用密度函数法对自由表面进行数值模拟

本文从二维非定常N-S方程出发，提出了一种处理自由表面问题的数值方法。引入密度函数法(density-function method)追踪自由表面，在密度函数的传输方程中采用TVD格式达到了二阶精度。对均匀来流中的非定常不可压方柱绕流进行了数值模拟。数值结果成功地捕捉了自由表面波动以及漩涡的生成、分离的时间发展历程。同时验证了本方法的有效性。     

基于CFD的船-舵水动力干扰数值研究

该文采用基于RANS方程求解的CFD方法,对匀速直航的船-舵系统的黏性绕流场进行数值模拟,计算了不同船速和不同舵角下的船-舵水动力干扰系数,计算中忽略了自由面兴波及螺旋桨的影响。文中以KVLCC1船、舵模型为研究对象,首先将数值计算的船舶水动力及船-舵水动力干扰系数结果与模型试验数据进行比较,验证了所采用数值方法的有效性;随后,对船-舵系统、裸船体和敞水舵分别进行计算,通过水动力计算结果的比较以及流场分析,对船-舵水动力相互作用进行了数值研究。     

Fully three-dimensional Reynolds-averaged Navier–Stokes modeling for solving free surface flows around coastal drainage gates

In this study we carry out numerical simulations of free surface flow through the drainage gates of the Saemangeum tidal barrier that is located in the west coast of South Korea and is also known as the world largest man-made tidal barrier. Instead of using depth-averaged numerical models, which have been widely used in hydraulic and coastal engineering, we employ the fully three-dimensional free surface flow model of Kang and Sotiropoulos (2012b) to simulate the flow around the gates. The numerical model is based on the two-phase level set method solving the air and water simultaneously and the curvilinear immersed boundary method that is able to handle arbitrarily complex geometries. In the simulations turbulent flows are also resolved by the shear stress transport $k-\omega$ model. The numerical model is applied to simulate fifteen different flow conditions with various gate opening scenarios, and for selected test cases laboratory experiments are also carried out. The computed flowfields at various flow conditions are compared with the laboratory measurements and the field observations and the comparisons showed satisfactory agreements both quantitatively and qualitatively. Using numerical simulation results, we elucidate the structures of turbulent flows associated with a high-speed jet-flow like structure and a hydraulic jump at the far downstream of the gates. The results presented in this paper demonstrate the predictive capabilities of the numerical model and its potential as a powerful engineering tool for estimating the discharge-water level relationship and the three-dimensional flowfield of real-life drainage gates.     

Numerical prediction of hydrodynamic forces on a ship passing through a lock with different configurations简

A CFD method is used to numerically predict the hydrodynamic forces and moments acting on a ship passing through a lock with a constant speed. By solving the RANS equations in combination with the RNG k-e turbulence model, the unsteady viscous flow around the ship is simulated and the hydrodynamic forces and moments acting on the ship are calculated. UDF is com-piled to define the ship motion. Meanwhile, grid regeneration is dealt with by using the dynamic mesh method and sliding interface technique. Under the assumption of low ship speed, the effects of free surface elevation are neglected in the numerical simulation. A bulk carrier ship model is taken as an example for the numerical study. The numerical results are presented and compared with the available experimental results. By analyzing the numerical results obtained for locks with different configurations, the influences of approach wall configuration, lock configuration symmetry and lock chamber breadth on the hydrodynamic forces and moments are demonstrated. The numerical method applied in this paper can qualitatively predict the ship-lock hydrodynamic interaction and pro-vide certain guidance on lock design.     

Hydrodynamic interaction.among hull,rudder and bank for a ship sailing along a bank in restricted waters

By applying a CFD tool to solve the RANS equations, the viscous flow around a model of hull-rudder system towed along a bank in shallow water is numerically simulated. Hydrodynamic forces and moments acting on the ship are calculated for different ship-bank distances and rudder angles. A container ship, KCS, is taken as an example for the numerical study. Under the assumption of low ship speed, the influences of free surface elevation and ship squat are assumed to be negligible. Based on the calculation results, the hydrodynamic interaction among the hull, rudder and bank is analyzed.     

Numerical simulation of rolling for 3-D ship with forward speed and nonlinear damping analysis

Strongly nonlinear characteristics of ship roll owing to viscous effect can be usually observed. To describe the nonlinear roll behavior, the CFD method has been frequently employed with obvious advantages compared with the traditional semi-empirical formula method in estimating the roll damping. Numerical simulations of free decay and forced rolling at various forward speeds and amplitudes for a 3-D ship hull are conducted in the present research to predict ship roll damping, in which a RANS solver is employed and a dynamic mesh technique is adopted and discussed in detail. Numerical results, including nonlinear flow characters around ships, rolling decay curves and damping coefficients, show that they are all in good agreement with available experimental data. The linear and nonlinear damping coefficients are estimated and analyzed by fitting with exponential functions for various rolling amplitudes, frequencies and speeds in the free decay simulations, and the damping coefficients are obtained by a polynomial fitting in the forced roll simulations. It is indicated that the damping coefficients increase with increasing rolling angle amplitude and velocity. It is also emphasized that the effect of forward speed is significant to roll damping and the nonlinear damping decreases with increasing velocity.     

