Journal of Hydrodynamics最新文献

A review of multi-chamber oscillating water column (OWC) device designs is presented. Two significant variations of these devices are discussed, onshore OWC (OOWC) and a floating OWC (FOWC). The efficiency results of several theoretical studies based on low- and high-fidelity numerical models are presented and compared with the model scale results. Generally, low-fidelity numerical models are very fast to run, but their accuracy is limited compared with high-fidelity numerical models. Scaled model experiments usually give results much more accurate than numerical models, but they need adequate facilities and are very expensive. In the case of the OOWC, all models show a similar trend of total efficiency, but while the analytical model shows a maximum value of around 90% efficiency, the CFD model shows 60%, and the experiments only go up to 40%. The main reason is connected with the mathematical simplifications and assumptions that do not represent all the hydrodynamic and aerodynamic processes between the water, air, and structure. For the case of the FOWC, interestingly, the experimental results show a maximum efficiency of almost 100%, while the analytical model only predicts a maximum of 80%. The efficiency seems highly dependent on the heave motion resonance of the entire device, where the analytical model fails to predict this natural frequency.

Wind turbines (WTs) face a high risk of failure due to environmental factors like erosion, particularly in high-precipitation areas and offshore scenarios. In this paper we introduce a novel computational tool for the fast prediction of rain erosion damage on WT blades that is useful in operation and maintenance decision making tasks. The approach is as follows: Pseudo-Direct Numerical Simulation (P-DNS) simulations of the droplet-laden flow around the blade section profile are employed to build a high-fidelity data set of impact statistics for potential operating conditions. Using this database as training data, a machine learning-based surrogate model provides the feature of the impact pattern over the 2-D section for given wind and rain conditions. With this information, a fatigue-based model estimates the remaining lifetime and erosion damage for both homogeneous and coating-substrate blade materials. This prediction is done by quantifying the accumulated droplet impact energy and evaluating operative conditions over time periods for which the weather at the installation site is known. In this work, we describe the modules that compose the prediction method, namely the database creation, the training of the surrogate model and their coupling to build the prediction tool. Then, the method is applied to predict the remaining lifetime and erosion damage to the blade sections of a reference WT. To evaluate the reliability of the tool, several site locations (offshore, coastal, and inland), the coating material and the coating thickness of the blade are investigated. In few minutes we are able to estimate erosion after many years of operation. The results are in good agreement with field observations, showing the promise of the new rain erosion prediction approach.

Oscillating water column (OWC) based wave energy absorption devices are classic which have been widely used for harnessing ocean wave energy. This paper presents a numerical study on a projecting wall (PW) type OWC wave energy converter in regular waves. The computational fluid dynamics (CFD) modelling of a stationary floating PW-OWC model in a three-dimensional wave flume is achieved by the software Flow-3D. Numerical analyses are carried out based on CFD simulations and the linear potential flow solutions with modifications to account for turbine-induced damping. The present numerical solutions are validated against our previous experimental data. It is found that both the CFD and modified linear potential flow predictions are in reasonably good agreements with the experimental data in the first order results of OWC and air pressure responses. When the nonlinear responses are included in the result, the modified linear potential flow solution is found to slightly under-estimate the wave energy conversion performance at long wavelengths. Regarding the airflows above and below the chamber orifice, the CFD results suggest that they are almost unidirectional, oscillating in not only the base frequency but also subharmonic and ultraharmonic frequencies. The evolution of the OWC responses during an entire period and the phase analysis based on CFD simulations are presented. The phase results provide the crucial evidence to the reasonability of the physics-based modification of the potential flow model in modelling of OWCs. The present results and analysis are expected to be beneficial to the understanding on the physical mechanism of OWCs and the design of phase control strategies.

