Water Exit Research Articles

Thermal performance prediction of a V-trough solar water heater with a modified twisted tape using ANFIS, G.L.R., R.T. and SVM models of machine learning

Four distinct neural models were used to evaluate the efficiency of a V-trough solar water heater (VTSWH) equipped with square-cut twisted tape (SCTT) and V-cut twisted tape (VCTT) at two different twist ratios, 3 and 5. The objective of this study was the use of ANFIS (Adaptive Neuro-Fuzzy Inference System), G.L.R. (Generalised linear regression), R.T. (Regression tree), and SVM (Support Vector Machine). A total of 162 data sets were acquired for these models through a variety of trials. Outdoor experiments were done using a twist ratio of Y = 3 and Y = 5, using both SCTT and VCTT. The models included eight distinct variables: ambient temperature, water mass flow rate, water intake temperature, water exit temperature, absorber plate temperature, tube temperature, solar intensity, and twist ratio. The dependent variables in this study are the Nusselt number (Nu), friction factor (FF), and efficiency (η). 130 datasets were chosen for training purposes, while 32 were used for testing. Using the ANFIS, G.L.R., R.T., and SVM techniques, the correlation coefficient (R2) values for Nusselt number were 0.9990, 0.9961, 0.9562, and 0.9280 for friction factor 0.9966, 0.9683, 0.9810, and 0.9560, and for efficiency 0.9997, 0.9976, 0.9845, and 0.9614, respectively. Comparing all models shows that ANFIS is the most effective of the four strategies studied. The ANFIS model outperformed the other models regarding Nu, FF, and η, with RMSE values of 0.0805, 0.0.0004, and 0.4534. According to the above data, the VTSWH thermal performance predicted using the ANFIS approach has the highest accuracy.

Study on vortex structure and hydrodynamic characteristics of water-exit projectile with shoulder ventilation

During the vertical motion of a water-exit projectile, the gas stored inside is expelled from the shoulder under the effect of pressure differential, forming a ventilated cavity covering the surface of the projectile. This paper explores the vortex structure characteristics of the surface ventilated cavity through numerical simulation and summarizes its impact on the hydrodynamic characteristics of the water-exit projectile. It was found that the end of the shoulder ventilated cavity presents a periodic vortex shedding form. And the existence of the shoulder ventilated cavity can reduce the vertical resistance of the projectile, improve the vertical movement speed, and play a role in load reduction and drag reduction. Additionally, the lateral displacement of the projectile at the moment of water exit decreases by approximately 45.2%, and the deviation angle reduces by about 55.6%, significantly improving the stabilization of its water exit posture. This indicates that the presence of the shoulder ventilated cavity can effectively enhance the hydrodynamic performance of the projectile.

Effects of axial launch spacing on cavitation interference and load characteristics during underwater salvo

This paper analyzes the effect of launch interval on cavitation flow interference and load characteristics during underwater salvo. The study employs the Improved Delayed Detached Eddy Simulation and the Schnerr-Sauer cavitation model, Volume of Fluid (VOF) multiphase flow model, and overlapping grid. Additionally, decompression experiment systems are designed, and numerical simulations are found to be in good agreement with experimental results, thus verifying the effectiveness of the simulation. Detailed discussions are provided on multiphase flow field and load distribution. The results reveal a top-down collapse process of the cavity, with collapse shrinking to an isolated bubble at the end. Synchronized collapse pressure is characterized by short pulse widths at the peaks, all located at the lowermost part of the cavity. During the underwater stage, when the axial launch spacing ranges between 0.5 times and 1.0 times the length of the projectile, the head of the second projectile acts on the area below the center of mass of the first. This leads to gradual stabilization of the initial cavity and a decrease in deviation of the center of mass toward the inside. Despite experiencing large-scale fracture and detachment due to interference from the wake of the first engine, the motion stability of the inside cavity of the second projectile remains intact. In the water exit stage, when the axial launch spacing ranges between 0.75 times and 1 time the length of the projectile, it causes expansion and contraction of the inside cavity of the second projectile. However, asymmetric synchronous collapse loads may occur, leading to unstable motion posture.

