Experimental insight for flood flow repartition in urban areas

Modeling floods in urban areas remains a challenge. To understand flow patterns in urban geometries better and constrain models, an experimental rig representing a 1/200 scale urban geometry with various street widths and angles is presented. Measurements of hydraulic variables for flow conditions ranging from moderate to extreme flooding were performed. Over this range, accurate inflow and outflow boundary condition measurements allow the geometry effect on inlet–outlet discharge conservation to be studied for each street. Froude numbers are found to be independent of the total flowrate. Interestingly, the flow distribution among all streets remains comparable over the range of boundary conditions. Moreover, three behaviors have been identified depending on street response as a function of the evolution of the upstream discharge distribution. Future measurements with high spatiotemporal sampling would allow possible coupling of flow features and energy dissipation to be studied at various scales and other flow configurations and district geometries to be characterized.     

Validation of a 1D-2D dual drainage model under unsteady part-full and surcharged sewer conditions

This paper presents a 1D-2D dual drainage model to compute the rainfall-runoff transformation in urban environments. Overland flow in major drainage systems is modelled with the 2D shallow water equations, whereas the flow in a sewer network is computed with the 1D Saint-Venant equations using the two-component pressure approach to model pressure-flow conditions. The surface and sewer network models are linked through manholes, which allow water interchange in both directions. A new series of rainfall–runoff experiments in a real-scale physical model of a street section is used to validate the model under unsteady part-full and pressure flow conditions. The experimental measurements of water depth and discharge at several locations in a drainage network show a very satisfactory performance of the numerical model.     

Wind conditions and ventilation in high-rise long street models

We regarded high-rise cities as obstacles and channels to wind. We first studied wind conditions and ventilations in idealized high-rise long street models experimentally and numerically with a constant street width (W = 30 mm), variable street heights (H = 2 W, 2.5W, 3W, 4W), variable street lengths (L = 47.4W, 79W, 333W, 667W) and a parallel approaching wind. The flow rates penetrating into windward entries are a little larger than the reference flow rate in the far upstream free flow through the same area with windward entries in all models. The stream-wise velocity decreases along the street as some air leaves upwardly across street roofs. Near the leeward entry, there is a downward flow which brings some air into the street and results in an accelerating process. In the neighborhood scale long streets (L = 47.4W and 79W), wind in taller streets is stronger and the ventilation is better than a lower one. For the city scale long streets (L = 333W and 667W), a constant flow region exists where the vertical velocity is zero and the stream-wise velocity remains constant. In such regions, turbulent fluctuations across the street roof are more important to air exchange than vertical mean flows. In a taller street, the process to establish the constant flow conditions is longer and the normalized balanced horizontal flow rate is smaller than those in a lower street. In the city scale long streets, the turbulence exchange rate can be 5–10 times greater than the mean flow rate.     

道路路面水流流态的研究

采用室外路面试验研究路面流的流态,在三种路面条件下,进行三种坡度,五种流量的放水冲刷实验,观察流速、水深和流态的变化。试验结果表明,平均流速和水深受路面粗糙度、坡度和流量共同影响,流量的影响最大,坡度和路面粗糙度影响较小;流态则只受流量的影响,坡度和路面粗糙度几乎没有影响。     

Experimental and numerical studies of flows through and within high-rise building arrays and their link to ventilation strategy

Urban ventilation implies that wind from rural areas may supply relatively clean air into urban canopies and distribute rural air within them to help air exchange and pollutant dilution. This paper experimentally and numerically studied such flows through high-rise square building arrays as the approaching rural wind is parallel to the main streets. The street aspect ratio (building height/street width, H/W) is from 2 to 5.3 and the building area (or packing) density (λ_p) is 0.25 or 0.4. Wind speed is found to decrease quickly through high-rise building arrays. For neighbourhood-scale building arrays (1-2 km at full scale), the velocity may stop decreasing near leeward street entries due to vertical downward mixing induced by the wake. Strong shear layer exists near canopy roof levels producing three-dimensional (3D) vortexes in the secondary streets and considerable air exchanges across the boundaries with their surroundings. Building height variations may destroy or deviate 3D canyon vortexes and induced downward mean flow in front of taller buildings and upward flow behind taller buildings. With a power-law approaching wind profile, taller building arrays capture more rural air and experience a stronger wind within the urban canopy if the total street length is effectively limited. Wider streets (or smaller λ_p), and suitable arrangements of building height variations may be good choices to improve the ventilation in high-rise urban areas.     

