability, and exceptional capability in handling complex geometries. The advancement of the weighted essentially non-oscillatory scheme has facilitated the development of high order finite volume methods, enabling high-precision sound field calculations. For the low Mach number flow, the fluid motion can be effectively approximated as an incompressible flow, allowing the separation of acoustic and incompressible flow field variables from the compressible flow field variables. The viscous/acoustic splitting method is employed, integrating advanced treatments for high-order convective schemes and implementing effective reflection-free boundary conditions. Furthermore, the computational efficiency of sound field calculations is enhanced by controlling acoustic field grid sizing. Its performance is validated through several cases, including: (1) flow past a fixed cylinder at Reynolds number 150 and Mach number 0.2, (2) flow past a rotating cylinder with a speed ratio of 0.2 at Reynolds number 160 and Mach number 0.2, and (3) flow past a cylinder with a splitting plate at Reynolds number 150 and Mach number 0.2. The results demonstrate that the proposed method accurately captures sound field propagation characteristics, with the reflection-free boundary effectively suppressing acoustic wave reflections. By reasonably selecting interpolation functions and maintaining a minimum of ten grid points per acoustic wavelength, the present method can predict the sound field quickly and accurately.

The outboard discharge of underwater vehicle forms a fluid structure in which the cross flow interacts with the jet. The hydrodynamic noise generated by the jet in cross flow(JICF) seriously affects the safety performance of the underwater vehicle. In this paper, the large eddy simulation method is used to establish the numerical model of the JICF, and compared with the literature experiment to verify the accuracy of the calculation results. Based on the theory of vortex sound, the sound source intensity and action area of the elliptical orifice and the circular orifice are compared, and the flow noise and fluctuating pressure characteristics of the elliptical and circular jet exits are obtained by using the Lighthill acoustic analogy method. The results show that the elliptical orifice structure with the same outlet area has obvious noise reduction function. The noise reduction area is reflected in the range of 10 times the aperture downstream of the orifice, and the main noise reduction frequency is reflected in the range of 100-1000 Hz.

The vibro-acoustic response of ring stiffened cylindrical shells with internal bulkheads under forced excitation is presented. The numerical analysis model is established using the Jacobi Ritz-Boundary element method. The first-order shear deformation theory, multi-segment technique and artificial spring technology are applied to establish the theoretical model, and the Jacobi orthogonal polynomials are introduced to represent the displacement functions. The Newmark-β integration method is used to obtain the vibration response of the structure, and the time-domain Kirchhoff boundary integral formulation is applied to describe the exterior acoustic field. The vibration and sound radiation of the ring stiffened cylindrical shell under impact and narrow band random loading were measured. The comparative study reveals that the results obtained from the proposed method agree well with the experimental results. The stiffeners and bulkheads have a significant effect on the vibration characteristics of the structure. Additionally, some physical insights into the resonant peaks and sound pressure directivity of the cylindrical shells are provided.

The study of the interaction between crossflow and jets has been ongoing for many years, however, the fluctuating pressure generated by jets in crossflow(JICF) is still rarely studied. In this work, the wall pressure fluctuation characteristics of JICF with three jet angles are analyzed in detail by large eddy simulation(LES). It is found that adjusting the injection angle effectively reduces the intensity and range of the fluctuating pressure. In order to reveal the mechanism of this process, the differences in the source terms of the Poisson equation under three cases are studied, and a visualization method for characterizing the source terms of partial Poisson equation by convective velocity is proposed. By analyzing the proper orthogonal decomposition (POD) modes of velocity and pressure, the reasons for the difference in the source terms of the Poisson equation are further revealed, and the noise reduction mechanism of adjusting the jet angle is explained from the perspective of eddy current transport. The results show that the main influence area of the fluctuating pressure is from 2 times the aperture upstream to 20 times the aperture downstream. The low jet angle condition has a lower mean shear-turbulence interaction(MTI) and turbulence-turbulence interaction(TTI) terms, and the MTI1 sub-term is the main contribution component of the wall fluctuating pressure. The POD spatial model shows that the vertical hanging wake vortex structure establishes the pressure relationship between the flow field and the wall surface in the process of providing vorticity for the main stage hairpin vortex. Reducing the jet angle effectively reduces the eddy current transport between the wall and the flow field, so the low jet angle condition has a lower fluctuating pressure level.

A numerical simulation based on the CFD method is used to study the interaction between a horizontal cylinder and wave flow. Firstly, a two-dimensional numerical calculation model of both a fixed and a rigid moving cylinder, with a free surface under varying wave flow conditions, is created. In the established model, the loads on the horizontal cylinder under different submergence depths, flow velocities, cylinder sizes, wave periods, and k values (spring stiffness) are analyzed and calculated. The results show that, when the cylinder is close to the free surface, its hydrodynamic load under wave flow conditions is more sensitive to changes in submergence depth, which essentially affects wave reflection and blockage. At different flow velocities, k values, cylinder radii, and arm lengths, the main frequency of the Fourier transform of the cylinder motion curve remains unchanged; however, the main frequency does change with the wave period and submergence depth. The efficiency of rotary cylindrical energy harvesting is influenced by various factors, among which an initial increase and then decrease are observed with a gradually increasing k value, arm length, period, and radius, in addition to an observed decrease with increasing flow velocity.