Overview of Theory, Simulation, and Experiment of the Water Exit Problem

The water exit problem, which is ubiquitous in ocean engineering, is a significant research topics in the interaction between navigators and water. The study of the water exit problem can help to improve the structural design of marine ships and underwater weapons, allowing for better strength and movement status. However, the water exit problem involves complex processes such as three-phase gas–liquid–solid coupling, cavitation, water separation, liquid surface deformation, and fragmentation, making it challenging to study. Following work carried out by many researchers on this issue, we summarize recent developments from three aspects: theoretical research, numerical simulation, and experimental results. In theoretical research, the improved von Karman model and linearized water exit model are introduced. Several classical experimental devices, data acquisition means, and cavitation approaches are introduced in the context of experimental development. Three numerical simulation methods, namely, the BEM (Boundary Element Method), VOF (Volume of Fluid), and FVM (Finite Volume Method) with LES (Large Eddy Simulation) are presented, and the respective limitations and shortcomings of these three aspects are analyzed. Finally, an outlook on future research improvements and developments of the water exit problem is provided.

Numerical study of vehicle motion during water exit under combined lifting force and wave action

During the retrieval process of the deep-sea mining vehicle (DSMV), the stability of the retrieval system is strongly influenced by the interaction between the vehicle body and the surrounding seawater due to the vehicle's complex shape and wave motion. Naturally, the negative side effects of significant changes in the vehicle's attitude and the water exit position can only increase retrieval's challenge. To investigate the characteristic of the flow field of the DSMV, this study employs the computational fluid dynamics method based on the Reynolds-averaged Navier–Stokes equations integrating the volume-of-fluid multiphase flow model with a fifth-order Stokes-wave model to explore the attitude and displacement changes of the vehicle during the water exit process in the ocean wave environment. The results indicate that the wave phase and lifting force are the major effect factors in the DSMV's water exit process. An appropriate lifting force under a specific wave phase can effectively reduce attitude changes and positional drift of the DSMV during water exit, thereby enhancing recovery efficiency and stability.

A parameter-free particle relaxation technique for smoothed particle hydrodynamics

In this paper, we present a parameter-free particle relaxation technique to improve the accuracy and stability of smoothed particle hydrodynamics (SPH). Instead of imposing a background pressure, particles are regularized following the criteria of 0th-order consistency, i.e., the gradient of a constant to be zero. Specifically, the modifications of particles' position are solved by a gradient decent method according to the error between zero value and the gradient of a constant. This modification decreases the integration error and leads a more uniform particles distribution. A set of challenging benchmarks including lid-driven cavity flow, Taylor-Green vortex, FSI (fluid-solid interaction) problem, 2D (two-dimensional) dam-break case, and water exit of a cylinder are investigated to validate the effectiveness of the present technique for addressing the well-known tensile instability and particle clumping problems. Finally, the study of 3D (three-dimensional) dam-break against an obstacle demonstrates the stability and versatility of the present method.

Gravity‐driven packed bed filter, with copper‐impregnated activated carbon, for continuous water disinfection in absence of electricity

AbstractAvailability of safe drinking water is a major concern in many parts of the world. While many filtration units operating on various principles are available to combat this, most require electricity, which may not be consistently available in such areas. In the present study, we have designed and demonstrated a water disinfection system that can operate purely on gravity, without any electricity. For this, a potassium hydroxide modified copper‐impregnated activated carbon (KOH‐Cu‐AC) hybrid was used as a filter medium for disinfection, because it is less expensive, with performance comparable to previously reported hybrids containing silver. To maintain a constant water flow rate under gravity, during disinfection, a Mariotte bottle was used as the reservoir of the contaminated water. Using this and a constant head between the bottle and the treated water exit point, the required water‐filter contact time of 25 min (for decontamination) is maintained in the filter column, regardless of tank‐fill level. The demonstrated lab‐scale system can perform disinfection of simulated contaminated water (with an initial concentration of 104 CFU mL−1 Escherichia coli), for at least 6 h, with a flow rate of 150 mL h−1. The disinfection performance from the gravity‐based filter was further validated with the conventional pump‐driven filter, used for continuous disinfection of drinking water. Equivalence of results between pump‐ and gravity‐driven operations helps us to eliminate the need for power, without any compromise in disinfection efficacy. Finally, copper concentration from treated water (106 ppb at steady state) remains very well within the safe limit (1000 ppb as per USEPA guideline). Hence, the lab‐scale design of gravity‐based packed bed filter will be useful for domestic and community‐based supply of safe drinking water in resource‐constrained areas, because it eliminated electricity requirement of conventional power‐driven systems.Practitioner Points Cost‐effective KOH‐Cu‐AC hybrid is developed as a disinfection material. Mariotte bottle used for maintaining constant disinfected water flow rate works without any electrical power supply. This system can be used for getting on‐spot, continuous disinfected water supply. The concentration of copper in the treated water is well within the safety limit. It can be applicable in rural and remote areas (no electric power source) as well as natural calamity‐affected areas.

