共查询到18条相似文献,搜索用时 187 毫秒
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综合糙率是采用曼宁公式确定河道水位和流量关系的关键参数。在河道冰封期,冰盖的出现增加了流动的阻力,明流条件下确定的综合糙率不再适用,需要重新估算。基于Einstein阻力划分过流断面的原理,冰盖下矩形河道的过水断面可划分为冰盖区、河床区和边壁区。根据总流的连续性方程,在确定各分区糙率系数、水力半径和断面面积的基础上,提出了冰盖下矩形河道综合糙率的计算公式。采用已有的试验水槽测量数据和天然河道实测资料对公式进行了验证,结果表明:公式计算的综合糙率与实测值吻合较好,与Einstein公式和Sabaneev公式相比,计算精度更高;对于冰封水流,宽浅河道采用分区水深代替水力半径进行简化计算的条件有别于明渠水流,在宽深比大于20时,计算结果才满足精度要求。 相似文献
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为进一步深化对明渠总流控制方程的认识,探究总流能量损失的构成与分布特点,通过理论分析,在黏性流体力学理论的框架下构建了描述明渠恒定紊流总体特性的总流积分模型与总流微分模型的控制方程,其模型参数能直接通过流动统计特征量在过流断面上的分布来获取,由此实现明渠流的流场特性描述与总流描述的统一,并得到了总流能量损失的显示表达式。同时指出,总流能量损失由黏性耗散与维持紊动两部分构成,在矩形明渠恒定均匀流紊流的分类能量损失构成中,壁面上以黏性耗散部分为主,维持紊动部分的能量损失密度随着离开壁面距离的增加而快速增加,且随着雷诺数的增大,其增加速度也越快。 相似文献
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为进一步深化对明渠总流控制方程的认识,探究总流能量损失的构成与分布特点,通过理论分析,在黏性流体力学理论的框架下构建了描述明渠恒定紊流总体特性的总流积分模型与总流微分模型的控制方程,其模型参数能直接通过流动统计特征量在过流断面上的分布来获取,由此实现明渠流的流场特性描述与总流描述的统一,并得到了总流能量损失的显示表达式。同时指出,总流能量损失由黏性耗散与维持紊动两部分构成,在矩形明渠恒定均匀流紊流的分类能量损失构成中,壁面上以黏性耗散部分为主,维持紊动部分的能量损失密度随着离开壁面距离的增加而快速增加,且随着雷诺数的增大,其增加速度也越快。 相似文献
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与顺直明渠水流相比,明渠交汇水流由于存在分离区和自由剪切面,其紊动特性引起的阻力较为复杂,交汇水流除床面阻力外还受到较强的阻力。基于数值模拟与实验比较分析表明,分离流与自由剪切流所产生的阻力(紊动粘性阻力)对于交汇水流数值模拟具有重要影响。同时,由于平面二维模型对二次流影响的忽略,断面环流较强时数值模拟会产生较大的阻力(环流阻力),而断面环流较弱时,交汇水流的三维特性相对较弱,采用平面二维模型和k-ε紊流模型能达到较好的模拟效果。因此,紊动及环流阻力的计算对交汇水流数值模拟的精度至关重要。 相似文献
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研究抽采过程中瓦斯运移特性有助于了解抽采气体来源、不同位置对抽采效果的贡献及抽采降压规律,为合理确定抽采时间、设计抽采位置和钻孔长度等提供依据。利用自主研发的多场耦合煤层气开采物理模拟试验系统,开展了卸压区不同钻孔长度条件下瓦斯抽采的物理模拟试验,分析了抽采过程中煤层瓦斯运移相对速度和方向特征。研究结果表明:抽采前期和钻孔周围区域分别是抽采量主要贡献时期和区域,瓦斯压力梯度大,流动快。卸压区瓦斯流动相对速度最快,应力集中区使得瓦斯相对流动速度衰减加速,且对原始区的瓦斯流动形成一道屏障,使其相对流动速度趋于0。随着抽采时间的增加,瓦斯相对流动速度逐渐衰减,对于瓦斯运移方向而言,抽采一旦开始便在煤层中形成了较为固定的运移通道,但在抽采后期和钻孔深部区域,由于瓦斯压力梯度小,流动缓慢,运移方向的不稳定性增强。而随着钻孔长度增加,卸压区内瓦斯相对流动速度表现出增大的趋势,因此,适当增大卸压区钻孔长度将有利于现场瓦斯开采。 相似文献
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中国东部地区的地质灾害多以坡面泥石流的形式发生,预测预报坡面泥石流的发生对于开展防灾减灾具有重要意义。地形、地质和降雨三大条件是影响坡面泥石流发生的主要条件。通过选择同样地质条件和基本相同降雨条件的区域,研究影响坡面泥石流发生的地形条件,并得出可以用于坡面泥石流预报的坡面泥石流地形条件。结果表明:地形条件由坡面坡度因子、泥石流上部因子、泥石流侧面因子和临空面因子组成;较大的地形条件T对应较大的泥石流发生可能性;降雨条件由泥石流发生前的降雨量与1 h降雨量组成;得出了由地形条件T和降雨条件R组成的坡面泥石流预报条件P,P值越大,坡面泥石流发生的可能性越大。预报条件P可以预报坡面泥石流的发生。 相似文献
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RANSOME K. T. CORNEY† JEFFREY PEAKALL DANIEL R. PARSONS LIONEL ELLIOTT† KATHRYN J. AMOS JAMES L. BEST GARETH M. KEEVIL DEREK B. INGHAM† 《Sedimentology》2006,53(2):249-257
