Journal of Hydrodynamics最新文献

Since 1970s, several experimental works revealed that the cavitation sheet inception does not occur at the minimum pressure location but further downstream at the location of a laminar/turbulent transition. Most of the cavitation models use the saturation vapour pressure as a threshold to initiate the production of vapour and therefore, are not able to capture such flows. In this paper, three modifications of the Schnerr and Sauer cavitation model are proposed and coupled with an algebraic laminar/turbulent transition model. Application to a NACA 16 012 profile shows the ability of the modifications to move the cavitation inception at the right location compared with the experiment. One of them, based on the multiplication of the evaporation term by the square of the turbulent intensity seems promising.

As one of the most common river patterns in nature, meandering river has very complex flow structures in its curved channel bends, including secondary flow structure and primary flow velocity redistributions. To date, most of the studies have been carried out on the flow structures in channel bends with unavoidable influences from inlet and outlet boundaries, while a streamwise periodic boundary can overcome this shortcoming elegantly. In this paper, large eddy simulations (LES) are employed to investigate the complex flow structures in periodically continuous sharp sine-generated bends. The influence of width-to-depth ratios and dimensionless curvature radiuses are studied. The results highlight two additional vortex structures beyond the commonly known secondary currents: The recirculation zone (RZ) and the inner bank cell (IBC). The width-to-depth ratio shows the determining effect on the recirculation zone. The size of recirculation zone is usually bigger in sine-generated-curve (SGC) channel with large width-to-depth ratios. The biggest recirculation zones appear between the zero-curvature section and the apex section. The inner bank cell only forms in SGC channels with small width-to-depth ratios and low curvature. For SGC channel with large width-to-depth ratios, only one circulation cell is observed near the inner bank. The spatial variations of turbulent features are also revealed by statistical analysis based on the LES sampling data. Results highlight remarkable effect of width-to-depth ratio and dimensionless curvature radius on the turbulent kinetic energy (TKE) and bed shear stress in SGC channels.

This study delved into the acoustic spectrum of bubble clusters, each consisting of 352 vapor bubbles across volume fractions ranging from 0.005% to 40%. The clusters, organized in five distinct layers, were modeled using the volume of fluid (VOF) method to capture the bubble interfaces, and the Ffowcs Williams-Hawkings (FW-H) methodology to compute the far-field acoustic pressure from bubble collapse. Further analysis revealed distinct sound pressure behaviors across different volume fractions: For 25%–40%, time-domain analysis shows that the peak acoustic pressure pulses from the two innermost layers of bubbles are significantly higher than those from the outer layers. In the frequency domain, the octave decay rate of the acoustic pressure levels is relatively low, around −3dB/octave. For 0.5%–25%, four acoustic pressure pulses with similar widths and peak values were observed in the time domain. In the frequency domain, there are three distinct peaks in sound pressure levels (SPL), directly linked to the difference in collapse times of bubbles within the cluster, and the octave decay rate accelerates as the volume fraction decreases, stabilizing at −6dB/octave when the volume fraction is reduced to 17.5%. For 0.005%–0.5%, as the volume fraction decreases from 0.5% to 0.1%, the number of acoustic pressure pulses significantly reduces. Below 0.1% volume fraction, only a single wider pulse is observed. In the frequency domain, the octave decay rate gradually increases with decreasing volume fraction, significantly exceeding −10dB/octave when it drops below 0.1%, reaching up to −11.7dB/octave.

In simulating vegetated flows using the porous approach, the reasonableness of the drag coefficient significantly impacts the calculation results. This study employs large eddy simulation (LES) to quantitatively investigate the effect of drag parameters on key flow characteristics in submerged vegetated flows. The results indicate that changes in the drag coefficient significantly alter the velocity in the middle of the vegetation layer and near the water surface in the free-flow layer. Compared with longitudinal velocity, the drag coefficient has a more pronounced effect on the vertical distribution of Reynolds stress, especially its peak at the top of the vegetation layer. The porous approach can accurately reproduce the vertical distribution of longitudinal velocity and Reynolds stress, consistent with experimental measurements, only when shear-scale flow dominates. Due to the high-intensity secondary flow under moderate vegetation density, fluctuations in the drag coefficient have a more significant impact on the numerical results than in very dense vegetation. Therefore, selecting the drag coefficient value should be done cautiously, especially in the absence of experimental measurements for validation.

Visual and pressurized pipeline systems with single- and multi-undulation layouts were used to study experimentally and analyze theoretically the transient characteristics of water-air two-phase flow during water fillings in undulation pipelines based on the combination action analyses of both the communicating pipe and the gravity of the water-air two-phase flows in the descending pipe. For the single undulation pipeline, the complex two-phase flow-pattern evolutions including full pipe flow and stratified flow for low, medium, high water-filling velocity cases, respectively, lead to a great difference in transient pressure, flow pattern and the water-filling duration. Especially for low and medium water-filling velocity cases, the hydraulic theories related to hydraulic drop and hydraulic jump were employed to investigate the entrapped air pocket evolutions in the descending pipe, and the mechanism of negative pressure at the top of the undulation pipes was analyzed. For the same multi-undulation pipeline, due to the different elevations of the three undulation points along flow direction, namely three different types of pipeline layout, high-medium-low case (high elevation undulation point, medium one, and low one), low-medium-high and high-low-medium ones, their water-filling durations are significantly different, i.e., approximately 80.02 s, 227.34 s and 617.78 s. Meanwhile, there are significant differences in flow patterns in water filling, namely larger entrapped air pockets in three descending pipes for the high-medium-low case, entrapped air pockets in the first two descending pipes and open channel stratified flow in the last one for low-medium-high case, some bubbles in three descending pipes for the high-low-medium case.

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.