公路施工放样中若干问题探讨

随着社会经济的大力发展,交通设施也不断的完善,对公路的需求也越来越大,近几年,我国大力修建公路,而公路施工放样在公路建设中至关重要。而在公路施工放样过程中经常会碰到一些通视条件与精度问题,从而影响工程进度。本文介绍了在公路施工中,偏角法放样在处理一些视线障碍问题时的可取之处,全站仪极坐标放样法在匝道中线定位中的应用以及对其进行精度分析。     

Natural convection in attics subject to instantaneous and ramp cooling boundary conditions

A fundamental study of the fluid dynamics inside an attic shaped triangular enclosure with cold upper walls and adiabatic horizontal bottom wall is reported in this study. The transient behaviour of the attic fluid which is relevant to our daily life is examined based on a scaling analysis. The transient phenomenon begins with the instantaneous cooling and the cooling with linear decreases of temperature up to some specific time (ramp time) and then maintain constant of the upper sloped walls. It is shown that both inclined walls develop a thermal boundary layer whose thicknesses increase towards steady state or quasi-steady values. A proper identification of the time scales, the velocity and the thickness relevant to the flow that develops inside the cavity makes it possible to predict theoretically the basic flow features that will survive once the thermal flow in the enclosure reaches a steady state. A time scale for the cooling-down of the whole cavity together with the heat transfer scales through the inclined walls has also been obtained through scaling analysis. All scales are verified by the numerical simulations.     

Langjährige Erfahrungen des Tragverhaltens von Bewehrte‐Erde‐Brückenwiderlagern

Das Bauverfahren „Bewehrte Erde“ wurde nach der Einführung in den frühen 60‐er Jahren des letzten Jahrhunderts zum Bau von zahlreichen Stützwänden und Brückenwiderlagern angewandt. Der Beginn der Erforschung des Tragverhaltens von Bewehrte‐Erde‐Brückenwiderlagern liegt mehr als 40 Jahre zurück. Seitdem sind durch Modellversuche, Messungen an Test‐ und reellen Bauwerken zahlreiche Verbesserungen und Optimierungen am entwickelten Berechnungsverfahren erreicht worden. Dieser Beitrag soll Bauherren, Ingenieurbüros und bauausführenden Firmen für den Anwendungsfall „Brückenwiderlager“ im Bauverfahren „Bewehrte Erde“ sensibilisieren und daran erinnern, dass, bedingt durch seine zahlreichen technischen und ökonomischen Vorteile, dieses Bauverfahren bei unseren europäischen Nachbarn sowie weltweit für Brückenwiderlager nahezu als Standardbauweise gilt. A long experience in structural behaviour of Reinforced Earth for the application of bridge abutments. Since Reinforced Earth was invented in the early 60s of the last century, many retaining walls and bridge abutments had been erected using this technique. The main research of the load bearing behaviour of reinforced earth bridge abutments started more than 40 year ago. Since that many improvements by exploring scale models, measurements on test and real structures had been executed. This article shall remind clients, contractors and engineers the Reinforced Earth is combining technical and economical advantages for the application of bridge abutments.     

永磁同步电机的双闭环调速系统设计

永磁同步电机因其优越的性能近年来得到了广泛应用。针对双闭环控制器参数整定困难所导致的控制效果不佳的问题,文中提出了基于极点配置和Ramp函数的改进型双闭环PI控制器。从永磁同步电机矢量控制算法的角度出发,建立了速度、电流双闭环解耦控制的系统模型,并在此模型下论述了速度环、电流环控制器的设计方法,给出改进后双闭环控制器参数的计算结果。对所研究方法分别进行了计算机仿真和实际试验,结果表明优化后的系统减小了系统过冲,缩短了稳定时间,提高了系统动态响应,具有良好的工程意义。     

Onset of buoyancy-driven convection in the horizontal fluid layer heated from below with time-dependent manner

The onset of buoyancy-driven convection in an initially isothermal, quiescent fluid layer heated from below with time-dependent manner is analyzed by using propagation theory. Here the dimensionless critical time Τ_c to mark the onset of convective instability is presented as a function of the Rayleigh number Ra_Ø and the Prandtl number Pr. The present stability analysis predicts that Τ_c decreases with increasing Pr for a given Ra_Ø. The present predictions compare reasonably well with existing experimental results. It is found that in deep-pool systems the deviation of temperature profiles from conduction state occurs starting from a certain time Τ?(2～4) Τ_c .     

