EXPERIMENTAL STUDY ON TURBULENCE IN VICINITY OF MID-CHANNEL BAR OF BRAIDED RIVER

Abstract

Alluvial streams with braided patterns are frequently encountered in nature. Braided streams are characterized by a random pattern of multi-thread channel networks owing to the appearance of braid bars within the overall waterway of the river. The presence of braid bars increases the total flow resistance and the energy losses along the boundary, thereby promoting the development of networks of hydraulically inefficient channels. The mid channel deposition is considered as one of the main reasons of initiation of braiding. Extensive laboratory investigations and stringent analysis have been carried out to study the turbulence flow hydraulics in the vicinity of mid-channel bar. Researchers such as Nakagawa and Nezu (1978), Wu and Jiang (2007) observed that the bursting events are more closely associated to the sediment entrainment rather than the total shear stress. Thus for thoroughly understanding the flow behaviour, the conditional technique is used for decomposing the Reynolds stress into the bursting events. Researchers Jennifer et al. (2011), Nezu et al. (1994) observed that only the extreme turbulent events contribute to the turbulent burst. They used the concept of Hole size for segregating the extreme quadrant events from the low intensity quadrant events. The effect of Hole size on the bursting events are studied in details. The velocity fluctuations are modelled using the Gram-Charlier method. The joint probability distribution of quadrant events is analysed. It will help in analysing the structure of turbulent burst generated due to the fluid bar interaction. The Hole size effect on joint probability distribution of velocity fluctuations is also studied. The extreme events are observed at sections near the upstream end of mid-channel bar. These extreme turbulent bursts are generated by the fluid bar interaction. The sweep events are found to be dominant near the boundary and ejection events are observed to be dominant away upward from the boundary. From the results, it was found that the dominance of sweep events decreases as the hyperbolic Hole size increases. The results bring out that the higher stresses are produced mainly due to ejection events. The transverse component of turbulent kinetic energy is found significant in the vicinity of mid-channel bar. Thus, the three-dimensional bursting analysis is carried out for analysing the turbulent flow structure. The ‘3-Dimensional Burst Index’ (3DBI) and ‘Transition Ratio’ (TR) parameters are proposed in this work for linking the bursting events with the local stream bed elevation change observed in the vicinity of mid-channel bar. The 3-Dimensional Burst Index and Transition Ratio exhibit linear relationship with the scouring/deposition phenomena observed in the vicinity of mid- ii channel bar with their Coefficient of Determination and Pearson R value are found to be above 0.9. The high value of correlation coefficients indicate that these parameters are closely related with the local stream bed elevation change observed in the vicinity of mid-channel bar. For studying the interaction of sweep and ejection events, a new parameter ‘3-Dimensional Dominance Function’ (3DDF) is proposed in this study. This parameter helps in analysing the kolks-boils phenomenon observed in the vicinity of mid-channel bar. Researchers have utilized the concept of Hole size for two-dimensional bursting events. The concepts of Hole size for octant events is yet to be developed till now as could be seen from literature. At region where the flow is three dimensional, the Hole size concept for octant events is necessary for segregating the extreme octant events from the low intensity octant events. Thus, for defining the Hole size concept for octant events, the new parameter ‘3-Dimensional Hole Size’ (3DHS) is proposed in this study. The variations of octant events with the 3DHS are analysed. The results indicate that the presence of mid-channel bar leads to the creation of high turbulent burst. The one more notable conclusion drawn from this analysis is that the dominant events becomes more dominant with increase in the 3DHS value. Three dimensional transitional movements are modelled using the first order Markov chain. The linkage of these transitional movements with the scouring/deposition occurred in the vicinity of mid-channel bar are analysed. The relationship is proposed between the stable transitional movements using the Nonlinear Fit Toolbox of MATLAB Software. The high value of correlation for these expressions indicates that they are correctly predicting the relationship between the stable movements. The spatial contours of turbulent parameters are studied for bar and no bar conditions. High level turbulent intensities are observed at region near upstream end of mid-channel bar and level of turbulent intensities decreases with increase in distance from the upstream end of mid-channel bar. The high level of turbulent intensities at region near upstream end of mid-channel bar is responsible for occurrence of scouring at that region. As could be seen from literature, hardly any work had been done on this type of experimental setup. Thus, for validating the experimental results, the laboratory model is developed in the Fluent Software. The Ansys Mesher is used for meshing the simulated model. The Reynolds stress model is used for modelling purpose. The three-dimensional implicit steady pressure-based solver is iii utilized for the computation. Discretization of pressure is done using the Presto method. The momentum and Reynolds stress are discretized using the second order upwind scheme. The depth-wise profile of velocity and turbulent parameters are investigated at locations upstream and downstream of mid-channel bar. The effect of submergence ratio on the flow structure is analysed by closely observing the depth-wise distribution profile of velocity and turbulent parameters. The results indicate that the increase in the height of mid-channel bar causes greater reduction in longitudinal velocity. The vertical velocity is negative at sections upstream of the mid-channel bar. The negative vertical velocity indicates the downflow at these sections. The magnitude of transverse velocity is significant at sections downstream of mid-channel bar. This highlights that the flow structure is three dimensional at downstream region of the mid-channel bar. The increase in height of mid-channel bar causes increase in the magnitude of longitudinal turbulent intensity. Results confirm that the experimental results are correctly validated by the Reynolds stress model simulation of Fluent Software. The deviation of velocity profile from logarithmic law is more discernible for sections located in close vicinity of the mid-channel bar. The longitudinal turbulent intensity at sections downstream of mid-channel bar is much more pronounced as compared to the upstream sections. This is mainly due to the interaction of surface wave with the downstream flow. The increase in vertical and transverse component of turbulent intensity is observed for higher submergence ratio. Third order moments of velocity fluctuations transmit the stochastic information of the turbulent fluctuation in terms of flux and advection of turbulent stresses. The behaviour of these third order moments of velocity fluctuations at locations upstream and downstream of mid-channel bar are analysed. The presence of mid-channel bar causes increase of skewness in the longitudinal velocity distribution. The value of 𝑆𝑆𝑢𝑢 is greater for higher submergence ratio. This indicates that the high submergence ratio creates greater skewness of longitudinal velocity. The magnitude of 𝑆𝑆𝑤𝑤 is more at sections in the close vicinity of mid-channel bar. This highlights that the presence of mid-channel bar causes greater skewness in the vertical velocity distribution. Presence of mid-channel bar causes redistribution of 𝐹𝐹𝐹𝐹𝐹𝐹 and 𝐹𝐹𝐹𝐹𝐹𝐹 coefficients. The magnitude of 𝐹𝐹𝐹𝐹𝐹𝐹 and 𝐹𝐹𝐹𝐹𝐹𝐹 are greater for higher submergence ratio. This indicates that the greater energy transfer takes place for higher submergence ratio. iv New parameters are developed in this study for an insight into turbulent flow structure in the vicinity of mid- channel bar. The concept of Hole size is proposed for the octant events in this study. The newly introduced Three-Dimensional Hole Size in this research helps in minutely studying the turbulent flow structure. Not many prominent studies have been done on the flow structure in the vicinity of mid-channel bar. Due to the lack of experimental data on this topic, the Reynolds stress modelling is done for validating the experimental results. The effect of submergence ratio on the turbulent flow structure in the vicinity of mid-channel bar are analysed. Effect of fluid-bar interaction on the third order moments of velocity fluctuations is investigated. The present study contributes towards better comprehension of the intricate linkage of turbulent burst events with the local stream bed elevation changes that are observed in the vicinity of mid-channel bar of braided river.