薄壁堰泄流能力的数值模型计算及模拟自由水面的评价

【目的】研究数值模拟技术对矩形薄壁量水堰自由水面和泄流量的模拟能力。【方法】应用基于CFD的流体分析软件FLUENT,采用带自由表面的k-ε气液两相紊流数学模型,并使用自定义UDF函数处理入口条件,对整个流场进行数值模拟计算,得到了整个流场的流态分布图、泄流量、自由水面和压力分布等水力参数。【结果】将模拟得到的自由水面和泄流量与实测值进行对比,二者吻合良好,说明所选数值模型能够准确模拟矩形薄壁量水堰的流场特性。【结论】本研究采用的数值模拟技术,能成功模拟矩形薄壁量水堰的自由水面和泄流能力,可以为量水堰的水力设计提供参考依据。     

Computational fluid dynamics characterization of a novel mixed cell raceway design

Computational fluid dynamics (CFD) analysis was performed on a new type of mixed cell raceway (MCR) that incorporates longitudinal plug flow using inlet and outlet weirs for the primary fraction of the total flow. As opposed to conventional MCR’s wherein vortices are entirely characterized by the boundary conditions at inlet nozzles and outlet center drains in the center of each cell, the new MCR design can develop a wider variety of fluid behaviors due to the additional boundary conditions at the inlet and outlet walls where the weirs are placed. In this study, we investigated how the primary longitudinal flow would affect vortex formations in the cells by designing three different MCR models and simulating three major cases for each model. Through this process, performances of two numerical CFD models (transition k-kl-ω vs. k-ε) were compared, along with two vortex quantification methods (Q-criterion vs. a proposed method). We found that the k-kl-ω CFD model more accurately predicted vortex formation than the k-ε model. The three MCR models differed only by weir geometry or drain size, in order to see their individual influence on cell vortex formation. Each case had its own unique weir flow rate and center drain loading rate values that combined to a total flow rate resulting in 15-min hydraulic retention time (HRT) for the MCR. The ratio of (expressed as percentage) of center drain loading rate to total flow rate (R = 7.5%, 12.5%, and 20.1%.) was defined to establish a relationship between R and vortex strength or size. Simulations demonstrated that inlet weir aspect ratio impacted cell vortex formation and strength. Unlike weir geometry effects, the drain size had non-significant impacts on fluid behavior other than the velocity very near the drains. While R did have positive correlations with vortex strength, vortex size, and self-cleaning performance, an R of 20.1% was sufficient to create uninterrupted vortex formations. Too low of a center drain rate or R value can result in lack of any meaningful cell vortex formation which then obviates any self-cleaning action in an MCR. Our key finding through extensive computational analysis was that an R value of 20% was required in order to maintain effective vortex formation. Expressed more explicitly, this can be described as maintaining a center drain loading rate of 0.010094 m³/s per cell (160 gpm), which correspond to unit loading rates of 16.3 lpm/m² per cell (0.40 gpm/ft² per cell).     

Use of vegetated drainage ditches and low-grade weirs for aquaculture effluent mitigation: II. Suspended sediment

Total suspended solids are a priority pollutant under the Clean Water Act and a point of concern for aquaculture facilities. The use of ubiquitous vegetated ditches on the aquaculture landscape may serve as an environmentally and economically sustainable practice for reducing suspended sediment contributions to downstream environments. This study assessed effects of consecutive low-grade weirs on suspended solids retention and settling rates of aquaculture pond effluent in a single drainage ditch. Two control and nine treatment discharges were conducted in September and October 2012 at the Mississippi State University South Farm Aquaculture Facility. All discharges decreased total and volatile suspended solid loads. Total suspended solids were decreased 72–94%, with a significant removal rate of 0.02 ± 0.01 mg L⁻¹ min⁻¹ in both control (F = 6.12, P < 0.001) and treatment discharges (F = 16.02, P < 0.001). Volatile suspended solids comprised 2–80% of total suspended solids and had a significant removal rate of 0.02 ± 0.001 mg L⁻¹ min⁻¹ in both control (F = 10.46, P < 0.001) and treatment discharges (F = 6.28, P < 0.001). There was no significant difference in overall settling rates between control and treatment discharges; however, prior to weir 1, both total and volatile suspended solid concentrations increased in control discharges. Treatment discharges decreased both total and volatile suspended solids significantly (P < 0.001) prior to weir 1. Further analysis revealed flow rate to be a significant (P < 0.001) variable in total suspended solid removal while initial concentrations affected reduction rates of volatile suspended solids significantly (P < 0.001). These results suggest that the use of low-grade weirs could be a viable best management practice that easily integrates within the aquaculture landscape and creates hydraulic conditions conducive to sediment retention.     