Numerical study on flow structure near two impermeable trapezoid submerged breakwaters on slop bottoms

Studying on hydrodynamic characteristics in water waves propagating over two impermeable trapezoid submerged breakwaters is of great significance to research design and construction of this type of submerged breakwaters. The turbulent characteristics are discussed with cnoidal waves in the wave flume with 1:20 smooth slope.The governing equations of the vertical 2D model are the Reynolds-averaged Navier-Stokes equations. The Reynolds stress terms are closed by a nonlinear (anisotropic) k − ε turbulence transportation model. Using a VOF method for tracking the free surface and source function wave generating method. Furthermore, the supplementary discussion is made by means of numerical results.

     

On the modeling of viscous incompressible flows with smoothed particle hydrodynamics

Smoothed particle hydrodynamics(SPH) is a Lagrangian, meshfree particle method and has been widely applied to different areas in engineering and science. Since its original extension to modeling free surface flows by Monaghan in 1994, SPH has been gradually developed into an attractive approach for modeling viscous incompressible fluid flows. This paper presents an overview on the recent progresses of SPH in modeling viscous incompressible flows in four major aspects which are closely related to the computational accuracy of SPH simulations. The advantages and disadvantages of different SPH particle approximation schemes, pressure field solution approaches, solid boundary treatment algorithms and particle adapting algorithms are described and analyzed. Some new perspectives and future trends in SPH modeling of viscous incompressible flows are discussed.     

The effect of free surface on cloud cavitating flow around a blunt body

In this study, the effect of the free surface on the cloud cavitating flow around a blunt body is investigated based on the water tank experiment and the CFD method. Numerical results are in good agreement with experimental data, and the mesh independence of the methods is verified. The cavity evolution process includes the cavity growth, the re-entrant jet, the cavity shedding, and the collapse, which can all be observed from the water tank experiment. The effects of the free surface on the cavity length, the thickness, and the cavity evolution period are analyzed by comparing the difference between the cavitating flows on the upper and lower sides of the body. This study also examines the effect of the distance between the free surface and the model through a series of water tank experiments and numerical simulations. The cavity stability and asymmetry, as well as the thickness and the velocity of the re-entrant jet inside the cavity, which varies with the submerged depth, are discussed with consideration of the effect of the free surface. The effect of the free surface on the cavitating flow around the blunt body is enhanced with the decrease of the submerged depth.     

EFFECT OF ESCAPE DEVICE FOR SUBMERGED FLOATING TUNNEL （SFT） ON HYDRODYNAMIC LOADS APPLIED TO SFT

This paper presents a potential approach to settle the problem of surviving major safety accidents in Submerged Floating Tunnel (SFT) that detachable emergency escape devices are set up outside SFT. The Computational Fluid Dynamics (CFD) technology is used to investigate the effect of emergency escape devices on the hydrodynamic load acting on SFT in uniform and oscillatory flows and water waves by numerical test. The governing equations, i.e., the Reynolds-Averaged Navier-Stokes (RANS) equations and standard turbulence equations, are solved by the Finite Volume Method (FVM). Analytic solutions for the Airy wave are applied to set boundary conditions to generate water wave. The VOF method is used to trace the free surface. In uniform flow, hydrodynamic loads, applied to SFT with emergency escape device, reduce obviously. But, in oscillatory flow, it has little influence on hydrodynamic loads acting on SFT. Horizontal and vertical wave loads of SFT magnify to some extend due to emergency escape devices so that the influence of emergency escape devices on hydrodynamic loads of SFT should be taken into consideration when designed.     

两层流体中潜艇水下运动尾迹的数值模拟

本研究使用基于RANS方程和VOF方法的非定常粘性数值方法对潜艇在两层流体中水下运动时的尾迹特征进行了探索性研究,成功的模拟出潜艇水下运动产生的表面波和内波,数值计算结果与理论分析和文献研究结果基本一致。     

Large eddy simulation of free-surface flows

This paper introduces and discusses numerical methods for free-surface flow simulations and applies a large eddy simulation(LES) based free-surface-resolved CFD method to a couple of flows of hydraulic engineering interest. The advantages, disadvantages and limitations of the various methods are discussed. The review prioritises interface capturing methods over interface tracking methods, as these have shown themselves to be more generally applicable to viscous flows of practical engineering interest, particularly when complex and rapidly changing surface topologies are encountered. Then, a LES solver that employs the level set method to capture free-surface deformation in 3-D flows is presented, as are results from two example calculations that concern complex low submergence turbulent flows over idealised roughness elements and bluff bodies. The results show that the method is capable of predicting very complex flows that are characterised by strong interactions between the bulk flow and the free-surface, and permits the identification of turbulent events and structures that would be very difficult to measure experimentally.     