The planform patterns of meandering submarine channels and subaerial fluvial bends show many similarities that have given rise to strong analogies concerning the fluid dynamics of these channel types. Existing models of helical motion in open‐channel bends depict flow that is characterized by surface flow towards the outer bank, and basal flow towards the inner bank. This paper investigates and compares, through an analytical model and physical experiment, flows within fluvial meanders, and submarine channel bends that contain density‐driven gravity currents. The results indicate that the sense of helical motion can be reversed in submarine bends that contain density currents when compared with fluvial bends, and that the orientation of the helical flow is dependent on the vertical distribution of downstream velocity. Specifically, the sense of helical motion is reversed in bends when the maximum downstream velocity is near the bed, resulting in near‐bed flow towards the outer bank. These findings suggest that the dynamics of sediment transport and deposition in curved channels with such velocity profiles will be fundamentally different to those currently assumed from sinuous open‐channels. 相似文献
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Tjalling de Haas Wilco van den Berg Lisanne Braat Maarten G. Kleinhans 《Sedimentology》2016,63(6):1596-1619
Alluvial fans develop their semi‐conical shape by quasi‐cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris‐flow fans, these quasi‐cyclic avulsions are poorly understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris‐flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris‐flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run‐out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris‐flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low‐gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in‐channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris‐flow avulsion cycles are governed by the same large‐scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris‐flow fans. This novel result provides a basis for modelling of debris‐flow fans with applications in hazards and stratigraphy. 相似文献
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以解决灌区普遍存在的通过增大水头损失来提高测流精度的问题为目标,探究梯形喉口无喉道量水槽在末级梯形渠道中量水的适用性。基于Flow-3D软件,采用RNG k-ε紊流模型和Tru-VOF方法对梯形喉口无喉道量水槽和矩形喉口无喉道量水槽水力性能进行了数值模拟,并通过物理原型试验验证了模拟结果的合理性。研究结果表明:梯形喉口无喉道量水槽测流过程中弗劳德数沿程呈单峰分布,在喉口下游附近出现临界流且喉口上游弗劳德数均小于0.5,满足灌区量水要求;测流过程中,最大流速位置随着量水槽边界变化而变化;对喉口收缩比相同的矩形喉口和梯形喉口无喉道量水槽测流精度、水头损失比较得出,梯形喉口有效改善了小流量时测流误差和大流量时的水头损失。 相似文献
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论动力相似预报的物理基础问题 总被引:1,自引:0,他引:1
为了更好地发展动力相似预报的策略和方法,基于非线性动力学成果,综合探讨了动力相似预报的物理基础问题。分析表明,“相似预报—低频流型—可预报性”在物理上存在密切联系,这在统计相似预报和动力相似预报问题中均适用。通过类比集合预报,并对实际大气不同尺度模式预报的个例分析,初步证实了相似初值对应的动力预报结果及其预报误差行为具有相似性。在此基础上,进一步提出三类相似性问题,并归纳了动力相似预报的物理基础:在初始状态相似性持续的情况下,动力预报结果以及预报误差行为具有相似性。 相似文献