Analysis Of Free-surface Flow Research Articles

Compatible interface wave–structure interaction model for combining mesh-free particle and finite element methods

This study presents a novel wave–structure interaction model, which is a compatible interface wave–structure interaction model that is based on mesh-free particle methods for free-surface flow analysis; the FEM for structural analysis. We adopt the explicitly represented polygon (ERP) wall boundary model, which is a polygon wall boundary model for mesh-free particle methods, to express the fluid–structure interfaces. The fluid–structure interfaces in the proposed model are geometrically compatible because the ERP model has advantages in dealing with complex-shaped and moving boundaries and it enables the direct use of surface meshes and shape functions given by finite element models of structures. This allows the automatic generation of polygons for free-surface flow analysis from finite elements, and therefore, it greatly increases the flexibility of the analysis. Based on the compatible interface between the polygons and finite elements, we propose a strong coupling algorithm based on an iterative partitioned scheme that defines the interacting models between the fluid and the structure. We model the force exerted by fluid particles on structures such that the kinetic boundary condition on the fluid–structure interface is satisfied. We perform the verification and validation tests of the proposed model by solving two benchmark problems.

Three-dimensional computational flow dynamics analysis of free-surface flow in a converging channel

Three-dimensional computational flow dynamics analysis of free-surface flow in a converging channel

Applying surface tension as pressure boundary condition in free surface flow analysis by moving particle simulation method

A model that introduces surface tension as a pressure boundary condition, named the surface tension as pressure (STP) model, was developed for free surface flow analyses by the moving particle simulation (MPS) method. The STP model assigns to surface particles the liquid pressure of Laplace’s formula. The model is an alternative to previous models that apply surface tension as volume force such as the continuum surface force model. Problems that appeared when using the volume force models, such as the dependencies of calculation results on particle resolution and pressure gradient accuracy, were solved by using the STP model. Calculations predicted the theoretical values of the internal pressure of a 3D spherical droplet and the oscillation period of a 2D elliptic droplet over a wide range of surface tension coefficients and droplet sizes with errors less than 10%. Since the STP model is easy to implement, does not increase computation cost from previous models, and does not require surface reconstruction or additional marker particles, the model is suitable for practical and large-scale free surface flow problems that involve violent deformation of the liquid surface such as liquid atomization.

Sharp Interface Methods for Simulation and Analysis of Free Surface Flows with Singularities: Breakup and Coalescence

A common feature of many free surface flows—drop/bubble breakup or coalescence and film/sheet rupture—is the occurrence of hydrodynamic singularities. Accurately computing such flows with continuum mechanical, multidimensional free surface flow algorithms is a challenging task given these problems’ multiscale nature, which necessitates capturing dynamics occurring over disparate length scales across 5–6 orders of magnitude. In drop breakup, the thinning of fluid threads that form and eventually pinch-off must be simulated until the thread's radius is about 10 nm. When two drops approach one another, the thickness of the fluid film separating them must fall below 10 nm before coalescence is said to have occurred. If the initial drop radii are 1 mm, simulations must remain faithful to the physics as thread radius or film thickness falls from 10−3 m to below 10−8 m. Here we review significant findings in interfacial flows with hydrodynamic singularities spearheaded by sharp interface algorithms. These multidimensional algorithms can achieve resolution that to date has only been possible with the use of simple 1D evolution equations.

Numerical aided design of Pelton nozzle jet deflector

Hydro energy still occupies an important place among renewable energy sources. In special operating conditions, Pelton turbines are irreplaceable and can be used for extremely small hydropower plants and also large hydro power plants. Pelton turbines can operate with high head and relatively small flow rates. In many cases, the height differences of the water are very large. Sometimes it is necessary to stop the operation of the turbine very quickly and the consequences of water hammer can be very severe. The responsible part to minimize the consequences of this phenomena is jet deflector, which can be in two different technical designs. The steps for 3-D geometry definition, pre-processing and post-processing, flow modelling and FEM analysis are presented. In the paper is presented the new optimized design of push-out jet deflector shape. Optimization consider 3-D CFD analysis of free surface flow and stress analysis. The main goal of the research was to minimize the influence of all force components on the torque of deflector servomotor. The final results present the geometry of the deflector, with a significant reduction in the stresses and deformations. These have been achieved with a crucial reduction in the hydrodynamic force and torque.

