Numerical study of sand scour with a modified Eulerian model based on incipient motion theory

Application of the standard Eulerian model to simulations of sand scour results in unrealistic phenomena. Therefore, the present work develops a modified Eulerian model based on sand incipient motion theory. The modified model is applied for simulating a two-dimensional single vertical jet and a moving planar jet. The simulation results generally demonstrate fairly good agreement with published results of scour profiles and the velocity contours of the water and sand phases. In addition, equations to describe self-similar scour profiles for the moving planar jet cases are given. The results demonstrate that the modified model efficiently and accurately simulates the two-dimensional sand scour produced by jets, particularly for the moving jet cases.     

The effects of blade twist and nacelle shape on the performance of horizontal axis tidal current turbines

The paper presents the effects of blade twist and nacelle shape on the performance of horizontal axis tidal current turbines using both analytical and numerical methods. Firstly, in the hydrodynamic design procedure, the optimal profiles of untwisted and twisted blades and their predicted theoretical turbine performance are obtained using the genetic algorithm method. Although both blade profiles produce desired rated rotational speed, the twisted blade achieves higher power and thrust performance. Secondly, numerical simulation is performed using sliding mesh technique to mimic rotating turbine in ANSYS FLUENT to validate the analytical results. The Reynolds-Averaged Navier-Stokes (RANS) approximation of the turbulence parameters is applied to obtain the flow field around the turbine. It is found that power and axial thrust force from BEMT (Blade Element Momentum Theory) method are under-predicted by 2% and 8% respectively, compared with numerical results. Afterwards, the downstream wake field of the turbine is investigated with two different nacelle shapes. It is found that the rotor performance is not significantly affected by the different nacelle shapes. However, the structural turbulence caused by the conventional nacelle is stronger than that by the NACA-profiled shape, and the former can cause detrimental effect on the performance of the downstream turbines in tidal farms.     

基于粗糙集与犹豫层次分析的北极东北航道风险评估

运用风险分析理论和方法,以北极东北航道6个关键海峡为研究目标,构建东北航道自然环境风险评价指标体系;针对全球气候变暖以及海冰融化背景下北极东北航道航行安全问题,引入犹豫层次分析方法确定指标主观权重、采用粗糙集思想来确定客观权重,集成得到组合权重。基于所建风险分析模型,对其中3个关键海峡的自然环境风险进行了量化评估和时空特征分析,同时计算了2013年8月份6个关键海峡的风险度,模拟了一条最优通航线路并与同时期永盛号轮船的实际航行线路进行了比对。研究表明:(1)2000-2014年东北航道3个关键海峡自然环境风险均呈现减小趋势,红军海峡自然环境风险减小趋势最大;(2)对比3个海峡海峡自然环境风险度,德朗海峡最低,维利基茨基海峡次之,二者均适合通航;红军海峡风险度最高,且不适宜通航;(3)模型模拟路线与实际航线较吻合,对实时航行路线的选择有参考意义。     

Lateral dynamic response of a partially embedded pile subjected to combined loads in saturated soil

In this article, an analytical solution is proposed to investigate the lateral dynamic response of a pile which is partially embedded in saturated soil layer and subjected to combined lateral and vertical loads. The saturated soil is described by Biot’s poroelastic theory and the resistance of soil is derived by potential function method. The governing equation of the pile is solved by coupling soil resistance and continuity conditions between the pile and the soil. The dynamic impedances of the pile are then obtained through transfer matrix method. To verify the validity of the proposed procedure, the present solution is compared with available solution for an idealized case. Finally, a parametric study is performed to investigate the effects of various parameters on the stiffness and damping properties of the pile-soil system. It is found that permeability of the soil and vertical load has significant effects on the dynamic response of the pile.     

On the criteria for the initiation of motion in tidal inlets,deterministic and stochastic approaches

Details are given herein of the refinement and application of a three-dimensional layer integrated numerical model to predict morphological changes in tidal basins. The solution of governing differential equations, which consist of the conservation of mass and momentum for the hydrodynamics, the transport equation for the suspended sediment fluxes and the sediment mass conservation equation for the bed level changes are carried out by the use of Alternating Direction Implicit (ADI) Finite Difference Method (FDM). The model includes different criteria for the initiation of motion namely Shields (1936, Application of Similarity Principles and Turbulence Research to Bed load Movement, Hydrodynamics Laboratory, California Institute of Technology, Pub. No. 167), Kolahdoozan (1999, Numerical Modelling of Geomorphological Processes in Estuarine Waters, PhD Thesis, Department of Civil and Environmental Engineering, University of Bradford, Bradford, UK, 288) and Zanke (2003, On the Influence of Turbulence on the Initiation of Sediment Motion, International Journal of Sediment Research, 18(1), 17–31), to compare different aspects of flow conditions. As the flow is highly turbulent with the random nature of its components, many researchers have tried to express sediment transport processes by using stochastic approaches. In the current study both deterministic and stochastic methods are included in the numerical model to evaluate their accuracy and efficiency. To validate the numerical model results, laboratory measurements are used, with these being obtained from an earlier experimental program undertaken by the authors. Results of a short term bed level changes in a laboratory model harbor are included for the model verification purposes. Comparisons are undertaken using different criteria for the initiation of motion, with the results highlighting that the unsteadiness in the flow parameters included in the numerical model has a major effect on the bed level changes inside the harbor, in compare with the turbulence structure of the flow. The model is then applied to a real case study of the Humber Estuary, located in the UK, with comparisons being undertaken for different criteria for the initiation of motion, using both deterministic and stochastic approaches for the long term bed level predictions.     

Settlement Prediction Under Linearly Loading Condition

The relationship between time and settlement is a key problem in geotechnical engineering. Current methods used to predict the time-dependant settlement are based mainly on instantaneous loading condition. In this paper, the settlement curve under linearly loading condition, which is suitable for building and embankment projects, is mathematically proved to be “S” shaped. Subsequently, a new model, the Poison model, is raised to predict the settlement, and the method of parameter solution is also introduced. Finally, several problems about the Poisson prediction model are discussed.     

Three-dimensional numerical simulation of oblique wave action on vertical walls

1 Introduction A vertical wall is one of the typical maritime sheltering structures in the coastal region, which is mainly subjected to wave forces. It has been a com- mon engineering assumption adopted for design pur- poses that normally incident wave fo…     

A rational approach to seepage flow effects on bottom friction beneath random waves

The bottom friction beneath random waves is predicted taking into account the effect of seepage flow. This is achieved by using wave friction factors for rough turbulent, smooth turbulent and laminar flow valid for regular waves together with a modified Shields parameter which includes the effect of seepage flow. Examples using data typical to field conditions are included to illustrate the approach. The analytical results can be used to make assessment of seepage effects on the bottom friction based on available wave statistics. Generally, it is recommended that a stochastic approach should be used rather than using the rms values in an otherwise deterministic approach.     

雷达VIL在局地强对流天气应用中应注意的几个问题

结合兰州新一代天气雷达的使用情况,介绍了其物理量产品中垂直积分液态含水量(Verti-cally Integrated L iqu id W ater Content,)的理论计算模式,从雷达观测距离对VIL计算的影响、VIL理论模式中的反射率强度和探测高度对VIL计算的影响及VIL在强对流天气识别中受季节变化的影响3方面提出了思考,以期为今后应用VIL识别强对流天气提供参考。