Unsteady RANS simulations of flow around a bridge section

This paper presents the results of unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of flow around a common bridge deck geometry. Computations are performed with a noncommercial unstructured flow solver using two-dimensional hybrid meshes with fine near-wall resolution. The influence of different simulation parameters (grid refinement, time-step size, turbulence modelling) is analyzed, in particular in order to obtain flow solutions independent of spatial and temporal discretization. Results given by a one-equation eddy-viscosity turbulence model and a two-equation explicit algebraic Reynolds stress model are compared. Despite the limits imposed by the URANS approach and the relatively inexpensive two-dimensional computations, satisfactory agreement is found with the experimentally measured forces and pressures. These simulations help to explain the discrepancy between the results of two wind-tunnel test campaigns and show the dramatic influence of non-perfectly sharp edges on the global flow field development. The capability of the numerical approach to capture complex Reynolds number effects is also discussed.     

Large-eddy simulation of flows past complex truss structures

Numerical simulation of turbulent flows past typical truss structures that are common in bridges and towers has been conducted. Large-eddy simulation (LES) method is used that has been verified in computing flows past simple bodies at moderate Reynolds numbers. Truss structures consist of thin members that are too small to be accurately resolved by numerical grids that can be handled by most computer systems. It has been found that when the individual members are long angular bars, the overall flow can be reproduced fairly well by approximating the members with rectangular cylinders whose cross sections are resolved by more than 2×2 computational cells and by taking computational regions extending at least four structure heights downstream and about three heights across the vertical and spanwise directions. The drag coefficient and the vortex shedding characteristics along with the complex wake structures associated with the truss structures are reproduced reasonably well.     

Simulation of mean flow and turbulence over a 2D building array using high-resolution CFD and a distributed drag force approach

A numerical simulation has been performed of the disturbed flow through and over a two-dimensional array of rectangular buildings immersed in a neutrally stratified deep rough-walled turbulent boundary-layer flow. The model used for the simulation was the steady-state Reynolds-averaged Navier-Stokes equations with linear and non-linear eddy viscosity formulations for the Reynolds stresses. The eddy viscosity was determined using a high-Reynolds number form of the k-ε turbulence-closure model with the boundary conditions at the wall obtained with a standard wall-function approach. The resulting system of partial differential equations was solved using the SIMPLE algorithm in conjunction with a non-orthogonal, colocated, cell-centered, finite volume procedure. The predictive capabilities of the high-resolution computational fluid dynamics (CFD) simulations of urban flow are validated against a very detailed and comprehensive wind tunnel data set. Vertical profiles of the mean streamwise velocity and the turbulence kinetic energy are presented and compared to those measured in the wind tunnel simulation.It is found that the performance of all the turbulence models investigated is generally good—most of the qualitative features in the disturbed turbulent flow field through and over the building array are correctly reproduced. The quantitative agreement is also fairly good (especially for the mean velocity field). Overall, the non-linear k-ε model gave the best performance among four different turbulence closure models examined. The turbulence energy levels within the street canyons and in the exit region downstream of the last building were underestimated by all four turbulence closure models. This appears to contradict the ‘stagnation point anomaly’ associated with the standard k-ε model which is a result of the excessive turbulence energy production due to normal straining. A possible explanation for this is the inability of the present models to account properly for the effects of secondary strains on the turbulence and/or for the effects of large-scale flapping of the strong shear layer at the canopy top.The results of the high-resolution CFD simulations have been used to diagnose values of the drag coefficient to be used in a distributed drag force representation of the obstacles in the array. Comparisons of the measured spatially-averaged time-mean mean velocity and turbulence kinetic energy in the array with predictions of the disturbed flow using the distributed drag force approach have been made.     

Re-examination of the effect of a plane boundary on force and vortex shedding of a circular cylinder

The hydrodynamic forces and vortex shedding of a smooth circular cylinder immersed in different boundary layers were experimentally investigated at Reynolds numbers from 1.30×10⁴ to 1.45×10⁴. The effects of the bed proximity, the thickness of the boundary layer, and the velocity gradient in the boundary layer on the pressure distribution, the hydrodynamic forces and the vortex shedding behavior were examined. The experimental results show that both the drag and lift coefficients strongly depend on the gap ratio, and are affected by the boundary layer. A downward lift is observed at certain gap ratios in rod-generated boundary layers, and an explanation of this downward lift is given. Two different criteria for calculating the Strouhal number in the literature are discussed in this paper. It is found that the variation of the root-mean-square (RMS) lift coefficient reveals the onset or suppression of the vortex shedding. A quantitative method for identifying the vortex shedding suppression point is proposed. The observations show that the vortex shedding is suppressed at a gap ratio of about 0.2–0.3, depending on the thickness of the boundary layer. This critical gap ratio decreases as the thickness of the boundary layer increases.     

高性能混凝土及防腐涂料在浙江某码头中的应用

依托舟山嵊泗县李柱山西侧客运码头工程,将高性能混凝土应用于部分码头结构,分析码头涂敷防腐涂料隔绝码头受海洋环境的影响。通过分析这两项措施对改善码头耐久性能的有效性和经济性,提出改善混凝土码头耐久性能的建议。研究认为,就原材料成本而言,高性能混凝土价格略微低于原普通混凝土,氯离子迁移系数仅为约原普通混凝土的一半,防腐涂料的采用将增加码头建设成本。建议防腐涂料仅在结构中重要的部位或容易受腐蚀的部位采用,而不必整个码头采用。     

Measurement of diffusion coefficients of VOCs for building materials: review and development of a calculation procedure

The measurement and prediction of building material emission rates have been the subject of intensive research over the past decade, resulting in the development of advanced sensory and chemical analysis measurement techniques as well as the development of analytical and numerical models. One of the important input parameters for these models is the diffusion coefficient. Several experimental techniques have been applied to estimate the diffusion coefficient. An extensive literature review of the techniques used to measure this coefficient was carried out, for building materials exposed to volatile organic compounds (VOC). This paper reviews these techniques; it also analyses the results and discusses the possible causes of difference in the reported data. It was noted that the discrepancy between the different results was mainly because of the assumptions made in and the techniques used to analyze the data. For a given technique, the results show that there can be a difference of up to 700% in the reported data. Moreover, the paper proposes what is referred to as the mass exchanger method, to calculate diffusion coefficients considering both diffusion and convection. The results obtained by this mass exchanger method were compared with those obtained by the existing method considering only diffusion. It was demonstrated that, for porous materials, the convection resistance could not be ignored when compared with the diffusion resistance.     

Unsteady natural ventilation at model scale—Flow reversal and discharge coefficients of a short stack and an orifice

Measurements of unsteady flow rates and pressures have been made on a scale model of a building fitted with two openings—a short stack and a sharp-edged orifice. The flow reversal characteristic of the stack is found to be different to that of a longer stack tested in an earlier study. The short stack displays a greater propensity to reversal, which is consistent with its lower inertia. The time-averaged discharge coefficients of both the stack and the orifice have been examined to determine the effects of external flow. It is found that the effects are similar to those observed in an earlier study that used a different experimental approach.