To analyze the noise induced by moving rigid structures in low Mach number flows, acoustic governing equations based on the viscous/acoustic splitting method and the arbitrary Lagrangian–Eulerian method are rigorously derived. In order to resolve the numerical instability generated in a non-uniform mean flow, the modified viscous/acoustic method, based on the filtering method, is developed. The acoustic equations are transformed into the same form as the incompressible flow equations by introducing the acoustic co-velocity and solved based on a collocated grid finite volume method. An approach for solving acoustic equation based on the PIMPLE algorithm is presented and computed in open-source computational fluid dynamics software OpenFOAM, which brings down communication costs and speeds up computing efficiency. Furthermore, the source term decomposition is extended to study the noise generated by each source term in a motion grid. Several examples including stationary and moving meshes have been designed to prove the accuracy of this approach. Finally, the aerodynamic and acoustic properties for the flow past a transversely oscillating cylinder at Re = 200, Ma = 0.2 in lock-in and non-lock-in regions is present.

Nonlinear aeroelastic system has the characteristics of complex structure, difficult modeling and difficult calculation of dynamic response. For the analysis of nonlinear aeroelastic systems, model identification is a very attractive method. However, the models identified by traditional methods are often relatively complex and limited in scope of use, so it is necessary to develop an interpretable equivalent reduced model. In this paper, sparse regression method and sequential threshold least squares technique are used to establish sparse identification method for complex aeroelastic systems. This method has the ability to identify reduced models containing only required nonlinear terms through measurement data. Then, the sparse identification method is used to identify the binary wing with dead zone nonlinearity and cubic stiffness nonlinearity. The obtained model can provide rapid and accurate prediction of the response of the system according to the sensor measurement, and can also be used as an explicit surrogate model for aeroelastic optimization design, thus verifying the superiority of the proposed method.

To obtain the hydrodynamic load characteristics of the floating horizontal-axis tidal turbine (HATT) with rotation and surging motion under wave-current conditions, a numerical method based on CFD is established to calculate and analyze the power and axial load characteristics of the HATT under different wave heights, surge periods, surge amplitudes and tip speed ratios. The results demonstrates that the power and axial load coefficients of the HATT fluctuate based on the relative wave and surge frequencies, and the fluctuation amplitude increases gradually with the increase of surge amplitude, surge frequency, and tip speed ratio. In this foundation, a fast prediction model of the power and axial load coefficients is proposed, and the correlation coefficients in the model can be obtained by fitting the CFD calculation results based on the least square method. The results show that the calculation results based on the fast prediction method are basically consistent with the CFD calculation results, verifying that the fast prediction method can effectively predict the hydrodynamic loads of the HATT with rotation and surging motion under wave-current conditions. Research findings can provide a fast calculation method of HATT axial load and power coefficients when the HATT and floating platform are coupled.

Simulation of the hydrodynamic performance of a floating current turbine in a combined wave and flow environment is important. In this paper, ANSYS-CFX software is used to analyse the hydrodynamic performance of a vertical-axis turbine with various influence factors such as tip speed ratio, pitching frequency and amplitude. Time-varying curves for thrust and lateral forces are fitted with the least squares method; the added mass and damping coefficients are refined to analyse the influence of the former factors. The simulation results demonstrate that, compared with non-pitching and rotating turbines under constant inflow, the time-varying load of rotating turbines with pitching exhibits an additional fluctuation. The pitching motion of the turbine has a positive effect on the power output. The fluctuation amplitudes of thrust and lateral force envelope curves have a positive correlation with the frequency and amplitude of the pitching motion and tip speed ratio, which is harmful to the turbine’s structural strength. The mean values of the forces are slightly affected by pitching frequencies and amplitudes, but positively proportional to the tip speed ratio of the turbine. Based upon the least squares method, the thrust and lateral force coefficients can be divided into three components, uniform load coefficient, added mass and damping coefficients, the middle one being significantly smaller than the other two. Damping force plays a more important role in the fluctuation of loads induced by pitching motion. These results can facilitate study of the motion response of floating vertical-axis tidal current turbine systems in waves.

The finite volume method, based on the dynamic mesh method, is used to investigate the transient viscous incompressible flow around an impulsively and translationally started cylinder with strips. The strips of different shapes are installed at different locations on the surface of the cylinder. The main purpose of this paper is to investigate the influence of the locations and shapes of strips on the flow caused by boundary motion. The present solutions agree well with the experimental results reported in literature. Six placement angles of strips were selected: 0°, 20°, 60°, 90°, 120° and 150°. The development of wake shows some new phenomena with different strip locations, and the significant difference appears at α = 90°. The vortex intensity is much larger than that of other locations. On the other hand, four shapes of strips were selected: arc, triangle, rectangle and trapezoid. The rectangular strips had the greatest influence on the drag coefficient and the maximum of the drag coefficient increased from 0.4 to 2.8, compared with the smooth cylinder. The maximum of negative velocity had the most significant change when the shape of strip is arc, increasing by 34% compared with the smooth cylinder, at T = 3.