近海水域三维水动力学和水质的精细模型研究

本文把研究近海水域水质污染与控制的三维精细预报系统作为目标。在分析近海环境中各种物理、化学和生物现象的基础上 ,针对近海水域水污染的特点 ,选定一个包含有二十多个状态变量及其相互作用的三维非线性系统作为研究和模拟对象 ,建立了一个统一考虑物理、化学和生物等过程综合作用的近海水域三维水动力学和水质的精细模型 ,并成功地应用于日本博多湾的水质模拟。     

The Modelling of Turbulence from Traffic in Urban Dispersion Models — Part I: Theoretical Considerations

The modelling of pollutant dispersion at the street scale in an urban environment requires the knowledge of turbulence generated by the traffic motion in streets. In this paper, a theoretical framework to estimate mechanical turbulence induced by traffic in street canyons at low wind speed conditions is established. The standard deviation of the velocity fluctuations is adopted as a measure of traffic-produced turbulence (TPT). Based on the balance between turbulent kinetic energy production and dissipation, three different parameterisations for TPT suitable for different traffic flow conditions are derived and discussed. These formulae rely on the calculations of constants that need to be estimated on the basis of experimental data. One such estimate has been made with the help of a wind tunnel data set corresponding to intermediate traffic densities, which is the most common regime, with interacting vehicle wakes.     

Characteristics of Resuspension,Settling and Diffusion of Particulate Matter in a Water Column

Experiments were conducted in order to characterize the distributions of concentrations of suspended particulate matter (SPM) in water columns of lakes and reservoirs. The experiments, in a reduced model of the water column, used a set of oscillating grids. Runs were done with particles denser than water as well as with light particles. The results were in good agreement with analytical solutions for steady-state, and non-steady-state conditions. An approximate analytical solution was derived and found to be in agreement with the full solution. The threshold for resuspension was measured, and characterized in terms of a modified Shields parameter, which is appropriate to a zero-shear environment. All experiments showed that the distribution of SPM exhibited a layer near the bottom that is thought to be analogous to the benthic nepheloid layer (BNL) observed in larger lakes. The thickness of the nepheloid layer increases with the turbulence intensity.     

Experimental study on unconfined methane explosion: Explosion characteristics and overpressure prediction method

Explosion accidents have become the main threat for the high-efficiency use of cleaner gas energy sources, such as natural gas. During an explosion, obstacle causing flame acceleration is the main reason for the increase of the explosion overpressure, which still remains to be fully understood. In this research, field experiments were conducted in a 1 m³ cubic frame apparatus to investigate the effect of built-in obstacles on unconfined methane explosion. Cage-like obstacles were constructed using square steel rods with different cross section size. The results demonstrated that the flame could get accelerated due to the hydrodynamic instability and obstacle-induced turbulence, which enhanced the explosion overpressure. In the near field, the overpressure wave travelled slower and the maximum overpressure could almost keep constant. Reducing the cross section size, or increasing the obstacle height or the obstacle number per layer could determine the rise of the maximum overpressure, the maximum pressure rising rate and the overpressure impulse. For uniformly constructed obstacles, self-similar theory was chosen to measure the influence of the hydrodynamic instability, and a parameter β was adopted to measure the flame acceleration caused by obstacle-induced turbulence, the value of which was 2 in this research. Based on the acoustic theory, an overpressure prediction model was proposed and the predicted results agreed with the measured values better than previous models, such as TNT equivalency model and TNO multi-energy model.     

人为扰动下河流复氧激增现象及机制分析

为探究河流复氧规律及其机制,研制出改变河道局部水流流速的深控型水平推流曝气装置,并使用该装置在天津市外环河中进行原位试验,探究人为扰动对河流复氧系数的影响.试验结果表明,装置工作时纵向流速提高至40cm/s,最大增幅22倍;水体溶解氧浓度提高0.6~1.7mg/L,约10%.研究发现,经典的Owens等5组河流复氧系数经验公式均低估了扰动下的复氧系数,试验均值约为49.5d^-1,是理论值的10~100倍,即河流复氧在人为扰动下发生激增现象.分析成因表明,湍流动能与河流复氧系数显著正相关,区别于自然流动,人为扰动提升了水体纵向和垂向湍流动能,加速了中下层水体溶解氧扩散,提高了复氧率,进而发生了复氧激增现象.研究成果为提升城市河流水质的水力学方法提供了科学依据.     

OHP–APT 2002 gravity wave campaign: waves,turbulence and forecasts

In July 2002, a multi-sensor campaign was conducted in southern France to investigate the hypothesized connection between gravity waves and optical turbulence. A generalized scidar (GS) was mounted on the 1.93 m diameter telescope at l’Observatoire de Haute-Provence (OHP). The GS provides continuous profiles of optical turbulence with 300 m vertical resolution from telescope altitude up to 25 km. Thermosondes, which provide in situ measurement of optical turbulence by measuring temperature variance, were launched at the OHP site and at a site approximately 20 km west-northwest of OHP. Gravity wave activity was deduced from temperature and wind velocity measured by radiosondes, which are part of the thermosonde system. In this paper, gravity waves were analyzed using techniques for simple two-dimensional mountain waves, with only fair results. Mesoscale models were run at moderately high resolution for the period. The forecasts were analyzed for wave activity, and a post processor model was used to diagnose the optical turbulence. Mountain waves were evident in the forecast, but quantitative comparison showed the forecast to be inadequate in predicting wave strength. The forecast optical turbulence was in fair agreement with measurements with notable exceptions. This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. The U.S. Government’s right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged. Presented at the Turbulence and Waves in Stably Stratified Atmospheric Shear Flows: Measurements, High Resolution Simulations and Numerical Prediction Challenges Conference, 13–15 September 2004, Lighthill Institute of Mathematical Sciences, University College London, London, UK.     

