粗糙底床对持续入流式异重流掺混和湍流特性的影响

doi:10.16183/j.cnki.jsjtu.2019.110

摘要/Abstract

摘要：

在自然环境和水利工程中，异重流现象广泛存在．现实工况中，大多数底床覆盖砾石及不同粒径大小的泥沙颗粒，可视为粗糙底床，因此研究异重流流过粗糙底床的动力学特性具有实际科学意义及工程应用价值．本文利用持续入流式异重流水槽试验，综合考虑底床粗糙度和异重流初始质量分数，分析异重流头部位置、头部速度、掺混系数等扩散特性，不同断面的湍流强度及雷诺应力等湍流特性，结合雷诺应力法和湍动能法计算底床剪应力．结果表明：异重流头部速度与底床粗糙度呈负相关，与初始质量分数呈正相关；在高粗糙度、高初始质量分数工况时，底床粗糙度是控制其运动特性的主要因素；当粗糙度增加到一定程度时，异重流纵向时均流速剖面(沿水深分布)的峰值点出现“爬升现象”；湍流强度剖面(沿水深分布)出现一个极小值及两个极大值，其中纵向湍流强度是异重流湍流结构的主导，粗糙底床上的垂向湍流强度相比于光滑底床增加幅度明显；靠近底床附近，雷诺切应力为正值，远离底床区域，雷诺切应力为负值；利用湍动能法计算的底床剪应力均小于同粗糙度下的雷诺应力法；在总体理查森数相同情况下，异重流掺混系数与底床粗糙度呈正相关．最后总结出粗糙底床对异重流的主要影响为：摩阻力增加、掺混作用强化、近底床区域密度重分布、湍流边界层变厚及纵向时均流速剖面峰值点“爬升现象”的出现．

关键词: 异重流, 持续入流, 粗糙底床, 湍流特性, 底床剪应力

Abstract:

In both the natural environment and hydraulic engineering, there widely exists the phenomenon of gravity current. In practical conditions, most beds are covered with gravel and sediment particles of different sizes, which can be regarded as rough bed conditions. Therefore, it is of practical scientific significance and engineering application value to study the dynamic characteristics of gravity current flowing over rough beds. By conducting flume experiments for continuous-flux gravity current, and considering the bed roughness and the current initial mass fraction comprehensively, the propagation characteristics such as head position, head velocity, and entrainment coefficient of gravity current are analyzed, the turbulence characteristics such as the turbulence intensity and Reynolds stress at different cross-sections are studied, and the bed shear stress is calculated by using the Reynolds stress method and the turbulent kinetic energy method. The results show that the velocity of the gravity current head is negatively correlated with the bed roughness, but positively correlated with the current initial mass fraction. In the experimental runs of conspicuous roughness and high initial mass fraction, the former is the dominant factor for the kinematic characteristics of gravity currents. When the bed roughness increases to a certain extent, the "lifting phenomenon" of the averaged longitudinal velocity profiles occurs in the gravity current body. One minimum and two maximun values are presented in the turbulence intensity profile of gravity current, and the longitudinal turbulence intensity is the main controlling factor for the current turbulent structure. Besides, the vertical turbulence intensity over rough beds increases significantly compared with that of smooth beds. Near the bed, the Reynolds shear stress is positive, whereas far away from the bed, the Reynolds shear stress is negative. The bed shear stress calculated by using the Reynolds stress method is higher than that by using the turbulent kinetic energy method at identical bed roughness. At the same bulk Richardson number, the entrainment coefficient of gravity current is positively correlated with the bed roughness. It can be concluded that the influence of rough beds on gravity current is mainly as follows: increasing friction resistance, reinforcing mixing effect, redistributing current density near the bed, thickening turbulent boundary layer, and "lifting phenomenon" of the averaged longitudinal velocity profiles.

Key words: gravity current, continuous-flux release, rough beds, turbulence characteristics, bed shear stress

中图分类号:

TV145.23

韩东睿, 余俊杨, 袁野平, 贺治国, 王雨杭, 林颖典. 粗糙底床对持续入流式异重流掺混和湍流特性的影响[J]. 上海交通大学学报, 2021, 55(1): 77-87.

HAN Dongrui, YU Junyang, YUAN Yeping, HE Zhiguo, WANG Yuhang, LIN Yingdian. Entrainment and Turbulence Characteristics of Continuous-Flux Release Gravity Current over Rough Beds[J]. Journal of Shanghai Jiao Tong University, 2021, 55(1): 77-87.

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工况	D₅₀/mm	w_ini/%	g′/(m·s^－2)	Re_head	Fr_d
1	0	1.75	0.129	5 456	0.671
2	0	2.50	0.185	6 248	0.642
3	0	3.25	0.241	7 232	0.651
4	3	1.75	0.129	4 600	0.566
5	3	2.50	0.185	5 464	0.561
6	3	3.25	0.241	5 488	0.494
7	10	1.75	0.129	4 560	0.561
8	10	2.50	0.185	5 088	0.523
9	10	3.25	0.241	5 352	0.482
10	30	1.75	0.129	3 536	0.435
11	30	2.50	0.185	4 192	0.431
12	30	3.25	0.241	4 592	0.413

D₅₀/mm	特征断面1			特征断面2
D₅₀/mm	RS法/Pa	TKE法/Pa	偏差/%	RS法/Pa	TKE法/Pa	偏差/%
3	0.110 4	0.058 9	46.64	0.091 2	0.066 5	27.12
10	0.125 0	0.074 1	40.73	0.105 5	0.085 0	18.95
30	0.156 0	0.096 9	37.88	0.132 6	0.114 0	14.02