Experimental investigations of effect of depth of Swiss roll combustor on its thermal performance as a heat generator

Hydrocarbon fuels have high energy density which is used to run small-scale mechanical systems. The present work emphasises on experimental investigations carried out on thermal characteristics of a small-size one turn Swiss roll combustor. Parameters investigated were namely equivalence ratio, flow velocity, combustors depth, flame stability, flame locations (flash back, stable and blowout), combustor surface temperatures, temperatures in reactants channel and temperatures in products channel. Flame stability was observed at the highest flow velocity of 32.75?m/s and at the maximum heat loss of 39.16?W. Values of volume flow rates of liquefied petroleum gas used during tests were 0.25 LPM, 0.3 LPM and 0.45 LPM. Combustor model with four depths was tested. Flame stability with higher heat loss and higher velocity was observed for the combustor of D21 and D5 models, respectively. The present combustor can be used as a heat source input to the thermoelectric device.

Abbreviations: LPG: Liquefied petroleum gas; MLPG: Volume flow rate of LPG, LPM; MAIR: Volume flow rate of air, LPM     

非饱和开裂混凝土中水分运移研究

根据双重孔隙介质模型将开裂混凝土视为由完好混凝土和裂缝组成的复合孔隙材料,基于非饱和混凝土的水分运动方程和裂隙渗流的立方定律提出非饱和开裂混凝土中的水分运动方程,同时采用渗水装置对预制裂缝混凝土试块进行渗水试验,研究混凝土初始饱和度、水胶比和裂缝宽度对渗水深度的影响;采用TOUGH 2软件数值分析开裂混凝土中饱和度和水分流速的变化,根据平均流速计算渗水深度并与试验结果比较.研究发现:混凝土初始饱和度越小、裂缝宽度越大,则渗水深度越大,且渗水深度增长先快后慢,反映了混凝土基质势是开裂混凝土中水分运移的驱动因素;裂缝宽度与混凝土渗透性正相关;水分入渗开裂混凝土后流速差和平均流速随时间延长逐渐降低;采用平均流速计算的渗水深度能更好地反映渗水试验中渗水深度的变化趋势.     

Skalenübergreifende Prozess-Studien zur Abflussbildung in Gebieten mit periglazialen Deckschichten (Sauerland, Deutschland)

Based on studies at different scales, runoff processes in the catchment “Bohläse” (Sauerland, Germany) were analysed during several rainfall-runoff-events. Using hydrological tracers and hydrometric methods, the influence of periglacial cover beds was determined for runoff at the catchment scale and for soil water flux at the point scale. The results show that the influence of the base layer depends on the current water content. If the water content in the base layer is low, vertical water movement is impeded. On the other hand, if the water content is high, the base layer becomes a preferential flow path for interflow. Due to the spatial variability of the soil physical properties, the base layer functions as a preferential flow path for interflow only if the bulk density is low. The results confirm the importance of periglacial cover beds for runoff processes in low mountain regions and represent an experimental basis for hydrological regionalisation depending on the spatial distribution of periglacial cover beds.     