Improved diffuse interface-immersed boundary method for three-dimensional multiphase fluids–structure interaction with moving contact lines

In the realm of numerical algorithms for investigating interactions between multiphase fluids and structure, the accurate computation of moving contact lines (MCLs) dynamics and the substantial computational resource demands bring challenges in three-dimensional simulations. In this study, an improved diffuse interface-immersed boundary method is proposed to efficiently investigate three-dimensional multiphase fluids–structure interactions. The present work commences with the development of the explicit correction scheme and the simplified Dirac function coefficient, designed to efficiently implement the immersed boundary technique. To validate the effectiveness and accuracy of the proposed method, the simulations of water entry and exit for a half-buoyant sphere are carried out, which conclusively demonstrate that the proposed method offers significant advantages in terms of both efficiency and accuracy, faithfully capturing the dynamic response of solid structures and the substantial deformation of free surfaces at the three-dimensional scale. Additionally, a comparative analysis of the results obtained using the proposed method for the free water exit of a hollow sphere against previous experimental data is conducted, highlighting the promising potential of the proposed method for applications.

Investigating Mountain Watershed Headwater‐To‐Groundwater Connections, Water Sources, and Storage Selection Behavior With Dynamic‐Flux Particle Tracking

AbstractClimate change will impact mountain watershed streamflow both directly—with changing precipitation amounts and variability—and indirectly—through temperature shifts altering snowpack, melt, and evapotranspiration. To understand how these complex processes will affect ecosystem functioning and water resources, we need tools to distinguish connections between water sources (rain/snowmelt), groundwater storage, and exit fluxes (streamflow/evapotranspiration), and to determine how these connections change seasonally and as climate shifts. Here, we develop novel watershed‐scale approaches to understand water source, storage, and exit flux connections using a dynamic‐flux particle tracking model (EcoSLIM) applied in California's Cosumnes Watershed, which connects the Sierra Nevada and Central Valley. This work develops new visualizations and applications to provide mechanistic understanding that underpins the interpretation of isotopic field data at watershed scales to distinguish sources, flow paths, residence times, and storage selection. In our simulations, streamflow comes primarily from snow‐derived water while evapotranspiration generally comes from rain. Most streamflow starts above 1,000 m while evapotranspiration is sourced relatively evenly across the watershed and is generally younger than streamflow. Modeled streamflow consists primarily of water sourced from precipitation in the previous 5 years but before the current water year, while ET consists primarily of water from precipitation in the current water year. ET, and to a lesser extent streamflow, are both younger than water in groundwater storage. However, snowmelt‐derived streamflow preferentially discharges older water from snow‐derived storage. Dynamic‐flux particle tracking and new approaches presented here enable novel model‐tracer comparisons in large‐scale watersheds to better understand watershed behavior in a changing climate.