预应力矩形渡槽温度应力有限元分析

结合南水北调沸河渡槽工程实例。通过运用三维有限单元法对预应力混凝土矩形渡槽的受力性能进行数值模拟。重点分析预应力和温度荷载对渡槽应力的影响。热应力耦合采用顺序耦合法来实现。计算结果表明：渡槽内力及预应力配筋分布合理,混凝土应力满足规范要求,温度荷载对槽身应力有较大影响。     

水果直径和缺陷面积的机器视觉检测

黄花梨是中国的一种重要水果,果径和果面缺陷面积是黄花梨分级的两项关键指标。通过研究黄花梨的分光反射特性,研制了一套适合黄花梨品质检测的机器视觉系统。为了适应实际生产中水果方向的随机性和水果外形的不规则性的要求,使水果尺寸检测的方法有更好的适应性,设计了一种利用水果的最小外接矩形(MER)法求最大横径的方法,并进行了试验验证,得出了表示实际最大横径与预测最大横径的关系的回归方程式,两者的相关系数为0.9962。分析了黄花梨缺陷区域的R、G、B各分量灰度的变化特点,利用R分量灰度和G分量灰度在缺陷区域和完好区域交界处有明显突变这一特点,采用梯度算法求得了可疑缺陷点,然后再用区域生长法,找出了缺陷点像素的最大连通集及所有的缺陷区域;采用像素点变换法,实现了根据三维物体的二维投影图像恢复物体表面的真实几何面积的设想,大大降低了缺陷面积计算的误差;另外,还提出了一种新的面积修正方法,即用实际缺陷面积等于经像素点变换后的缺陷面积减去缺陷区域周长的一半加上1个像素点的面积来进行修正,进一步提高了缺陷面积计算的精度,而且该修正方法同样适用于其它图像面积的计算     

Adaboost算法在人脸检测中的应用

针对人脸检测的特点，详细叙述了检测中应用的特征值的计算以及积分图的使用，给出了基于Adaboost算法的弱分类器选取和单个强分类器的训练过程，以及最后的层叠分类器的具体组成形式．基于此算法的人脸检测具有检测率高、速度快、对于光照变化适应性强的特点，能够达到实时检测的要求．     

Performance evaluation of rice–fish integration system in rainfed medium land ecosystem

This study was carried out for three successive years during 1999–2001 to evaluate growth and yield performance of fish, prawn and paddy under rice–fish integration system in rainfed medium land ecosystem. Irrespective of stocking density, faster growth rate was recorded for Catla catla followed by Cyprinus carpio, Cirrhinus mrigala, Labeo rohita and Macrobrachium rosenbergii. C. carpio and C. mrigala performed better growth rate against that of L. rohita probably due to the fact that being bottom dwellers, C. carpio and C. mrigala are more tolerant to fluctuation of oxygen concentration. Productivity of fish and prawn was, however, higher (p<0.05) in refuges with 10-cm weir height plots, irrespective of stocking density, while overall yield performance was good at stocking density of 25,000 ha⁻¹. It was observed that, even with supplemental feeding, with increase in stocking density, biomass yield increased up to an optimum and then decreased. An average minimum and maximum yield of 906.6–1282.3 kg ha⁻¹ of fish and prawn has been achieved, which was much higher than the earlier recorded productivity in a season under rice–fish integration system. Highest grain yield was recorded at 15-cm weir height plot (3629 kg ha⁻¹), probably contributed by higher number of panicles per square meter (235.5) and number of filled grains per panicle (121.7). Percentage increase in rice yield under rice–fish integration system was 7.9–8.6% against control, where paddy was cultivated without integration of fish and prawn probably due to better aeration of water, greater tillering effect and additional supply of fertilizer in form of leftover feed and fish excreta. Irrespective of stocking density, the overall rice equivalent yield (REY) of the system was high (4.22–4.55 tons ha⁻¹) at 12.5-cm weir height plots-cum-refuge, without using any pesticide, herbicide, etc.     

都江堰内外江分水鱼嘴工程变迁考述

内外江分水鱼嘴是都江堰三大渠首工程之一,其功能主要是分水分沙。自战国时期李冰创建以来,鱼嘴位置不断移动,修筑材料结构则从战国唐宋时期的竹笼卵石发展到元明时期的铸铁条石,再到清代至民国初期的条石结构,直至当今的混凝土结构,即都江堰分水鱼嘴如今呈现的面貌是历代不断探索、改进的结果,其中包含的经验教训值得总结与借鉴。     

车辆导航立体视觉测量技术特征点匹配方法

车辆跟踪定位中的立体视觉算法,特征点匹配计算量大,误匹配概率高,实时性不强。对特征点匹配技术进行了研究,通过分析传统特征点提取方法的不足,提出了基于四边形约束的彩色标示特征点的匹配方法。试验表明,该方法匹配的准确率为100%,计算速度快、适应性强。