Pollutant dispersion near roadways — Experiments and modeling

Since the 1970's, many field and wind tunnel experiments have been conducted to study pollutant dispersion from roadways. For an at-grade situation, field experiments have revealed that mechanical mixing dominates effects due to ambient stability, that plume rise is important under very low crossroad winds, that regions of large shear enhance the mixing volume, and that the wake region grows rather slowly in the vertical direction. The models that have been developed based on recent experimental results are briefly described. For the street canyon situation, both field and wind tunnel experiments have revealed that ambient stability does not play an important role, that corner vortices near an intersection cause an increase in pollutant concentrations near the bottom corners of the leeside buildings, that in the midsection of a street block the vortex circulation causes high pollutant concentrations to be advected toward and up the leeside wall. No general street canyon models are available except an empirical model for the midsection of the street block. The complicated flow field must first be ascertained before a reliable concentration model can be developed.     

Sewer Discharge Estimation by Stereoscopic Imaging and Synchronized Frame Processing

A system for fully automatic contact‐less image‐based measurement of volumetric flow rate in urban drainage structures is presented. The hardware includes two original equipment manufacturer cameras and a single‐board computer on which our custom image processing software is running. The value of water discharge depends on the surface velocity, water level and channel's geometry. The level of the flow is estimated as the difference between distances from the camera to the water surface and from the camera to the channel's bottom. Camera‐to‐water distance is recovered automatically using large‐scale stereo‐matching, whereas the distance to the channel's bottom is measured upon installation. Surface velocity is calculated using cross‐correlation template matching: individual natural particles in the flow are detected and tracked throughout the sequence of images recorded over a fixed time interval. The relative discharge computation error is lower than 1.34% of the theoretical maximal discharge for a given location, which makes our system competitive to commercial components such as ultrasonic flow meters, while using cheaper technologies.     

Backflow air and pressure analysis in emptying a pipeline containing an entrapped air pocket

ABSTRACT

The prediction of the pressure inside the air pocket in water pipelines has been the topic for a lot of research works. Several aspects in this field have been discussed, such as the filling and the emptying procedures. The emptying process can affect the safety and the efficiency of water systems. Current research presents an analysis of the emptying process using experimental and computational results. The phenomenon is simulated using the two-dimensional computational fluid dynamics (2D CFD) and the one-dimensional mathematical (1D) models. A backflow air analysis is also provided based on CFD simulations. The developed models show good ability in the prediction of the sub-atmospheric pressure and the flow velocity in the system. In most of the cases, the 1D and 2D CFD models show similar performance in the prediction of the pressure and the velocity results. The backflow air development can be accurately explained using the CFD model.     

Hydraulic experiments on spillway weir profiles

Spillway weirs are the system for regulating the flow in open channel flow passages. Weirs are commonly used to alter the flow of rivers to prevent floods, measure discharge and also in nuclear reactors for uniform cooling purposes. This paper describes the hydraulic experiments conducted for finding the air entrainment on water under laminar flow conditions in open channel flow passage with various profiles of spillway at flow rates in the range of 100–650?lpm. The concept of the spillway system allows stabilisation of the free level of water and avoids variation in the free level of water along the flow passage as a function of flow rate. The main problem in the spillway is the profiling of weir crest. The criteria that need to be satisfied are: (i) there should be no flow separation from the crest and (ii) there should be a uniform circumferential flow to avoid flow asymmetry in the passage. Separation of flow leads to large impact velocity of the falling water, which would lead to large-scale entrainment of air. When flow adheres to the weir crest and upstream, the impact velocity of water with the free surface would be less and hence entrainment would also be less.     

An investigation of turbulent accelerated flow along smooth prismatic chutes

Turbulent chute flow was investigated experimentally and numerically for various flow conditions. The Navier‐Stokes equations are solved with the k ? ε turbulence model on a structured non‐orthogonal grid. A method based on water continuity was used to calculate the movement of the water surface. Using an adaptive grid in the vertical direction, the location of the water surface was recalculated from an initially horizontal profile. After several iterations a steady solution emerged. The velocity distribution in longitudinal and vertical directions and pressure distribution along the chute were calculated. The numerical model was calibrated and verified using experimental data model studies. Reasonable agreement was found between the experimental results and that from the numerical model. Multiple‐regression equation was developed for computing the water surface profile along chute.     