COMPUTATION OF SHIP MANOEUVRING RELATED VISCOUS FLOW AND HYDRODYNAMIC FORCES

An efficient multi block incompressible viscous flow solver based on solving the Reynolds Averaged Navier Stokes (RANS) equations numerically has been developed that can be applied to simulation of a variety of ship maneuvering related flows and calculation of hydrodynamic forces. Valida tion and verification of the solution procedure were carried out on several model problems with good agreement to experimental and numerical results.     

Entropy generation in bypass transitional boundary layer flows

The primary objective of this study is to evaluate the accuracy of using computational fluid dynamics(CFD) turbulence models to predict entropy generation rates in bypass transitional boundary layers flows under zero and adverse pressure gradients. Entropy generation rates in such flows are evaluated employing the commercial CFD software, ANSYS FLUENT. Various turbulence and transitional models are assessed by comparing their results with the direct numerical simulation(DNS) data and two recent CFD studies. A solution verification study is conducted on three systematically refined meshes. The factor of safety method is used to estimate the numerical error and grid uncertainties. Monotonic convergence is achieved for all simulations. The Reynolds number based on momentum thickness, Re_θ, skin-friction coefficient, fC, approximate entropy generation rates, S'', dissipation coefficient, dC, and the intermittency, γ, are calculated for bypass transition simulations. All Reynolds averaged Navier-Stokes(RANS) turbulence and transitional models show improvement over previous CFD results in predicting onset of transition. The transition SST k-ω 4 equation model shows closest agreement with DNS data for all flow conditions in this study due to a much finer grid and more accurate inlet boundary conditions. The other RANS models predict an early onset of transition and higher boundary layer entropy generation rates than the DNS shows.     

Investigations into salt wedge dynamics at a bar blocked estuary using front tracking approach

The interfacial flow simulations using a robust interface-tracking method are presented. The method is based on one fluid formulation, where a single set of governing equations for the whole computational domain with variable material properties is solved. Interfacial terms, at the boundary separating the phases, are accounted for by adding the appropriate sources as δ functions. The representation of the moving interface and its dynamic restructuring, as well as the transfer of information between the moving front and the fixed grid, is discussed. This method has been applied to density stratified flows, and interfacial movements are then presented. An experimental study on salt wedge movement, conducted in the University of Dundee, has been simulated using the above algorithm. The numerical results are compared with the experimental results and those of a semi-empirical model developed earlier by the authors.     

A hybrid RANS-LES model for combining flows in open-channel T-junctions

In the present study, a hybrid RANS-LES (Reynolds Averaged Navier-Stokes equation — Large Eddy Simulation) model is developed for the simulation of open-channel T-junction flows. The hybrid approach can save computational effort comparing to the LES approach. The comparison between the computational results and the detailed experimental data shows that this relatively new modeling approach is more accurate than the RANS approach in the prediction of the combining flows in T-junctions.

     

Surface Water Quantity for Drinking Water during Low Flows - Sensitivity Assessment Solely from Climate Data

The future sensitivity of the surface water supply of Québec City is assessed in this paper using two methodologies: the methodology that has prevailed since the publication of the AR4 report, the hydroclimatological modeling framework, and an alternative approach adapted from Foulon et al. (2018). This alternative approach captures past relationships between climate data indices (CDIs), such as cumulative rainfall, and hydrological data indices (HDIs), such as 7-day low flows, and applies these relationships to assess future trends. Future climates were built for two emission scenarios, RCP-4.5 and???8.5, and the uncertainty of climate change was addressed through the use of 16 climate models. Overall, both methodological frameworks predicted similar low flow trends for the reference and future horizons (2016–2045 and 2046–2075). The future pressure on the surface water supply of Québec City should raise concerns. Indeed, for RCP-8.5, results indicated a decrease in the PI₁ values (ratio of 2-year low flow to water abstraction rate) of around 20% (2016–2045) and 35% (2046–2075) with a fairly high confidence (around 90% of models agreeing on the direction of change); leading to values less than 1; indicating an insufficient water supply with respect to available water during 2-year low flows. These results demonstrate the capacity of the method to provide a screening assessment of future drought-prone-watersheds. Furthermore, the application of the alternative approach, given climate simulations, would help early implementation of good management practices even for municipalities that do not have the capacities to conduct the more conventional approach.

     

UNSTEADY FREE-SURFACE WAVES GENERATED BY BODIES IN A VISCOUS FLUID

BRIEFINTRODUCTIONOFTHEPAPER :　Inthefirstpartofthisthesis ,thefundamentalsolutionsforthesingularStokesandOseenflowsinanun boundedfluidarederivedinauniversalformwhichinvolvestheHamiltonian ,Hessian ,andLaplacianoperators ,andelementaryfunctions .Inthesecondpart,theinteractionofunsteadylow Reynolds numberflowswithafreesurfaceisinvestigatedanalytically .Thedisturbedflows ,generatedbysubmergedbodiesmovingverticallydownwardsawayfromthesurfaceofthefluid ,aregovernedbytheunsteadyStokeseq…     

CALCULATION OF VISCOUS FLOW AROUND CIRCULAR CYLINDER WITH THREE-DIMENSIONAL NUMERICAL SIMULATION

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