Performance and Flow Field of Gravitation Vortex Type Water Turbine using Volute Tank

Gravitation vortex-type water turbines use gravitational vortex generated when water is guided into the tank and drained from the hole at the bottom of the tank. When a circular tank is used, a runner inlet flow becomes uneven circumferentially, resulting in poor performance. To control the flow in the tank and make it uniform in the circumferential direction, we used a volute tank to elucidate the impact of tank geometry on the performance and flow field of the gravitation vortex type water turbines. We further investigated the performance and flow field of the water turbine using volute and circular tanks through experiments and free surface flow analysis. We compare the performance and flow field of the volute tank to those of the circular tank. We found that the effective head and turbine output of the volute tank decreased more than those of the circular tank, whereas the water turbine efficiency of the volute tank improved more than that of the circular tank at low to medium rotational speeds. This is because the theoretical head of the volute tank was smaller than that of the circular tank, but the runner inlet flow was uniform in the circumferential direction, and the loss in the tank, which is the dominant loss in the circular tank, was greatly reduced.

Remote Sensing of Aerated Flows at Large Dams: Proof of Concept

Dams are important for flood mitigation, water supply, and hydroelectricity. Every dam has a water conveyance structure, such as a spillway, to safely release extreme floods when needed. The flows down spillways are often self-aerated and spillway design has typically been investigated in laboratory experiments, which is due to limitations in suitable full scale flow measurement instrumentation and safety considerations. Prototype measurements of aerated flows are urgently needed to quantify potential scale effects and to provide missing validation data for design guidelines and numerical simulations. Herein, an image-based analysis of free-surface flows on a stepped spillway was conducted from a top-view perspective at laboratory scale (fixed camera installation) and prototype scale (drone footage). The drone videos were obtained from citizen science data. Analyses allowed to remotely estimate the location of the inception point of free-surface aeration, air–water surface velocities, and their fluctuations, as well as the residual energy at the downstream end of the chute. The laboratory results were successfully validated against intrusive phase-detection probe data, while the prototype observations provided proof of concept at full scale. This study highlights the feasibility of image-based measurements at prototype spillways. It demonstrates how citizen science data can be used to advance our understanding of real world air–water flow processes and lays the foundations for the remote collection of long-missing prototype data.

Numerical Analysis of Free-Surface Flows over Rubber Dams

This study integrates a large eddy simulation (LES) model and volume of fluid (VOF) method to simulate the free-surface flows over inflexible circular-crested dams of different shapes. The simulated water depths and pressures on the dam surface are validated by the results of laboratory experiments. Then the numerical model examines the effects of the water depths and the Reynolds number on the hydrodynamic force and the discharge coefficient. The simulation results reveal that the time-averaged drag coefficient decreases as the downstream water depth H2 increases, while the influence of water depth H2 on the lift coefficients is less significant. Furthermore, the discharge coefficients of circular and elliptical dams, computed from the simulated velocity profiles over the crest of the dam, agree with the formulae suggested by previous studies when the downstream depth H2/H1 < 0.90. In contrast, the discharge coefficient of a tear-shape dam is slightly larger than those of circular dams.

An equivalent pipe network model for free surface flow in porous media

Free surface flow analysis in porous media is challenging in many practical applications with strong non-linearity. An equivalent pipe network model is proposed for the simulation and evaluation of free surface flow in porous media. On the basis of representative elementary volume with homogeneous pore-scale patterns, the pore space of the homogeneous isotropic porous media is conceptualized as a collection of capillary tubes. According to Hagen-Poiseulle's law and flux equivalence principle, equivalent hydraulic parameters and unified governing formulations for the pipe network model are deduced. The two-dimensional free surface flow problem is reduced to a one-dimensional problem of pipe networks and a one-dimensional procedure based on the finite element method is then developed by introducing a continuous penalized Heaviside function. The proposed equivalent pipe network model is verified with results from numerical solutions and laboratory-measured data available in the literature, and good agreements are obtained. The proposed equivalent pipe network model is shown to be effective in analyzing the free surface flow in porous media. The numerical results also indicate that the proposed equivalent pipe network model has weak sensitivity of the mesh size and penalty parameters.