An experimental investigation of the wind environment and air quality within a densely populated urban street canyon

This paper presents an experimental investigation of wind flow characteristics and air quality along a street canyon located within a dense urban area. Four typical models of a highly populated urban area are studied and wind tunnel experiments are carried out over an extended range of the applied wind directions. The building patterns are represented by 1:100 scale models, where wind velocity and tracer gas concentrations are measured along the two sides of the street. The study results provide evidence that building configurations and wind directions are very important factors in determining both wind flow and pollutant dispersion characteristics within urban domains. Also, the results demonstrate that gaps between buildings are a very important factor to be considered by urban planners and designers, because, for a given building height, larger gaps induce more wind in urban canyons, thus improving the ventilation process.     

A mixing‐length‐scale‐based analytical model for predicting velocity profiles of open‐channel flows with submerged rigid vegetation

In open‐channel flows with submerged vegetation, the vertical velocity profile can often be described by two layers: the vegetation layer in the lower region and the surface layer in the upper non‐vegetated region. In this paper, a new mixing‐length scale of eddy is proposed for predicting the vertical velocity profile of flow in an open‐channel with submerged rigid vegetation. The analytical model of velocity profile is based on the momentum equation of flow where the turbulent eddy viscosity is assumed to have a linear relationship with the local velocity. The proposed model was tested against different datasets from the literature. The 22 datasets used cover a range of submergence [flow depth (H)/vegetation height (h) = 1.25 ~ 3.38], different vegetation densities of ah = 0.11 ~ 1.85 (a defined as the frontal area of the vegetation per unit volume) and bed slopes (S_o = 1.8 × 10^?6 ~4.0 × 10^?3). This study showed that the proposed model can predict the velocity profiles well against all datasets, and that the mixing length scale of eddies (λ) is well related with both vegetation height (h) and flow depth of surface layer (i.e. the height of non‐vegetation layer, H–h). Close examination of λ in the proposed model showed that when λ = 0.03, the model predicts vertical velocity profiles well for all datasets used except for very shallow submergence (i.e. H/h < 1.5).     

Experimental study on average movement characteristics of bed-load particles

The three-dimensional movements of bed-load particles in an open channel with different grain diameters and different specific gravities under the conditions of different hydraulic gradient are measured by means of particle tracking velocimetry (PTV) technique. The average movement characteristics of the particles are analyzed by statistics. The results show that the longitudinal average velocities of the particles neither agree with the law suggested by Einstein nor obey the law assumed by Bagnold. Einstein’s law is that the particle velocity is equal to 11.6 times of shear velocity, and the Bagnold’s law assumes the velocity equal to the difference between flow velocity at corresponding water depth and settling velocity of particle. Two formulas for determining the velocity of particle in the forms of expression given by Einstein and Bagnold are obtained according to the statistical results of experimental data, respectively.     

Phosphorus dynamics observed through increasing scales in a nested headwater-to-river channel study

