In the marine ecological system, the prime role of water management and durability of an ecosystem is being played by the vegetation patches. The vegetation patches in open channels can significantly affect the flow velocity, discharge capacity and hinder energy fluxes, which ultimately helps in controlling catastrophic floods. In this study, the numerical simulation for turbulent flow properties, i.e. velocity distribution, Reynolds stresses and Turbulent Intensities (TI) near the circular vegetation patches with progressively increasing density, were performed using the computational fluid dynamics (CFD) code ANSYS FLUENT. For examination of the turbulent flow features in the presence of circular patches with variable densities, Reynolds averaged Navier-Stokes equations, and Reynolds stress model (RSM) were employed. The numerical investigation was performed in the presence of in-line emergent and submerged patches having variable vegetation density in the downstream direction. Two of the cases were investigated with three circular patches having a clear gap to patch diameter ratio of La/D = 1 (where La is the clear spacing between the vegetation patches and D is the diameter of the circular patch), and the other two cases were analyzed with two patches having a clear gap ratio of La/D = 3. The case with a clear gap ratio (La/D = 3) showed 10.6% and 153% inflation in the magnitude of longitudinal velocity at the downstream of the sparse patch (aD = 0.8) and upstream of the dense patch (aD = 3.54), respectively (where aD is the flow blockage, in which “a” represents the patch frontal area and “D” represents the patch diameter). The velocity was reduced to 94% for emergent and 99% for submerged vegetation due to successive increase in vegetation density made by introducing a middle patch which reduced the clear gap ratio (La/D = 1). For La/D = 1, the longitudinal velocities at depth z = 15cm were increased by 319% than at depth z = 6cm at the downstream of the dense patch (aD = 3.54). Whereas it was observed to 365% higher in the case of La/D = 3. The magnitude of turbulent characteristics was observed 36% higher for submerged vegetation cases having a clear gap ratio of La/D = 1. The successive increase in the patch density reduced the Reynolds stresses, turbulent kinetic energy and turbulent intensities significantly within the gap region. The major reduction in the flow velocities and turbulent properties in the gaps provides a stable environment for aquatic ecosystems nourishment and fosters sediment deposition, and supports further vegetation growth.
The paper describes a mathematical and physical modelling of flow of complex slurries in pipelines, i.e. a flow of slurries composed of solids covering a very broad range of particle sizes that overlaps more than one flow patterns – non-Newtonian, pseudohomogeneous, heterogeneous and fully-stratified. A typical examples are residual products (“tailings”) from mining industry with normal average particle sizes of 20 to 100 μm or more. Experimental results of flows of complex slurries composing of non-Newtonian carrier fluid and three fractions of glass particles in 50 mm pipe are presented. Depending on the particle size, particles show different flow patterns and therefore considerable differences in pressure drops. Fine particles tend behave as a homogeneous matter, while coarser particles exhibit heterogeneous behaviour and even coarser particles form a sliding bed. A mathematical 3-component predictive model for turbulent flow of complex slurries is presented based on well-established semi-empirical formulae developed originally for flows with Newtonian carrier. The predicted values of pressure drops show very reasonable agreement with experimental results and indicate suitability of the model for engineering practice.
In this work the performance of Reynolds Averaged Navier-Stokes (RANS) simulations to predict the flow structure developed by the presence of a sidewall obstacle in a uniform open-channel shallow flow is discussed. The tested geometry was selected due to its important role in several fluvial applications, such as the control of riverbank erosion and the creation of improved ecological conditions in river restoration applications. The results are compared against experimental laboratory velocity fields obtained after Large Scale Particle Image Velocimetry (LSPIV) measurements. It is shown that the length of reattachment of the separated shear layer generated by the obstacle is well predicted by a Reynolds Stress Model, while classical two-equation models show important limitations. All the performed RANS simulations are unable to properly predict the formation of a secondary gyre region, which develops immediately downstream the obstacle.
This paper explores the impacts of reconfiguration and leaf morphology on the flow downstream of a flexible foliated plant. 3D acoustic Doppler velocimetry and particle image velocimetry were used to experimentally investigate the hydrodynamic interaction between a foliated plant and the flow, testing two plants with different leaves morphology under different bulk flow velocities. The model vegetation was representative of riparian vegetation species in terms of plants hydrodynamic behavior and leaf to stem area ratio. To explore the effects of the seasonal variability of vegetation on the flow structure, leafless conditions were tested. Reconfiguration resulted in a decrease of the frontal projected area of the plants up to the 80% relative to the undeformed value. Such changes in plant frontal area markedly affected the spatial distributions of mean velocity and turbulence intensities, altering the local exchanges of momentum. At increasing reconfiguration, the different plant morphology influenced the mean and turbulent wake width. The leafless stem exhibited a rigid behavior, with the flow in the wake being comparable to that downstream of a rigid cylinder. The study revealed that the flexibility-induced reconfiguration of plants can markedly affect the local distribution of flow properties in the wake, potentially affecting transport processes at the scale of the plant and its subparts.
Turbulence of flow over mobile bedforms in natural open channels is not yet clearly understood. An attempt is made in this paper to determine the effect of naturally formed mobile bedforms on velocities, turbulent intensities and turbulent stresses. Instantaneous velocities are measured using a two-dimensional particle image velocimetry (PIV) to evaluate the turbulence structure of free surface flow over a fixed (immobile) bed, a weakly mobile bed and a temporally varying mobile bed with different stages of bedform development. This paper documents the vertical distribution of velocity, turbulence intensities, Reynolds shear stress and higher-order moments including skewness and turbulent diffusion factors. Analysis of the velocity distributions shows a substantial decrease of velocity near the bed with increasing bedform mobility due to increased friction. A modified logarithmic law with a reduced von Kármán constant and increased velocity shift is proposed for the case of the mobile bedforms. A significant increase in the Reynolds shear stress is observed in the mobile bedforms experiments accompanied by changes over the entire flow depth compared to an immobile bed. The skewness factor distribution was found to be different in the case of the flow over the mobile bedforms. All higher-order turbulence descriptors are found to be significantly affected by the formation of temporally varying and non-equilibrium mobile bedforms. Quadrant analysis indicates that sweep and outward events are found to be dominant in strongly mobile bedforms and govern the bedform mobility.