Scientific computing libraries, whether in-house or open-source, have witnessed enormous progress in both engineering and scientific research. Therefore, it is important to ensure that modifications to the source code, prompted by bug fixing or new feature development, do not compromise the accuracy and functionality that have been already validated and verified. This paper introduces a method for establishing and implementing an automatic regression test environment, using the open-source multi-physics library SPHinXsys as an illustrative example. Initially, a reference database for each benchmark test is generated from observed data across multiple executions. This comprehensive database encapsulates the maximum variation range of metrics for different strategies, including the time-averaged, ensemble-averaged, and dynamic time warping methods. It accounts for uncertainties arising from parallel computing, particle relaxation, physical instabilities, and more. Subsequently, new results obtained after source code modifications undergo testing based on a curve-similarity comparison against the reference database. Whenever the source code is updated, the regression test is automatically executed for all test cases, providing a comprehensive assessment of the validity of the current results. This regression test environment has been successfully implemented in all dynamic test cases within SPHinXsys, including fluid dynamics, solid mechanics, fluid-structure interaction, thermal and mass diffusion, reaction-diffusion, and their multi-physics couplings, and demonstrates robust capabilities in testing different problems. It is noted that while the current test environment is built and implemented for a particular scientific computing library, its underlying principles are generic and can be easily adapted for use with other libraries, achieving equal effectiveness.

Effective urban land-use re-planning and the strategic arrangement of drainage pipe networks can significantly enhance urban flood defense capacity. Aimed at reducing the potential risks of urban flooding, this paper presents a straightforward and efficient approach to an urban distributed runoff model (UDRM). The model is developed to quantify the discharge and water depth within urban drainage pipe networks under varying rainfall intensities and land-use scenarios. The Nash efficiency coefficient of UDRM exceeds 0.9, which indicates its high computational efficiency and potential benefit in predicting urban flooding. The prediction of drainage conditions under both current and re-planned land-use types is achieved by adopting different flood recurrence intervals. The findings reveal that the re-planned land-use strategies could effectively diminish flood risk upstream of the drainage pipe network across 20-year and 50-year flood recurrence intervals. However, in the case of extreme rainfall events (a 100-year flood recurrence), the re-planned land-use approach fell short of fulfilling the requirements necessary for flood disaster mitigation. In these instances, the adoption of larger-diameter drainage pipes becomes an essential requisite to satisfy drainage needs. Accordingly, the proposed UDRM effectively combines land-use information with pipeline data to give practical suggestions for pipeline modification and land-use optimization to combat urban floods. Therefore, this methodology warrants further promotion in the field of urban re-planning.

In winter, rivers in cold regions often experience flood disasters resulted from ice jams or ice dams. Investigations of the variation of ice jam thickness and water level during an ice jammed period are not only a practical need for ice prevention to avoid disaster and plan water resource, but also essential for the development of any mathematical model for predicting the evolution of ice jam. So far, some equations based on the energy equation have been proposed to describe the relationship between ice jam thickness and water level. However, in the derivation of these equations, the local head loss coefficient at the ice jam head and the riverbed slope factor were neglected. Obviously, those reported equations cannot be used to preciously describe the flow energy equation with ice jams and accurately calculate the ice jam thickness and water level. In the present study, a more comprehensive theoretical model for hydraulic calculation of ice jam thickness has been derived by considering important and essential factors including riverbed slope and local head loss coefficient at the ice jam head. Furthermore, based on the data collected from laboratory experiments of ice jam accumulation, the local head loss coefficient at the ice jam head has been calculated, and the empirical equation for calculating the local head loss coefficient has been established by considering flow Froude number and the ratio of ice discharge to flow discharge. The results of this study not only provide a new reference for calculating ice jam thickness and water level, but also present a theoretical basis for accurate CFD simulation of ice jams.