The hypothesis that the dynamic patterns of phosphorus (P) transport at plot scale are similar to the patterns that could be observed quasi-simultaneously (i.e., approximately at the same time) at a river basin scale, in terms of inputs and dilution of P, across a range of rainfall and runoff conditions, was tested. From this information, it was hoped to be able to make some simple inferences about the connectivity or mass flux of P transport between the different scales of observation. An intensive study using 30-m2 plots, 1-ha plots and nested river channel sites ranging in scale from 20 ha up to a maximum of 834 km2 in the River Taw basin, South West England, UK, was conducted with three campaigns under differing flow conditions: (1) a zero rainfall base flow period, (2) a 10-mm rainfall residual flow period, and (3) a 42-mm rainfall storm flow period. The mass flux of total P ranged from 49 kg during base flow to 4 tonnes during the storm period at the largest 834 km2 scale. During base flow conditions, total phosphorus (TP) concentrations from diffuse sources were low (26 microg L-1 in the upper catchment) and reactive P forms dominated the fractions filtered <0.45 microm. During storm flow, concentrations of TP increased at all scales within the drainage basin, to a maximum of 500 microg L-1 and were sufficient to override the effect of any point source inputs of P. Unreactive (i.e., mostly 'organic') forms of P dominated the fractions that were >0.45 microm during residual flows and storm flows. Spatially normalised discharge apparently decreased with increasing scale, most notably during storm flow conditions and this implies that there is some storage of water through the catchment and in part may reflect varying velocities of water in different pathways. Most attenuation and dilution of P appeared to occur at larger scales, whilst the hydrological connectivity between source areas and receiving waters was greatest at smaller scales (<20 ha), and during the highest flows. The importance of diffuse agricultural sources in contributing to P export through the basin was dominated by dynamic temporal changes in hydrological activity, with a 'piston pushing' effect of particulate associated P through the basin as it wets up in response to rainfall input. We concluded that the processes of P transfer are different at different scales. However, some uncertainties of spatial heterogeneity around the catchment underlie the difficulties in dealing with scale and thus more data and studies of this nature are required.     

Experimental study on average movement characteristics of bed-load particles

The three-dimensional movements of bed-load particles in an open channel with different grain diameters and different specific gravities under the conditions of different hydraulic gradient are measured by means of particle tracking velocimetry (PTV) technique. The average movement characteristics of the particles are analyzed by statistics. The results show that the longitudinal average velocities of the particles neither agree with the law suggested by Einstein nor obey the law assumed by Bagnold. Einstein’s law is that the particle velocity is equal to 11.6 times of shear velocity, and the Bagnold’s law assumes the velocity equal to the difference between flow velocity at corresponding water depth and settling velocity of particle. Two formulas for determining the velocity of particle in the forms of expression given by Einstein and Bagnold are obtained according to the statistical results of experimental data, respectively.     

Removal of heavy metals from storm and surface water by slow sand filtration: the importance of speciation

The removal of heavy metals from storm and surface waters by slow sand filtration is described. The importance of speciation as a technique for exploring and improving the mechanisms of removal is identified. Laboratory-scale slow sand filters operating at conventional flow rate and depth were shown to be able to reduce concentrations of selected heavy metals (Cu, Cr, Pb and Cd) found in road runoff, surface water and sewage effluents to drinking water standard. Nitrogen, volatile solids and modified Stover speciation were used to differentiate between the potential mechanisms of removal, i.e. active biomass, organic adsorption and simple adsorption or precipitation on the surface of the sand. The data presented show that adsorption via organic ligands was the predominant mechanism for metal removal at the surface of the filter but chemical adsorption was the more important deeper in the filter. In the lower layers the adsorbed metals were more easily exchanged than the organically bound metals. The precise chemical ligands were not identified and varied from metal to metal. The most important operational factors affecting performance were therefore the concentration of organic matter, filter depth and the flow velocity.     

城市街道视域景观指数与邻里尺度特征效应

城市街道景观指数提取为城市街景研究量化途径之一。结合已有指数,提出用全景静态图的不同数据类型,分类计算视域景观指数的优化方法;提出基于计算机视觉尺度不变特征转换(SIFT)的关键点邻域尺度区间频数;采用层次聚类分析指数不同的邻里尺度最优簇数、贡献度,确定分布特征重要指数组成;探索不同指数作用于不同邻里尺度的特征效应。研究发现视域景观指数具有不同邻里尺度效应。研究指数中城市街道空间的绿视率和天空开阔度(空间组成层面)、关键点邻域尺度(0,10]和(10,20]区间频数(对象尺度层面),以及色彩丰富度指数(颜色层面)是构成城市街道分布特征的重要特征指数。通过确定不同邻里尺度城市街道特征分布,可以为城市街道空间的景观质量提升、量化管理和城市微更新提供参照。