避险车道制动床上失控车辆车轮沉陷深度研究

避险车道制动床上失控车辆车轮沉陷深度是制动床铺设厚度设计的主要理论依据.为此,建立悬架-车轮-集料三维离散元模型并开展车轮沉陷深度数值模拟.首先在PFC3D中利用Fish语言建立悬架-车轮-集料三维离散元模型;其次通过静力三轴压缩试验标定集料离散元颗粒参数;通过实车足尺试验数据标定车轮离散元颗粒参数;最后利用标定的模型...     

斜坡海涂上修建海堤的爆炸挤淤施工技术

采用爆炸挤淤置换法在较陡的斜坡海涂上修筑海堤,因堤身内外侧涂面高差大,施工过程中抛填堤身容易滑塌。最初在浙江舟山某海堤工程中遇到了这个难题。通过对堤身稳定性的计算分析,将堤身抛填方法调整为内侧突前、外侧迟后,炸药布置侧重于内侧。既保证了施工期抛填堤身的稳定,又使得爆炸处理后形成的堤身断面满足设计要求。此后该方法又在多条类似海堤的施工中得到成功的应用,由此总结出了在斜坡海涂上修建海堤的爆炸挤淤技术,并申报了发明专利(201510908384.1)。     

Characterizing acrylic foam pressure sensitive adhesive tapes for structural glazing applications-Part II: Creep rupture results

The objective of this study was to determine relevant, primary mechanical characteristics of high-performance acrylic foam pressure sensitive adhesive (PSA) tapes, an alternate fastening method for structural glazing applications. Traditional fastening methods for structural glazing typically employ structural silicones and, consequently, relevant ASTM test standards and specifications appear to be more appropriate for silicones and similar products that exhibit limited time dependence. This study was aimed at characterizing and evaluating acrylic foam PSA tapes for structural glazing applications, and because of their time dependence, viscoelastic characterization was included. Since there may be significant differences in the viscoelastic responses of acrylic foam PSA tapes and silicones, the study may account for important differences between acrylics and silicones in structural glazing applications. The acrylic foam PSA used in the study was 3M™ VHB™ Tape G23F. For comparison, parallel ramp-to-fail tests were also conducted on 3 silicones: two one-part and one two-part compositions. Mechanical characteristics determined for the VHB™ Tape included the viscoelastic properties over a range of temperatures and test rates. Mechanical characteristics available in the literature were assumed for the silicones and confirm their limited time dependence. Ramp-to-fail strength data (Part I paper) and creep rupture (accompanying paper, Part II paper) data was collected for the VHB™ Tape and the silicones. Using the time–temperature superposition principle, master curves of VHB™ Tape storage and loss moduli in shear and tension were developed with data from a dynamic mechanical analyzer (DMA). The thermal shift factors obtained from these constitutive tests were successfully applied to the VHB™ Tape creep rupture and ramp-to-fail data collected at 23, 40, and 60 °C, resulting in master curves of ramp-to-fail strength (in Part I of paper) and creep rupture durability (Part II paper) in shear and tension.     

Scaling of free convection heat transfer in a triangular cavity for Pr > 1

Unsteady natural convection inside a triangular cavity subject to a non-instantaneous heating on the inclined walls in the form of an imposed temperature which increases linearly up to a prescribed steady value over a prescribed time is reported. The development of the flow from start-up to a steady-state has been described based on scaling analyses and direct numerical simulations. The ramp temperature has been chosen in such a way that the boundary layer is reached a quasi-steady mode before the growth of the temperature is completed. In this mode the thermal boundary layer at first grows in thickness, then contracts with increasing time. However, if the imposed wall temperature growth period is sufficiently short, the boundary layer develops differently. It is seen that the shape of many houses are isosceles triangular cross-section. The heat transfer process through the roof of the attic-shaped space should be well understood. Because, in the building energy, one of the most important objectives for design and construction of houses is to provide thermal comfort for occupants. Moreover, in the present energy-conscious society it is also a requirement for houses to be energy efficient, i.e. the energy consumption for heating or air-conditioning houses must be minimized.     

一种新型高精度斜坡补偿电路

针对采用高端功率MOSFET的导通电阻作电流取样电阻的同步整流电流模式降压DC-DC变换器,设计了一种新型高精度斜坡补偿电路.该电路采用有一定死区时间的同步整流,防止主功率管和续流管同时导通.死区过后,主功率管给电感充电,此时,续流管漏极寄生电容还在放电,造成电流采样误差.针对该问题,提出一种斜坡补偿电路,进行误差消除...