Erosion Induced Flow Changes in Pelton Bucket: A Numerical Approach

Abstract The hydro-turbines working in sediment-laden conditions often experience some material loss from their surface, which modifies the surface design. This material loss alters the mechanical strength of the turbine component as well as the corresponding flow phenomenon. These changes in the flow pattern can lead to efficiency losses, but they can also be exploited to determine the actual condition of the component in real time. In fact, if detected, they can be useful for developing maintenance strategies to predict the turbine conditions in real time. To compare the flow parameters, a numerical simulation has been performed for a 2D stationary Pelton bucket under both eroded and uneroded scenarios considering clean water as the working fluid. The 2D middle section of a reference Pelton bucket has been considered in two configurations: the uneroded bucket case and eroded, whose uneven surface patterns were created taking inspiration from real erosion patterns inside Pelton buckets observed in sediment-laden power plants. The CFD simulations have been carried out using OpenFOAM multiphase solver interFoam. The two cases were then compared based on several flow parameters such as water film thickness, velocity, and exit flow angle. The result analysis confirmed flow disturbances due to erosion causes the decrement in the overall flow velocity and the increment in water film thickness. The velocity reduction effect, as well as the water film thickness increment effect, strengthens approaching the outlet of the bucket. The results also showed a significant alteration of exit flow angle due to erosion. Specifically, for the erosion pattern considered in this study, the exit flow angle was observed to be larger compared to the uneroded bucket case.

Experimental study on cavitation characteristics and pressure load of actively ventilated double-vehicle configuration during water exit

The cavitation phenomenon is widely observed in the cross-medium movement of vehicles, seriously affecting their structural safety and motion stability. Attempts to control these effects include active ventilation, although existing research on active ventilation techniques has focused on single-vehicle configurations. In real situations, multiple vehicles may exit the water at the same time. However, the cavitation development process of the double vehicles during the water exit is completely different from that of the single vehicle, and the flow field around it is also quite different. Therefore, it is necessary to research the water-exit process of actively ventilated double-vehicle configurations. In this article, a double-vehicle water-exit device that injects high-pressure gas is designed, and the factors influencing changes in the cavitation bubbles are studied, such as the launch speed and environmental pressure. The water exit of a single vehicle under the same conditions is considered as the control case. According to our results, active ventilation effectively relieves the cavitation suppression phenomenon between the two vehicles, and reduces the pressure peak at which the cavitation bubbles collapse. Factors such as the launch speed, environmental pressure, and ventilation rate have a significant influence on the development of cavitation bubbles, especially their size. The influence of these factors on the pressure is mainly realized by changing the thickness of the cavity and the initial pressure.

Experimental study on wave-induced loads and nonlinear effects for pier-pile group foundations of sea-crossing bridges: Wave slamming and suction effect

Strong nonlinear effects, such as wave slamming, suction effects, and wave overtopping, may be induced during wave-bridge interaction. However, an accurate understanding of these nonlinear effects is still insufficient. This study comprehensively investigates the wave slamming and suction effects on pier-pile group foundations of sea-crossing bridges in a 1:50 scale laboratory experiment. Random and regular waves with different strengths are generated based on wave conditions at the bridge site. Five water depths with a 2 cm spacing are designed to simulate varying pile cap clearances. Various pile arrangements are also set up to explore the influence of piles. Results show that wave slamming is low-aeration and is triggered mainly on the cap bottom wall, whereas the wave action on the vertical wall of the pile cap is quasi-static. The suction effect is generally recorded in the quasi-static phase of pressures and depends on the water exit process and the wave crest height. Moreover, the pressure oscillation after the impact phase results from the flow separation and rotation at structure corners. The wave slamming enhances with the increasing wave frequency when the impact area is constant, otherwise, it depends on the impact area. On the cap bottom wall, the wave slamming on inter-pile areas is enhanced, but the suction effect is less affected by pile arrangements. The increase in pile cap clearances reduces vertical forces but has less effect on horizontal forces. Additionally, a sustained increase in the wave crest height may reduce the slamming force. An empirical method for predicting wave forces is finally proposed based on the water entry theory and experimental findings. This work enhances the physical understanding of nonlinear effects during wave-bridge interaction and aims to support the design of substructures for sea-crossing bridges.