Mass Transport in Vegetated Shear Flows

Submerged aquatic vegetation has the potential to greatly improve water quality through the removal of nutrients, particulates and trace metals. The efficiency of this removal depends heavily upon the rate of vertical mixing, which dictates the timescale over which these constituents remain in the canopy. Continuous dye injection experiments were conducted in a flume with model vegetation to characterize vertical mass transport in vegetated shear flows. Through the absorbance–concentration relationship of the Beer–Lambert Law, digital imaging was used to provide high-resolution concentration profiles of the dye plumes. Vertical mass transport is dominated by the coherent vortices of the vegetated shear layers. This is highlighted by the strong periodicity of the transport and its simple characterization based on properties of the shear layer. For example, the vertical turbulent diffusivity is directly proportional to the shear and thickness of the layer. The turbulent diffusivity depends upon the size of the plume, such that the rate of plume growth is lower near the source. In the far-field, mass is mixed more than twice as rapidly as momentum. Finally, plume size is dictated predominantly by X, a dimensionless distance that scales upon the number of vortex rotations experienced by the plume.     

A Fine-Scale <Emphasis Type="Italic">k</Emphasis>−ε Model for Atmospheric Flow over Heterogeneous Landscapes

A multi-purpose model for small-scale atmospheric flows over heterogeneous landscapes is being developed. The aim of this research is to build a tool able to predict the dynamical (wind, turbulence) and diffusive (gases, particles) fields over landscapes characterised by heterogeneous plant cover. In its present stage of development the model is based on the numerical integration of neutral atmospheric flow equations, using an energy-dissipation closure scheme and over a domain that may include vegetation layers. Three validation cases of the model are presented: (i) response of the airflow to a change in surface roughness; (ii) airflow within and above a horizontally homogeneous plant canopy; (iii) airflow over two complex forest-to-clearing and clearing-to-forest transitions. All simulations provide results in good agreement with the experimental data, except for turbulent kinetic energy just after a clearing-to-forest transition. This result is not surprising for a statistical k– model in a flow region characterised by strong distorsion and intermittent turbulence. However the overall good performance of the model is promising for environmental research at fine scales over heterogeneous landscapes.     

A Simple Model to Determine In Situ Mixing Height Growth from Surface Measurements

A generic In Situ Mixing Height Growth (IMG) model, capable of predicting the real-time growth of the mixed layer and its diurnal evolution from routinely observed simple surface meteorological is developed. The algorithm for the determination of temporally growing daytime mixing height includes both convective and mechanical turbulence contributions. It accounts for neutral as well as height varying potential temperature gradients above the mixed layer. For thermally stable and mechanically dominated unstable night time Atmospheric Boundary Layer (ABL) the module uses similarity formulae based on the wind velocity [1]), the Monin—Obukhov length [2], and the Coriolis parameter. In the convective case simple slab model is integrated, based on initial lapse rate and the surface heat flux. The lapse rate is evaluated on the basis of vertical atmospheric stability, surface type and surface temperature. This differentiates the IMG model from other existing mixing height models that need initial measured lapse rate for calculation. IMG model is site specific as it calculates the radiative incoming heat flux depending on the solar declination estimates based on-site latitude and longitude. The IMG model is applied to calculate mixing height for India by using surface data (viz. wind speed, surface temperature, surface type) from 152 surface meteorological stations. Results have been evaluated with radiosonde mixing height data procured from 18 upper air stations. Sensitivity analysis of the model with respect to various parameters is performed. The model is formulated after reviewing presently available radiosonde mixing height data in India and can satisfactorily provide an alternative means of estimating mixing height for air pollution dispersion models.     

A Modelling Study of Transient,Buoyancy-Driven Exchange Flow over a Descending Barrier

Results are presented from a series of model studies of the transient exchange flow resulting from the steady descent of an impermeable barrier separating initially-quiescent fresh and saline water bodies having density ₀ and ₀+()₀, respectively. A set of parametric laboratory experiments has been carried out (i) to determine the characteristic features of the time-dependent exchange flow over the barrier crest and (ii) to quantify the temporal increase in the thickness and spatial extent of the brackish water reservoir formed behind the barrier by the outflowing, partly-mixed saline water. The results of the laboratory experiments have been compared with the predictions of a theoretical model adapted from the steady, so-called maximal exchange flow case and good qualitative agreement between theory and experiment has been demonstrated. The comparisons indicate that head losses of between 7% and 3% are applicable to the flow over the ridge crest in the early and late stages, respectively, of the barrier descent phase, with these losses being attributed to mixing processes associated with the counterflowing layers of fresh and saline water in the vicinity of the ridge crest. The experimental data show (and the theoretical model predictions confirm) that (i) the dimensionless time of detection t _det(g/H _b)^1/2 of the brackish water pool fed by the dense outflow increases (at a given distance from the barrier) with increasing values of the descent rate parameter g'H _b/(dh _b/dt)² and (ii) the normalised thickness (x,t)/H _b of the pool at a given reference station increases monotonically with increasing values of the modified time (t–t _det)/(H _b/g)^1/2, with the rate of thickening decreasing with increasing values of the descent rate parameter g'H _b(dh _b/dt)². Here, g=(g/₀)()₀ is the modified gravitational acceleration, H _b is the mean depth of the water and dh _b/dt denotes the rate of descent of the barrier height h _b with elapsed time t after the two water bodies are first brought into contact.