Reduction of skin friction by microbubbles and its relation with near-wall bubble concentration in a channel

Determination of the flow structure near the wall is essential for a clear insight into the phenomenon of skin friction reduction by microbubbles in a turbulent boundary layer. An important parameter, is the bubble concentration or void fraction in the wall region in drag-reducing conditions. The purpose of this paper is to show drag-reducing effects due to microbubbles in a water channel and, more importantly, to show the dependence of the drag-reduction values on the near-wall void fraction. A two-dimensional channel with an aspect ratio of 10 was specially built for this purpose with provisions for air injection through porous plates. Skin friction was directly measured by a miniature floating element transducer with a 5-mm circular sensing disk mounted flush on the top wall 67 channel-heights downstream of the injector. The wall friction in the presence of air bubbles was found to be reduced under the same bulk velocity when compared with the value without air. Detailed void fraction profiles across the channel were obtained by a sampling probe and a fiber-optic probe. Better collapse of the drag reduction data, independent of different profile shapes, was found when plotted against the near-wall void fraction than against a cross-sectional mean void fraction. While this dependence reconfirms that the phenomena are essentially inner-region dependent, the lack of influence of the bubble distribution patterns away from the wall implies lack of outer region influence.     

三跨无背索斜拉桥的动力特性分析

湖南路大桥主桥为一座斜塔无背索斜拉桥,跨径组合为30 m+92 m+30 m,全长152 m。建立该桥空间有限元动力分析模型,重点研究该桥的自振频率和振型特征。通过与一座同类型方案桥的对比,总结出该桥动力特性及结构体系的主要特点,为后续同类桥梁的设计和相关研究提供借鉴。     

中新天津生态城市政配套工程中创新理念的应用

该文阐述了在中新天津生态城市政道路及排水工程设计中,所采用的生态、节能、环保的设计理念和思路,对其他市政工程的建设具有借鉴意义。     

Sideslip angle estimation based on input–output linearisation with tire–road friction adaptation

An adaptive sideslip angle observer considering tire–road friction adaptation is proposed in this paper. The single-track vehicle model with nonlinear tire characteristics is adopted. The tire parameters can be easily obtained through road test data without using special test rigs. Afterwards, this model is reconstructed and a high-gain observer (HGO) based on input–output linearisation is derived. The observer stability is analysed. Experimental results have confirmed that the HGO has a better computational efficiency with the same accuracy when compared with the extended Kalman filter and the Luenberger observer. Finally, a road friction adaptive algorithm based on vehicle lateral dynamics is proposed and validated through driving simulator data. As long as the tires work in the nonlinear region, the maximal friction coefficient could be estimated. This algorithm has excellent portability and is also suitable for other observers.     

Effective assessment of tyre–road friction coefficient using a hybrid estimator

Vehicle stability and active safety control depend heavily on tyre forces available on each wheel of a vehicle. Since tyre forces are strongly affected by the tyre–road friction coefficient, it is crucial to optimise the use of the adhesion limits of the tyres. This study presents a hybrid method to identify the road friction limitation; it contributes significantly to active vehicle safety. A hybrid estimator is developed based on the three degrees-of-freedom vehicle model, which considers longitudinal, lateral and yaw motions. The proposed hybrid estimator includes two sub-estimators: one is the vehicle state information estimator using the unscented Kalman filter and another is the integrated road friction estimator. By connecting two sub-estimators simultaneously, the proposed algorithm can effectively estimate the road friction coefficient. The performance of the proposed estimation algorithm is validated in CarSim/Matlab co-simulation environment under three different road conditions (high-μ, low-μ and mixed-μ). Simulation results show that the proposed estimator can assess vehicle states and road friction coefficient with good accuracy.     

The mechanisms involved during the wet braking of new and worn tires

ABSTRACT

Tires are used by the customers during several tens of thousands of kilometres, and before their replacement, the driver will encounter a continuous variation of tread depth due to the tire wearing. Although the wet braking labelling demonstrates the performance of the tire in the new stage, it is known that the wet traction evolves with tire wear. In this paper, an in-depth comparison of the wet grip performance of new and worn tires will be conducted, based on the regulatory wet braking test. For this purpose, we propose an original approach to analyse braking test results, which allows breaking down and quantifying the relative importance of the mechanisms involved during this test. This study demonstrates that two main mechanisms are taking place during the entire test: rubber friction and hydroplaning mechanisms. The µ value obtained at low speeds reflects the friction potential of the tested tires while the decline of performance at higher speeds is attributed to hydroplaning mechanisms. This analysis is conducted on numerous tires and demonstrates that current regulatory test applied on new tires is focussing mainly on the rubber friction mechanism. The same test applied on worn tires exhibits both rubber friction and hydroplaning mechanisms. The mechanisms decomposition shows that the source of the performance decline from new to worn status varies greatly, some tires having most of their performance loss due to hydroplaning, some others due to rubber friction drop.     

预应力混凝土连续曲线箱梁预应力效应分析

曲线箱形梁桥是空间复杂受力结构体系,预应力钢束产生的径向分布力是预应力混凝土曲线箱梁产生扭矩的主要原因之一。采用组合有限元法和简化方法分析曲线箱梁中预应力所产生的效应,得出预应力作用产生的内力和变形的变化趋势,为进一步完善曲线预应力混凝土箱梁桥的设计提供了依据。     

无灯控行人过街元胞自动机交通模型研究

以无灯控行人过街对交通流影响为分析目标,在综合考虑行人穿越行为及车辆行为的基础上,建立了相应的行人及车辆元胞自动机模型规则,尤其是在模型中提出了行人等待区的概念及基于等待区内有无行人的车辆行为规则。以上述模型为基础,通过对不同行人产生概率及速度快的行人比例情况时路段交通流演化趋势进行了仿真。     

采用改进真空预压技术加固软土地基的试验研究

通过改进后的真空预压技术与传统"砂垫层+塑料排水板"工艺的对比试验,研究在软基处理工程中采用"无砂直连"和变孔径滤膜排水板技术的加固效果,并在试验成果的基础上分析改进技术的特点和工作机理。采用"无砂直连"技术可减少真空度损失,选择合适孔径的滤膜排水板可减少滤膜的淤堵,这些技术改进均有利于软土地基的排水固结。     

城市高架桥无支架施工研究分析

近年来,随着城市化进程的不断推进,城市人口呈现日益增多的趋势,机动车保有量显著提升,城市交通日益拥堵.高架快速路由于具备占地小,立体化交通功能强大等优点而被广泛采用.而城市高架桥施工对现状交通必然存在一定程度的影响,因此开展高架桥结构无支架施工设计的研究,旨在探究出不同结构形式及施工方案在高架桥建设中的适用特点,可为今...