基于掺气水流的窄缝挑坎水力特性研究

The air concentration of flow in conventional hydraulic model experiment always has an obvious scale effect. Several accident analysis of the slope bank destruction by flip-nappe revealed that, the air concentration of flow in prototye is much larger than that measured in model experiment is a principal accident cause. Slit-type bucket is widely used in the energy dissipation of side spillway or sluice tunnel, especially in those with high water head and long flow path. Its hydraulic characteristics are more complex than that of the normal flip bucket. Currently, there has been no systemic research reported about the characteristics of aerated water flow on slit-type bucket. In this project, the hydraulic characteristics of slit-type bucket based on aerated water flow will be investigated through model experiment on chute flow. On basis of ordinary layout of slit-type bucket (straight symmetric contraction, contraction coefficient from 0.2 to 0.3, flip angle from -10 to 0 ), controlling the entrained air quantity of the flow before the slit-type bucket contraction section by artificial aeration, the influence law of the air aeration on the hydraulic factors of slit-type bucket will be illustrated, and a calculation express taking account the air quantity could be derived through the measurement of the time-averaged pressure, fluctuating pressure, water surface profile, velocity distribultion of outlet flow, jet trajectory and et al. This work will enrich the experimental results on slit-type bucket and promote the understanding of the influence of air aeration on the hydraulic characteristics of slit-type bucket, which will also provide theoretical and practical references to the engineering applications in hydraulic design and construction.

常规水工模型实验中的水流掺气浓度存在较大的比尺效应。几起挑流鼻坎水舌冲毁岸坡事故的调查分析表明，原型挑流鼻坎前水流掺气浓度远大于其模型实验值是主要原因。窄缝挑坎是高水头、长流程岸边溢洪道或泄洪洞常用的消能型式，其水力特性比一般挑坎更加复杂。目前尚未见到窄缝挑坎掺气水流特性研究的相关报道。 本项目拟采用陡槽水流模型实验对窄缝挑坎掺气水流特性进行研究。在窄缝挑坎常用的布置体型（直线对称收缩，收缩比0.2～0.3，挑角-10 ～0 ）条件下，通过调控窄缝挑坎前来流断面的模型水流掺气浓度，重点研究掺气水流的窄缝挑坎时均压力、脉动压力、水面线、出坎水流纵向流速分布、水舌空中运动轨迹及挑距等，揭示掺气水流的窄缝挑坎水力要素变化规律，提出考虑水流掺气浓度影响因子的相关水力参数计算关系式。 该研究将丰富窄缝挑坎相关实验成果，加深对窄缝挑坎掺气水流相关特性的认识，对完善工程设计理论有重要的科学意义。

窄缝挑坎是大型水利水电工程常用的泄洪消能结构型式，其水力特性复杂，通常需要借助水力模型试验进行相关设计；而常规水力模型由于水流自掺气的比尺效应，不能真实反映水流掺气条件下的相关水力特性。.本项目选取窄缝挑坎常用的结构形式（收缩比β=0.2～0.3、挑角θ=-10°～0°），通过人工干预模型泄槽水流紊动、促使水流表面自掺气提前发生的方式，使模型窄缝挑坎前泄槽形成不同掺气浓度的水、气两相均匀流；系统研究了不同掺气水流的窄缝挑坎相关水力特性，提出了考虑水流掺气浓度影响因子的各水力参数计算修正式。.（1）通过陡槽水力模型试验，发现了水流掺气使窄缝段冲击波交汇位置提前、冲击波交汇角增大的规律，建立了窄缝冲击波交汇相对位置、冲击波相对波角与水流掺气浓度、窄缝体型参数β、θ的计算关系式。.（2）揭示了窄缝出口掺气相对水深hc/h0只与水流掺气浓度、窄缝收缩比相关、与窄缝挑角和来流佛氏数Fr无关的基本规律，建立了窄缝出口掺气相对水深与水流掺气浓度的关系式，为窄缝边墙最大高度的设计提供了理论依据。.（3）发现了水流掺气使窄缝边墙竖向压力分布趋向坦化的规律，最大动水压力随掺气浓度减小、水流紊动强度随掺气浓度增大的规律；建立了以水流掺气浓度为影响因子的动水压力及水流紊动强度修正关系式；得出了水流充分掺气会加剧结构物振动的结论。.（4）通过水流掺气后窄缝挑坎出口断面水流平均流速和垂线流速分布的研究分析，得到了窄缝出口断面平均流速随水流掺气浓度增大、出口顶部流速随水流掺气浓度减小的规律；建立了窄缝出口断面各特征流速与水流掺气浓度的关系式。.（5）探明了水流掺气浓度对窄缝挑坎水舌外缘挑距有较大影响的规律，提出了基于水流掺气浓度的水舌外缘挑距计算修正式。.该项目填补了窄缝消能工在掺气水流条件下相关水力特性研究的空白，提出了对常规水力模型试验相关成果进行水流掺气浓度影响修正的计算式；丰富了窄缝挑坎相关水流理论，对指导工程设计具有重要的科学意义。