Part I presents the statistical theory of turbulence, and Part 2 the coherent structures in open-channel flows and boundary layers. The book is intended for advanced students and researchers in hydraulic research, fluid mechanics, environmental sciences and related disciplines. References Index.

Turbulence in open-channel flows

The most immediate impression of turbulent flow is, probably, that it is an exceedingly complicated, tortuous and chaotic phenomenon. The experimenter equipped with instruments capable of recording the parameters of a turbulent flow will soon conclude that all these parameters fluctuate in an irregular fashion.

Statistical Description of Turbulence

Experimental investigation on dispersion mechanisms in rigid and flexible vegetated beds

In this study, a computational method is used to investigate the flow properties and turbulence characteristics through discontinuous and vertically double layered vegetation patches of finite width in an open channel. A three dimensional Reynolds stress turbulence model was implemented utilizing FLUENT (ANSYS). After the validation of the numerical model, flow characteristics were investigated against varying vegetation density (St/d and Ss/d, where St = spacing between taller vegetation elements, d = diameter of vegetation, Ss = spacing between smaller vegetation elements) and vegetation patch width. Various profiles and contour plot distributions of mean stream-wise and depth averaged velocities, turbulent kinetic energy and turbulent intensities at specified critical locations and sections simulated by the model are presented in this paper. In almost all the cases, mean stream-wise velocities directly behind the smaller and larger vegetation elements showed fluctuations, with sharp inflection points at the top of the smaller vegetation elements. Velocities were amplified above the vegetation zones, whereas deceleration in the velocity magnitudes was observed within the vegetation zones i.e., below the top of vegetation. Inside the vegetation patch zones, the turbulent intensity was higher as compared to that in the gap zones. The turbulent intensity was increased for dense vegetation case (St/d = 6.66 and Ss/d = 3.34) as compared to that of sparse arrangement of vegetation (St/d = 10 and Ss/d = 5). Hence, a clear difference in flow characteristics between the vegetated and non-vegetated zones was observed.

Numerical investigation of the flow characteristics through discontinuous and layered vegetation patches of finite width in an open channel

An open channel with an emergent vegetation patch: predicting the longitudinal profiles of velocities based on exponential decay

Flow characteristics in an alluvial channel covered partially with submerged vegetation

The flow characteristic in a partly vegetated channel with emergent condition is studied by covering a finite length of the channel with natural Oryza sativa (rice) stems with downward seepage. Spatial study of velocity profiles shows that vegetation reduces the flow velocity lengthways of vegetation zone even with the presence of downward seepage. Reynolds stress increases at the downstream end of the interface section because of higher velocity fluctuations between vegetated and un-vegetated zone. The increase in Reynolds stress at the downstream of interface section is attributed to higher lateral flow and momentum exchange in this transition zone between vegetated and un-vegetated area. For the vegetated section, vegetation serves as a barrier for the vegetated section in reducing the maximum Reynolds stress in the range of 17–20% along the flow direction. It is also observed that downward seepage increases the Reynolds stress as well as turbulence intensities. Integral scales increase with increase in percentage of downward seepage resulting in higher Reynolds stresses with downward seepage. Presented work will help in analyzing the changes in flow field with partially emergent vegetation in alluvial channel subjected to downward seepage. The overall results of this study may enhance the comprehensive understanding of the vegetation effects on flow characteristics in partly emerged vegetated channel and may also provide some guidance for the practice of river ecological restoration on river bed or banks.

Flow characteristics in a partly vegetated channel with emergent vegetation and seepage

Experimentation on submerged flow over flexible vegetation patches with downward seepage

Turbulent flow statistics of vegetative channel with seepage

The presence of aquatic vegetation changes the flow characteristics in a channel. The impact of the presence of submerged, flexible vegetation in a channel with an additional parameter such as downward seepage is analyzed in the present work. Two patterns, staggered and uniform, and two different vegetation spacing, 15 and 10 cm center–center, were adopted. Experiments were conducted for the case of no seepage, 10% seepage, and 15% seepage. Velocity profiles show the presence of an inflection point near the top of the vegetation where maximum Reynolds stress is achieved. The velocity profiles measured in the unobstructed region of uniform pattern are higher compared to the velocity measured in line with the vegetation stems. Downward seepage increases the velocity, Reynolds stress, and turbulent intensities in the average range of 10–15% as compared to the no-seepage case. Higher-order turbulent statistics such as moment analysis, quadrant analysis, and turbulent kinetic budget are derived through...

Thokchom Bebina Devi

Papers

Flow characteristics in an alluvial channel covered partially with submerged vegetation

Flow characteristics in a partly vegetated channel with emergent vegetation and seepage

Experimentation on submerged flow over flexible vegetation patches with downward seepage

Turbulent flow statistics of vegetative channel with seepage

Channel Hydrodynamics of Submerged, Flexible Vegetation with Seepage