Numerical study on the cavity dynamics of water entry and exit for a high-speed projectile crossing a wave

The high-speed projectile moving near the sea level will significantly suffer from the effects of waves. The water entry and exit of a high-speed projectile crossing a wave are investigated by detached-eddy simulation. Three simulations with different altitudes through the wave are conducted to analyze the altitude's influence on the cavity dynamics. To validate the numerical model, a water-entry experiment is carried out in a wave tank for comparison. The projectile crossing the wave forms a cavity channel from water entry to exit. Because the water below the cavity is more difficult to displace than the atmosphere with the water surface under the cavitation effects, the downward expansion of vapor is blocked, and the wave surface is lifted. Consequently, the cavity above the projectile expands more strongly until breaking through the water surface, while the cavity below the projectile keeps closed, and the projectile is wetted. Thus, a nose-up pitching moment is generated at water entry, while the drag force is gradually enhanced during the water exit, and a lift force acts toward the atmosphere. As the altitude increases, the upper cavity becomes more open, but the lower cavity shrinks, leading to the augmentation of the lift force.

Research on the multiphase flow interference and motion characteristics of vehicles during an underwater salvo

A cavitation flow can greatly impact a vehicle's attitude and stability when leaving water. This paper adopts an improved delayed detached eddy turbulence model and Schnerr–Sauer cavitation model as well as the volume-of-fluid method and an overlapping grid technique to investigate this effect. The simulation method used for the cavitation model is validated. The interference effects of a transient multiphase flow, collapse loads, and the motion instability of vehicles during an underwater salvo are studied. The results show multiple obvious pressure peaks during the process of cavity collapse, which do not overlap significantly. Instead, they are sequentially arranged from the top to the end of the bubble, and the synchronous collapse pressure peak is much stronger than the other pressure peaks. The synchronous collapse pressure has a high peak and a short pulse width, and its action position is at the bottom of the shoulder cavity. The salvo time interval is zero, the launch depth is equal to the length of the vehicle, the initial cavitation number is 0.233, and the lateral launch spacing is varied from 2 times the diameter to 5 times the diameter. When the lateral spacing is in the range of 4 times the diameter to 5 times the diameter, the effect of flow interference on the underwater travel and water exit stages disappears.

Swimming competence of 9–10-year-old Norwegian primary school children: A cross-sectional study of physical education

Swimming is a profound source of joy in life. The impact of swimming competence extends beyond leisure, encompassing aquatic skills crucial for the prevention of drowning incidents. The World Health Organization (WHO) strongly advocates for the proactive initiative of teaching basic swimming and water safety skills to school-aged children, which is recognized as a direct and effective measure in mitigating the risk of drowning. This article aims to investigate and quantify aquatic skills and swimming competence in 9–10-year-old primary school children. A study was conducted throughout the academic year of 2021–2022, as an integral component within the primary schools' physical education. The study design was tailored to facilitate large-group assessment, encompassing children from 69 primary schools ( n = 2421) situated across three Norwegian municipalities. The assessments were administered upon the culmination of the fourth-grade learn-to-swim programs and carried out using the Swimming Competence Assessment Scale, involving six consecutive aquatic skills: water entry, swimming on the front, surface diving, float/rest, swimming on the back, and water exit. The results indicated that 62.5% of the children successfully met the predetermined criteria for swimming competence according to the Norwegian standard. Among the six assessed aquatic skills, proficiency in swimming on the front emerged as the most influential factor contributing to the overall competence level. This study emphasizes the pivotal role of swimming education for school-aged children. It highlights the need to prioritize swimming and water safety education, initiating children's learning journey toward being water-competent.

Efficient prediction method for the water-exit characteristics of unmanned aerial–underwater vehicles

In the design of unmanned aerial–underwater vehicles (UAUVs), the performance during the transition from water to air is a crucial aspect requiring considerable attention. During water exit, UAUVs transition from water to air, crossing two vastly different mediums. Even after emerging from the water, the vehicle is still subjected to the forces exerted by the attached water. These challenges pose significant difficulties in predicting the water-exit performance. This study investigated a UAUV equipped with foldable wings and introduced a novel method for forecasting its water exit and flight characteristics using relay calculations. The method considered the complex force changes when crossing the water–air interface, as well as the force changes caused by wing unfolding and the influence of water attached to the vehicle after exiting the water. Additionally, an efficient aerodynamic coefficient identification method was adopted, and a novel approach for capturing and describing the state of the attached water was introduced. The accuracy and efficiency of the proposed method were demonstrated through comparisons with computational fluid dynamics methods. This method can be applied to the rapid iterative design of new UAUVs and provides a foundation for investigating water-exit motion control.

