Investigation of the secondary current pattern in the open channel with different roughness forms on the bed using Flow3D

Document Type : Original Research

Authors
B. Pirzadeh
N. Shahbeygi
University of Sistan and Baluchestan
Abstract
It is important in Hydraulic and river engineering to estimate the mean velocity and turbulence intensity to identify the presence of secondary currents, its shape and position. The flow of channels consists of three components of velocity: one in the direction of flow ,one in the transverse and the orther in the vertical directions of the channel. Due to the heterogeneity of the velocity fluctuations, a series of vortex vortices in the channel section to be formed which is called secondary currents cells. The secondary currents are dependent on factors such as bed roughness, channel slope and bed shear stress. The present study investigates the effect of bed roughness form on the pattern of secondary currents with numerical modeling in Flow-3D software by using RNG turbulent model. In order to successfully model and reduce the cost of simulating the near wall region, mesh sensitivity analysis has been done and numerical domain has been divided into some subdomains. This research has been carried out according to the data of the Negara laboratory model (for triangular roughness) carried out at the Hydraulic Laboratory of Singapore National University. In the results obtained from the mean velocity profile, the mean error for the roughness trough was 9.94% and for roughness crest was 3.71%. in the case of shear velocity, the error was obtained at three cross sections x=4m, x=5m and x=6m respectively 6.58%, 6.86% and 5.67% which demonstrated the good fit of the numerical model results with the reference laboratory model. The flow conditions in the channel were designed and studied for three geometries of roughness (with same height) ,i.e., rectangular roughness, trapezoidal roughness with an internal angle of 80 degrees and trapezoidal roughness with an internal angle of 55 degrees that are most used in hydraulic structures. The results of the study on the turbulence intensity, secondary currents and turbulence kinetic energy showed the effect of trapezoidal roughness with an internal angle of 55 degrees relative to the other two forms of roughness. The difference in the turbulence intensity in trapezoidal roughness with an internal angle of 55 degrees relative to a triangular roughness was obtained about 4.54%. The location of the center of the secondary current cell was in the depth of about 0.2 meters. The tendency of cells to the side walls of the channel is also affected by roughness geometry. In trapezoidal roughness with an internal angle of 55 degrees, there is a tendency to both walls. The turbulence kinetic energy contour with the center of the nucleus begins at the roughness crest with an approximate value of 0.0002 for all roughness, but its location was different in relation to the roughness geometry. In fact, the turbulence kinetic energy is the effect of fluctuating components of the velocity that the existence of external rotations is the effect of the redistribution of energy by velocity tensor, which is responsible for the formation of secondary current cells. As has been pointed out in the literature, results showed the role of roughness shape in the appearance of the secondary currents.

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Modares Civil Engineering journal
Volume 19, Issue 4
November and December 2019
Pages 97-108