Course stability and yaw motion of a ship in steady wind

In order to achieve safe navigation, it is important to be able to understand and calculate the effects of an external force on the maneuvering behavior of a ship. This paper analyzes the course stability and yaw motion of a ship traveling under steady wind conditions. A course stability criterion and approximate formulae for the yaw motion in steady wind, including the aero/hydrodynamic force derivatives for the ship, are derived. To confirm the reliability of the criterion and formulae, they were used to investigate a pure car carrier in steady wind. The results of this investigation revealed that course instability appears in the head and following wind directions, mainly under the influence of aerodynamic derivatives with respect to the yaw restoring forces. However, this course instability can be reduced by applying steering control. For winds ranging from head winds to beam winds, yaw oscillation appears when the period is relatively long and the damping is small. The analytical formulae derived here can be used to gain a better understanding of ship maneuvering behavior in steady wind.     

Mixing over the steep side of the Cycladic Plateau in the Aegean Sea

Intensive microstructure sampling over the southern slope of the Cycladic Plateau found very weak mixing in the pycnocline, centered on a thin minimum of diapycnal diffusivity with Kρ=1.5×¹⁰−6 m² s^− 1. Below the pycnocline, Kρ increased exponentially in the bottom 200 m, reaching 1 × 10^− 4 m² s^− 1 a few meters above the bottom. Near-bottom mixing was most intense where the bottom slope equaled the characteristic slope of the semi-diurnal internal tide. This suggests internal wave scattering and/or generation at the bottom, a conclusion supported by near-bottom dissipation rates increasing following rising winds and with intensifying internal waves. Several pinnacles on the slope were local mixing hotspots. Signatures included a vertical line of strong mixing in a pinnacle's wake, an hydraulic jump or lee wave over a downstream side of the summit, and a ‘beam’ sloping upward at the near-inertial characteristic slope. Because dissipation rate averages were dominated by strong turbulence, ?/νN² > 100, the effect on Kρ of alternate mixing efficiencies proposed for this range of turbulent intensity is explored.     

New method for predicting full-scale power performance of pumpjet propulsion system based on statistical learning北大核心CSCD

[目的]针对“适配于螺旋桨的船尾线型+泵喷推进器”构成的船舶泵喷推进系统,提出一种基于统计学习的实船快速性预报新方法。[方法]以某大型水面船舶泵喷推进系统为对象,通过神经网络学习典型推进泵的推力系数图谱曲线,综合运用船-桨配合时的K_(T)-J曲线和船体-喷泵配合时的推力特性曲线,建立“仅需船舶阻力曲线就能实现船舶泵喷推进系统实船快速性预报”的新方法,并基于船模阻力试验、泵喷模型敞水试验及船体-泵喷自航试验的测量换算结果对实船推进性能的预报结果开展精度校验。[结果]校验结果表明:在航速18~30 kn范围内,船舶泵喷推进系统的自航转速、推力和功率的预报误差可控制在5.4%以内,其中设计航速附近的误差甚至小于2%;船体-泵喷的相互作用程度介于船-桨与船体-喷泵之间且幅值相对较小,推力减额系数为趋向于0的极小值,故船舶泵喷推进系统是介于桨轴推进系统和喷水推进系统之间的产物。[结论]该预报方法有利于提升船舶泵喷推进系统实船快速性预报的能力,可为新型舰艇泵类推进系统总体设计/研究提供参考。     

Experience as a conditioning effect on choice: Does it matter whether it is exogenous or endogenous?

Previous choice studies have proposed a way to condition the utility of each alternative in a choice set on experience with the alternatives accumulated over previous periods, defined either as a mode used or not in a most recent trip, or the mode chosen in their most recent trip and the number of similar one-way trips made during the last week. The paper found that the overall statistical performance of the mixed logit model improved significantly, suggesting that this conditioning idea has merit. Experience was treated as an exogenous influence linked to the scale of the random component, and to that extent it captures some amount of the heterogeneity in unobserved effects, purging them of potential endogeneity. The current paper continues to investigate the matter of endogeneity versus exogeneity. The proposed approach implements the control function method through the experience conditioning feature in a choice model. We develop two choice models, both using stated preference data. The paper extends the received contribution in that we allow for the endogenous variable to have an impact on the attributes through a two stage method, called the Multiple Indicator Solution, originally implemented in a different context and for a single (quality) attribute, in which stage two is the popular control function method. In the first stage, the entire utility expression associated with all observed attributes is conditioned on the prior experience with an alternative. Hence, we are capturing possible correlates associated with each and every attribute and not just one selected attribute. We find evidence of potential endogeneity. The purging exercise however, results in both statistical similarities and differences in time and cost choice elasticities and mean estimates of the value of travel time savings. We are able to identify a very practical method to correct for possible endogeneity under experience conditioning that will encourage researchers and practitioners to use such an approach in more advanced non-linear discrete choice models as a matter of routine.