Water-exit impact of deep-sea mining vehicles: Experimental and numerical investigations

The operation of the deployment and retrieval system of a deep-sea mining vehicle (DSMV) is closely related to its water exit process; rapid water exit processes under wave conditions tend to exert negative influences on the safety and reliability of the entire system. To evaluate the specific effect of the interaction between the DSMV and the surrounding seawater, computational fluid dynamics (CFD) was used to predict the hydrodynamic characteristics and water-exit impacts of the DSMV at different retrieval speeds. First, a stokes-wave-free surface based on an overset mesh method integrating the volume of fluid (VOF) method was developed to reproduce level 4 sea conditions. Subsequently, the water surface deformation and water resistance were recorded to verify the proposed numerical method. These numerical and experimental findings indicate that when a vehicle body passes through wavy water at a relatively high speed, its lifting cable experiences a water-esistance force that is several times its weight. Finally, a vehicle body with a small retrieval speed might undergo several wave cycles, resulting in its lifting cable becoming loose.

Combined force decomposition approach and CFD simulation methods for 2D water entry and exit problem

The force decomposition approach is extended to solve the two-dimensional (2D) water entry and exit problems. Two typical scenarios, direct water exit and continuous water entry and exit, are examined. It reveals that the total force is irrelevant to the body velocity but to the acceleration. More specifically, the force can be expressed as the multiplication of constant values, body acceleration and a non-dimensional coefficient, and the coefficient is related only to the displacement and immersion condition of the body while independent of its motion condition. Two typical body shapes, a widely used wedge model and a practical ship section model, are examined here. CFD simulation is employed here to directly extract the non-dimensional coefficients, which avoids complex calculations for the wetted length of body by analytical models. The force decomposition approach with the obtained coefficients provides a means to quickly and accurately predict the hydrodynamic force acting on body for water exit scenarios. The proposed method is verified by comparing its results with CFD simulations in the complicated motion cases with varying acceleration. Furthermore, this work provides a potential tool to calculate the total force acting on any entire 3D hull that is divided into several independent 2D slices.

Water-exit dynamics of a ventilated underwater vehicle in wave environments with a combination of computational fluid dynamics and machine learning

A ventilated vehicle exiting water in a wave environment is a complex nonlinear process, and the mechanism by which the wave conditions influence this process remains poorly understood. This paper describes realistic simulations of a ventilated vehicle exiting a water body under various wave conditions. Comprehensive analysis is conducted for a range of distinct wave scenarios, and a machine learning-based method is developed for the rapid forecasting of vehicle-related parameters. A three-layer backpropagation neural network is constructed, and its prediction performance is verified. Subsequently, predictive and optimization procedures are employed to determine the optimal wave phase for the water exit of the vehicle. Different wave conditions are shown to significantly affect the evolution of the ventilated cavity as well as the kinematic and loading characteristics of the vehicle. The pitch angular velocity and angle at the moment when the head of the vehicle reaches the free surface exhibit a positive cosine trend under different wave conditions. No regularity of the pitch angular velocity at the moment when the tail reaches the free surface is evident. The neural network exhibits exceptional proficiency in predicting the motion parameters and load characteristics of the vehicle. The optimal point for the vehicle to exit the water is determined to be at a wave phase of 0.125π, while the most hazardous point occurs when the wave phase is 1.1875π.

Experimental study on water-exit of cylinder

In this paper, the hydrodynamic characteristics of the cylinder water-exit are studied by high-speed camera experiments. The characteristics of cylindrical motion trajectory, velocity and acceleration are analyzed by image processing algorithm. The emission voltage (i.e. initial velocity) and emission depth are important influencing factors. Based on different emission voltage and depth, the hydrodynamic coefficient is compared and analyzed. The results show that the new research method has good adaptability to the experimental study of cylindrical water-exit. The emission voltage directly affects the initial velocity of the cylinder, which in turn affects the force changes during the water exit process. The increase in emission depth exacerbates the uncertainty of the moment when the cylinder exits the water, thereby affecting the changes in the wake and motion stability during the cylinder water exit process.