scholarly journals Free-Surface Aeration, Turbulence, and Energy Dissipation on Stepped Chutes with Triangular Steps, Chamfered Steps, and Partially Blocked Step Cavities

2017 ◽  
Author(s):  
Gangfu Zhang
Keyword(s):  
Free Surface ◽  
Energy Dissipation ◽  
Surface Aeration

scholarly journals Experimental Characterization of the Hydraulic Jump Profile and Velocity Distribution in a Stilling Basin Physical Model

Water ◽  
2020 ◽  
Vol 12 (6) ◽  
pp. 1758
Author(s):  
Juan Macián-Pérez ◽  
Francisco Vallés-Morán ◽  
Santiago Sánchez-Gómez ◽  
Marco De-Rossi-Estrada ◽  
Rafael García-Bartual
Keyword(s):  
Free Surface ◽  
Energy Dissipation ◽  
Velocity Distribution ◽  
Physical Model ◽  
Hydraulic Jump ◽  
Surface Profile ◽  
Air Entrainment ◽  
Stilling Basin ◽  
Free Surface Profile ◽  
Stilling Basins

The study of the hydraulic jump developed in stilling basins is complex to a high degree due to the intense velocity and pressure fluctuations and the significant air entrainment. It is this complexity, bound to the practical interest in stilling basins for energy dissipation purposes, which brings the importance of physical modeling into the spotlight. However, despite the importance of stilling basins in engineering, bibliographic studies have traditionally focused on the classical hydraulic jump. Therefore, the objective of this research was to study the characteristics of the hydraulic jump in a typified USBR II stilling basin, through a physical model. The free surface profile and the velocity distribution of the hydraulic jump developed within this structure were analyzed in the model. To this end, an experimental campaign was carried out, assessing the performance of both, innovative techniques such as the time-of-flight camera and traditional instrumentation like the Pitot tube. The results showed a satisfactory representation of the free surface profile and the velocity distribution, despite some discussed limitations. Furthermore, the instrumentation employed revealed the important influence of the energy dissipation devices on the flow properties. In particular, relevant differences were found for the hydraulic jump shape and the maximum velocity positions within the measured vertical profiles, when compared to classical hydraulic jumps.

scholarly journals Investigation of Free Surface Damping Models With Applications to Gap Resonance Problems

2017 ◽  
Author(s):  
Kevin Markeng ◽  
Torgeir Vada ◽  
Zhi Yuan Pan
Keyword(s):  
Free Surface ◽  
Energy Dissipation ◽  
Resonance Problem ◽  
Surface Boundary ◽  
Flow Solver ◽  
3 Dimensional ◽  
Energy Devices ◽  
Dissipation Term ◽  
Free Surface Boundary Condition ◽  
Damping Model

In this paper two methods for modelling the damping in a narrow gap are investigated. The first method is called the Pressure Damping Model. This method has been used in studies of wave energy devices. An attractive feature of this model is that the modified input is directly related to the energy dissipation in the gap, which means that if this dissipation is estimated the input to the model can be obtained directly. The idea of the method is to add a pressure input in the gap to suppress the resonant motion. A challenge with the method is that it contains a non-linear term. The second method is the Newtonian Cooling damping model. The method is based on introducing a dissipation term in the free surface boundary condition. This dissipation term contains a coefficient which is not directly related to the energy dissipation. Hence this method is not so easy to relate directly to the estimated energy dissipation. An advantage with this method is that it is linear and hence can be expected to be more robust. In the first part of the paper a 2-dimensional problem is investigated using both methods. In addition to the numerical performance and robustness, much focus is put on investigation of the energy balance in the solution, and we attempt to relate both models to the energy dissipation in the gap. In the second part the Newtonian cooling method is implemented in a 3-dimensional potential flow solver and it is shown that the method provides a robust way to handle the resonance problem. The method will give rise to a modified set of equations which are described. Two different problems are investigated with the 3D solver. First we look at a side-by-side problem, where the 3D results are also compared with 2D results. Finally, the moonpool problem is investigated by two different 3D solvers, a classical Green’s function based method and a Rankine solver. It is also shown how this damping model can be combined with a similar model on the internal waterplane to remove irregular frequencies.

Hydrodynamics of Side-by-Side Fixed Floating Bodies

2016 ◽  
Author(s):  
Kie Hian Chua ◽  
Rodney Eatock Taylor ◽  
Yoo Sang Choo
Keyword(s):  
Free Surface ◽  
Energy Dissipation ◽  
Skin Friction ◽  
Viscous Dissipation ◽  
Potential Flow ◽  
Flow Model ◽  
Time Window ◽  
Distance Estimation ◽  
Linear Potential ◽  
Potential Flow Model