     

Residual life prediction for marine diesel turbocharger based on Wiener process北大核心CSCD

[目的]针对船舶柴油机增压器难以收集到全生命周期性能退化数据的问题,提出一种基于维纳过程的寿命预测模型。[方法]首先,采用K-Means模型对增压器实际运行工况进行聚类,提取出典型工况数据;然后,使用贝叶斯突变点检测模型识别增压器的缺陷点;最后,建立基于维纳过程的退化模型,并以某型船用柴油机增压器为应用对象,预测增压器的剩余使用寿命。[结果]结果显示,基于维纳过程的寿命预测方法能够在不需要同类设备历史退化数据的情况下对增压器的剩余寿命进行预测。[结论]所提方法对缺少故障样本的船舶柴油机增压器寿命预测具有一定的参考价值。     

Easing or tightening control strategies: determination of COVID-19 parameters for an agent-based model

Some agent-based models have been developed to estimate the spread progression of coronavirus disease 2019 (COVID-19) and to evaluate strategies aimed to control the outbreak of the infectious disease. Nonetheless, COVID-19 parameter estimation methods are limited to observational epidemiologic studies which are essentially aggregated models. We propose a mathematical structure to determine parameters of agent-based models accounting for the mutual effects of parameters. We then use the agent-based model to assess the extent to which different control strategies can intervene the transmission of COVID-19. Easing social distancing restrictions, opening businesses, speed of enforcing control strategies, quarantining family members of isolated cases on the disease progression and encouraging the use of facemask are the strategies assessed in this study. We estimate the social distancing compliance level in Sydney greater metropolitan area and then elaborate the consequences of moderating the compliance level in the disease suppression. We also show that social distancing and facemask usage are complementary and discuss their interactive effects in detail.

     

The Gini index of demand imbalances in public transport

Transportation - The paper studies a general bidirectional public transport line along which demand varies by line section. The length of line sections also varies, and therefore their contribution...     

厚壁球壳的杂交／混合有限元分析

本文对８一节点四边形轴对称杂交／混合元ＡＸＨＭ１６进行了系列考核：（１）考查ＡＸＨＭ１６是否含有零能变形模式，以查明它的含秩状况；（２）通过受内压作用的厚壁球壳考题，观察ＡＸＨＭ１６的转轴不变性和其它性能表现。事实表明：ＡＸＨＭ１６满足单元合理足秩的必要条件和充分条件，不含零能变形模式，单刚合足秩。数值考核表明：ＡＸＨＭ１６具有很好的位移精度和应力精度，对坐标转轴的敏感程度并不明显，且在近乎相同的     

自主神经功能检测对早期糖尿病自主神经功能损害的诊断价值

目的评价自主神经功能检测对早期糖尿病自主神经功能损害的诊断价值。方法50例2型糖尿病患者进行自主神经功能检查,即交感皮肤反应和心血管自主神经功能检测,并与30名健康人进行对比分析。结果糖尿病患者自主神经功能检测中交感皮肤反应,瓦氏比值和深呼吸时心率变异与对照组比较均有显著性差异(P<0.05),三者的异常率分别为70%,46%和50%;以交感皮肤反应的异常率最高,尤其以双下肢明显。立卧位血压差和冰水试验与对照组比较无显著性差异(P>0.05),其异常率分别为12%和16%。结论检测外周器官自主神经功能状态的方法中以交感皮肤反应的阳性率最高,可以作为检测早期糖尿病自主神经功能的常规筛选检查。     

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熵最大化方法被广泛应用于交通规划，为交通需求预测模型提供了理论依据.本文使用信息论中的熵概念描述居民出行分布，并根据最大信息熵理论提出了基于原点矩典型特征量约束条件下的出行分布模型，详细阐述了模型中各个参数的实际含义，该模型是一个普适性模型，其具体形式决定于最高阶原点矩阶数的取值，此值对模型精度及参数标定难易程度有重要影响.为确定模型参数，提出了实用的参数标定方法.由于模型中原点矩的最高阶数对模型精度有较大影响，为找到两者之间的关系，利用长春市出行调查数据研究了四种出行方式分别时应于五种原点矩最高阶数的模型拟合情况.因文中所建立的模型是一系列模型，为比较其描述问题的优劣差异，按照最大信息熵原理给出了确定系统分布的熵方法，并运用该方法对前述模型进行了比较.调查数据的验证结果表明，出行分布的信息熵模型可以正确地描述出行分布规律，熵方法可以简单有效地评价不同模型对同一问题描述的优劣差异.本文提出的熵模型和熵方法对城市交通规划具有指导意义.