Double bubbles near a rigid wall surface collapse to produce a significant jet impact, with potential applications in surface cleaning and ultrasonic lithotripsy. However, the dynamic behaviors of near-wall bubbles remain unexplored. In this study, we investigate the jetting of a near-wall bubble induced by another tandem bubble. We define two dimensionless standoff distances, γ₁, γ₂, to represent the distances from the center of the near-wall bubble to the rigid wall and the center of controlling bubble to the center of the near-wall bubble, respectively. Our observations reveal three distinct jetting regimes for the near-wall bubble: transferred jetting, double jetting, and directed jetting. To further investigate the jetting mechanism, numerical simulations are conducted using the compressibleInterFoam solver in the open-source framework of OpenFOAM. A detailed analysis shows that the transferred jet flow is caused by the pinch-off resulting from the axial contraction velocity at the lower end of the near-wall bubble being greater than the vertical contraction velocity, leading to a maximum jet velocity of 682.58 m/s. In the case of double jetting, intense stretching between the controlling bubble and the wall leads to a pinch-off and a double jetting with a maximum velocity of 1 096.29 m/s. The directed jet flow is caused by the downward movement of the high-pressure region generated by the premature collapse of the controlling bubble, with the maximum jet velocity reaching 444.62 m/s.

In this study, direct numerical simulations were conducted to investigate the compressible flow around a circular cylinder near a heated wall at a Reynolds number (Re) of 500 and a Mach number (Ma) of 0.4. The heating ratio T* ranging from 1.0 to 1.6 represents the different situations of a heated wall, whereas the gap ratio ranges from 0.3 to 1.0. This study analyzed the impact of heating effect and wall proximity on flow characteristics and aerodynamic forces. The results indicated that the stability of the flow was enhanced as the gap ratio decreased or the heating ratio increased. Through the calculation of enstrophy, it was found that the strength of shedding vortices weakens with a decrease in gap ratio or an increase in heating ratio. Furthermore, the mean drag coefficient decreases as the heating ratio increases or the gap ratio decreases. In contrast, the mean lift coefficient initially decreases and then increases as the gap ratio decreases. Finally, the drag reduction mechanism was analyzed by examining the pressure distribution on the surface of the cylinder.

To investigate the energy partition in laser-induced cavitation bubbles near the rigid wall with a gas-containing hole, we utilized a nanosecond resolution photography system based on a Q-switched Nd: YAG laser and conventional industrial camera to carefully observe the transient process of bubble collapse near the rigid wall with a gas-containing hole. We analyzed the generation of collapse microjets and the emission of collapse shock waves. We found that the cavitation bubble near the rigid wall with a gas-containing hole collapsed at different times and space, and produced various types of shock waves. Based on the far field pressure information of the shock waves measured by hydrophone, the energy of the shock waves generated by the bubble collapse near the rigid wall with a gas-containing hole is calculated for the first time. The results show that the ratio of collapse shock wave energy to bubble energy is approximately between 0.7 and 0.8.

Due to vegetation drag and vegetation-generated turbulence, bedload transport in vegetated channels is more complicated than that in nonvegetated channels. It is challenging to obtain accurate predictions of bedload transport in vegetated channels. Previous studies generally used rigid circular cylinders to simulate vegetation, and the impact of plant morphology on bedload transport was typically ignored; these methods deviate from natural scenarios, resulting in prediction errors in transport rates of more than an order of magnitude. This study measured bedload transport rates inside P. australis, A. calamus and T. latifolia canopies and in arrays of rigid cylinders for comparison. The impact of plant morphology on bedload transport in vegetated channels was examined. Inside the canopies of natural morphology, the primary factor driving bedload transport is the near-bed turbulent kinetic energy (TKE), which consists of both bed-generated and vegetation-generated turbulence. A method was proposed to predict the near-bed TKE inside canopies with natural morphology. For the same solid volume fraction of plants, the transport rate inside canopies with a natural morphology is greater than or equal to that within an array of rigid cylinders, depending on the plant shape. This finding indicates that plant morphology has a significant impact on transport rates in vegetated regions and cannot be ignored, which is typical in practice. Four classic bedload transport equations (the Meyer-Peter-Müller, Einstein, Engelund and Dou equations), which are suitable for bare channels (no vegetation), were modified in terms of the near-bed TKE. The predicted near-bed TKE was inserted into these four equations to predict the transport rate in canopies with natural morphology. A comparison of the predictions indicated that the Meyer-Peter-Müller equation had the highest accuracy in predicting the transport rate in vegetated landscapes.