Safety of cargo transfer operations between side-by-side vessels depends on accurate modelling of hydrodynamic behavior, especially in terms of predicting the gap free surface elevations between the two vessels. The common industry practice of using linear potential flow models to study these interactions over-predicts the free surface elevations, due to the fact that potential flow does not include viscous dissipation effects such as flow separation at hull corners and skin friction. This may result in inaccurate projections of the time-window when these operations can safely take place. This is an important aspect for developments such as Floating Liquefied Natural Gas (FLNG) platforms, where side-by-side cargo offloading is an essential operation. In a recent research [1], an approach of splitting the amount of energy lost through viscous dissipation (calculated from three-dimensional viscous CFD simulations) into components representative of the flow phenomena has been proposed. Using the approach, referred to as component energy dissipation, the amount of energy lost due to vortex shedding and skin friction can be estimated. Modifications to linear potential flow were also proposed in the referenced research, such that the energy loss components can be converted into dissipative coefficients that are used in terms added to the free surface and body boundary conditions. By combining use of the component energy dissipation approach and the modified dissipative potential flow model, better predictions of gap hydrodynamic interaction can be obtained, compared to using conventional potential flow. In this paper, results from viscous simulations of two identical fixed-floating side-by-side barges of 280m (length) × 46m (breadth) × 16.5m (draught) under excitation from regular incident waves are presented, and compared with corresponding results from the modified dissipative potential flow model. Two types of side-by-side hull configurations were investigated, the first using rectangular barges with sharp bilge corners at varying gap distances and the second using barges with rounded bilge corners of varying radii at a fixed gap distance. Estimation of the dissipative coefficients used in the modified potential flow model, calculated from the viscous results, will also be discussed. The comparison of results serves both as a validation of the modified potential flow model, and to highlight the importance of including viscous dissipation when analyzing hydrodynamic interactions.

Effect of Free Surface Nonlinear and Fluid Viscosity on Fluid Resonance in the Narrow Gap

2017 ◽  
Author(s):  
Sheng-chao Jiang ◽  
Li Zou ◽  
Tie-zhi Sun ◽  
Chang-feng Liu
Keyword(s):  
Free Surface ◽  
Energy Dissipation ◽  
Resonant Frequency ◽  
Incident Wave ◽  
Wave Amplitude ◽  
Relative Energy ◽  
Dominant Factor ◽  
Fluid Viscosity ◽  
Narrow Gap ◽  
Resonant Amplitude

Numerical simulations are carried out for gap resonance problem between two side-by-side non-identical boxes. The linear potential model over-predicts the resonant amplitude in the narrow gap because it not only neglects the energy dissipation due to vortical motion, but also neglect the nonlinearity due to free surface. More relative energy are reflected with the increase of incident wave amplitude, leading to the decrease of relative resonant amplitude and relative energy dissipation in the narrow gap at resonant frequency. When the incident wave frequency is outside a little band to resonant frequency, relative energy dissipation becomes the dominant factor for the decrease of relative wave amplitude in the narrow gap with the increase of incident wave amplitudes. In a word, both the free surface nonlinearity and fluid viscosity play the important, but different, roles on wave resonances in the narrow gap.

scholarly journals Experimental investigations of air entrainment in transition and skimming flows down a stepped chute

2002 ◽  
Vol 29 (1) ◽  
pp. 145-156 ◽  
Author(s):  
H Chanson ◽  
L Toombes
Keyword(s):  
Free Surface ◽  
Water Flow ◽  
Air Entrainment ◽  
Stepped Spillway ◽  
Transition Flow ◽  
Flow Properties ◽  
Flow Measurements ◽  
Surface Aeration ◽  
Stepped Chute ◽  
Uniform Equilibrium

Stepped spillways have been used for about 3500 years. The last few decades have seen the development of new construction materials, design techniques, and applications, for example, embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes (α ~ 50°). This paper presents an experimental study conducted in a large-size stepped chute (α = 22°, h = 0.1 m, W = 1 m). Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed air–water flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterization is developed for the distributions of void fraction, bubble count rate, and velocity, and flow resistance data are compared with other studies. Transition flows exhibit significantly different air–water flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalls.Key words: stepped spillway, air entrainment, two-phase flow properties, skimming flow, transition flow.

scholarly journals PERFORMANCE ANALYSIS OF COMPOSITE SEMICIRCULAR BREAKWATERS OF DIFFERENT CONFIGURATIONS AND POROSITIES

2012 ◽  
Vol 1 (33) ◽  
pp. 38 ◽  
Author(s):  
Hee Min Teh ◽  
Vengatesan Venugopal ◽  
Tom Bruce
Keyword(s):  
Free Surface ◽  
Energy Dissipation ◽  
Performance Analysis ◽  
Physical Modelling ◽  
High Energy ◽  
Immersion Depth ◽  
Irregular Waves ◽  
Hydrodynamic Characteristics ◽  
Semicircular Breakwater ◽  
High Energy Dissipation

The perforated free surface semicircular breakwater developed by Teh et al. (2010) was experimentally proven to be an effective anti-reflection structure with high energy dissipation ability. However, the performance characteristics of the breakwater deteriorated with a decrease in the immersion depth and an increase in wavelength. To enhance the performance of the breakwater with limited immersion depth, wave screens of different configurations and porosities were introduced below the free surface semicircular caisson. The hydrodynamic characteristics of these composite breakwaters were investigated in irregular waves using physical modelling. Comparisons of the experimental results showed that the semicircular caisson with a double screen of 25% porosity was a better breakwater configuration compared to that with a single screen. The extension of wave screen was also found to be particularly helpful in attenuating longer waves.

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