Loss analysis of gravitation vortex type water turbine and influence of flow rate on the turbine’s performance

A gravitation vortex type water turbine is a water turbine that uses gravitational vortices generated upon water draining from a tank bottom and can generate power at low head and low flow rate. In our case, given its operation in a free surface flow field, it is critical to quantitatively understand the performance and hydraulic loss at various flow rates for its design and operation. Accordingly, we investigated the influence of flow rates on the performance of the gravitation vortex type water turbine by conducting experiments and free surface flow analysis. Using the analysis results, we proposed a loss analysis method and quantitatively evaluated the hydraulic loss. We found that the effective head and the turbine efficiency increased as the flow rate increased; hence, the turbine output increased at a rate greater than the increase rate of the flow rate. Our study revealed that among the losses that occurred in the water turbine, the tank loss and tank outlet loss were the most dominant, followed by the friction loss inside the tank, whereas the runner loss and friction loss in the runner were small.

Improvement of Performance of Undershot Cross-Flow Water Turbines Based on Shock Loss Reduction

Undershot cross-flow turbines are applicable to shallow open channel flows and are characterized by a simple structure free of guide vanes or a casing. Since these water turbines operate in a free surface flow field, the optimum inlet blade angle and outlet blade angle are considered to vary with the flow rate in the open channel; however, guidelines for setting these angles are yet to be established. In this study, a method to improve the performance of an undershot cross-flow water turbine based on shock loss reduction was proposed. First, by varying the blade angle of the straight blades of the turbine, the inlet blade angle and outlet blade angle which reduced shock loss in the first-stage inlet region and second-stage inlet region were investigated through free surface flow analysis. From the results, it was shown that by taking the inlet and outlet blade angles which minimize the first-stage and second-stage shock losses for straight blades and applying them to curved blades, shock loss is reduced, thereby improving turbine efficiency and turbine output.

SPH Analysis of Free Surface Flow over Pivot Weirs

Pivot weirs have been proved efficient to measure, control and stabilize open channel flows in an easy-to-use manner. This work presents a numerical investigation of free surface overflow from full-width rectangular pivot weirs based on the hydrostatic-corrected smoothed particle hydrodynamics (SPH). In verification stage, the draining process of a reservoir-vertical weir system is separately analyzed by activating and deactivating the hydrostatic correction. The SPH results are benchmarked against a simplified analytical solution derived herein. It is shown that the declining reservoir head is more accurately predicted by activating the correction term. The SPH simulations are then validated for five weir inclination angles, ranging from a very mild to vertical configurations. Comparison with relevant experimental data demonstrates satisfactory agreement in each case. It is observed that the location of vena contracta in the overtopping jet shifts further downstream at steeper inclination angles. Moreover, a simple relation among discharge coefficient and weir inclination angle is proposed based on the curve-fitting of SPH results. These two parameters appeared to be negatively correlated so that an increase in the inclination angle poses a decrease in the discharge coefficient and vice-versa. The present analysis can be useful for estimating flow discharge if a simple head measurement is available for a given geometry of the pivot weir.

Analysis of the Flow in a Typified USBR II Stilling Basin through a Numerical and Physical Modeling Approach

Adaptation of stilling basins to higher discharges than those considered for their design implies deep knowledge of the flow developed in these structures. To this end, the hydraulic jump occurring in a typified United States Bureau of Reclamation Type II (USBR II) stilling basin was analyzed using a numerical and experimental modeling approach. A reduced-scale physical model to conduct an experimental campaign was built and a numerical computational fluid dynamics (CFD) model was prepared to carry out the corresponding simulations. Both models were able to successfully reproduce the case study in terms of hydraulic jump shape, velocity profiles, and pressure distributions. The analysis revealed not only similarities to the flow in classical hydraulic jumps but also the influence of the energy dissipation devices existing in the stilling basin, all in good agreement with bibliographical information, despite some slight differences. Furthermore, the void fraction distribution was analyzed, showing satisfactory performance of the physical model, although the numerical approach presented some limitations to adequately represent the flow aeration mechanisms, which are discussed herein. Overall, the presented modeling approach can be considered as a useful tool to address the analysis of free surface flows occurring in stilling basins.

Effect of flow rate on performance and flow field of an undershot cross-flow water turbine

Undershot cross-flow water turbines are characterized by their being easily applicable to open channels with various widths and depths by changing their blade width and/or the outer diameter of the runner. On the other hand, as this water turbine operates in a free-surface flow field, it is expected that when the flow rate changes, the flow field around the runner including the water depth changes dramatically, and that the turbine performance also changes. Thus, the objective of this study is to elucidate the effect of flow rate on the performance and flow field of an undershot cross-flow water turbine. By changing the flow rate, we conduct experiments and free surface flow analysis on the water turbine, as well as torque component analysis. The result showed that, among the torques in the respective flow regions, that torque resulting from the second stage cross-flow was dominant. Furthermore, it was found that among the respective dynamic components of the torque, the most dominant component was the gravitational component for low flow rates and the angular momentum component for medium-to-high flow rates. We expect these results to contribute to the establishment of higher performance design guidelines, such as optimum shapes for the flow rates.

