Shanghai Jiaotong Daxue xuebao (Jan 2021)
Entrainment and Turbulence Characteristics of Continuous-Flux Release Gravity Current over Rough Beds
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.
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