Numerical Analysis of Free Surface Flow with Multiple Moving Fish along a Curved Path

Numerical Analysis of Free Surface Flow with Multiple Moving Fish along a Curved Path

Moving Particle Semi-implicit法によるノズル内キャビテーションと液体噴流のシミュレーション

The fact that the spray characteristics of automotive fuel injectors of direct injection type are substantially influenced by cavitation in the nozzles is widely known. Many researchers have been engaged in understanding the relation between a cavitating flow and characteristics of fuel spray using a numerical simulation till now. In a free surface flow analysis and a multiphase flow analysis, advantages of using a particle-based meshless method are utilized. However, particle-based meshless methods have not been used to simulate cavitation. In this study, the cavitation in a two-dimensional nozzle and the water jet were simulated using Moving Particle Semi-implicit (MPS) method employing a cavitation model. The simulation results are relatively close to experimental data.

Numerical Analysis of Free Surface Flow over a Submerged Rectangular Bridge Deck

AbstractThis study integrates a large eddy simulation model to investigate the effect of a submerged rectangular cylinder on the hydrodynamics of free surface flows. The simulation results are validated by the results of flume experiments. Then the numerical model is utilized to examine the influences of Reynolds number, Froude number, and blockage ratio on the flow field and the force coefficients of the deck. The simulation results reveal that the drag coefficient is dependent on the deck’s Froude number and blockage ratio of the bridge deck, rather than the Reynolds number. For both subcritical and transcritical flows, the drag coefficient increases as the blockage ratio increases. However, due to the wave-induced drag, the drag coefficient of the cylinder in transcritical flows is greater than that in subcritical flows with the same blockage ratio. On the other hand, the lift coefficient is a function of the submergence ratio and the deck’s Froude number. The separation shear flow on the upper side of...

Analysis of free-surface flows through energy considerations: Single-phase versus two-phase modeling.

The study of energetic free-surface flows is challenging because of the large range of interface scales involved due to multiple fragmentations and reconnections of the air-water interface with the formation of drops and bubbles. Because of their complexity the investigation of such phenomena through numerical simulation largely increased during recent years. Actually, in the last decades different numerical models have been developed to study these flows, especially in the context of particle methods. In the latter a single-phase approximation is usually adopted to reduce the computational costs and the model complexity. While it is well known that the role of air largely affects the local flow evolution, it is still not clear whether this single-phase approximation is able to predict global flow features like the evolution of the global mechanical energy dissipation. The present work is dedicated to this topic through the study of a selected problem simulated with both single-phase and two-phase models. It is shown that, interestingly, even though flow evolutions are different, energy evolutions can be similar when including or not the presence of air. This is remarkable since, in the problem considered, with the two-phase model about half of the energy is lost in the air phase while in the one-phase model the energy is mainly dissipated by cavity collapses.

313 表面張力を考慮した自由表面流れのフリーラグランジュ法による数値解析

313 表面張力を考慮した自由表面流れのフリーラグランジュ法による数値解析

Axis-switching and breakup of low-speed elliptic liquid jets

Theoretical and experimental investigations are conducted to analyze the instability of a low-speed liquid jet emerging from an elliptic nozzle. The complexity of viscous free surface flow analysis for an asymmetric geometry is simplified using an approach based on the Cosserat theory (also called director theory) which reduces the exact three-dimensional equations to a system depending only on time and on a single spatial variable. This work is mainly focused on the spatial instability analysis to examine the key characteristics of an elliptic jet such as jet profile, axis-switching and breakup length. In the experimental part, both natural (free) and excited (forced) breakup behaviors are studied. In the natural breakup, the effects of nozzle’s ellipticity and length to diameter ratio are examined. In the forced breakup case, disturbances are applied to the jet, by modulating the jet exit velocity using a piezoelectric actuator with given sinusoidal perturbations. The spatial evolution of the jet shape is captured with a high speed camera. Liquid jet instability is studied for various nozzle geometries over a specific range of jet velocities and excitation frequencies. Results are compared with conventional circular nozzles which can be considered as a special case of an elliptic jet.