Sharif Digital Repository

There is an importance issue for preservation and survival of ecological systems by vegetation. A study focusing on the hydrodynamics of these complex ecosystems could help to explain the filtering ability of vegetation in wetlands and estuaries. Wetlands assist in flood control, shoreline protection, and groundwater recharge.In addition, they play a purification role, removing sediments and contaminants from polluted waters, nutrient uptake, and oxygen production. Laboratory experiments have been designed to study the influence of marsh vegetation on lateral dispersion. The flow through vegetation is modeled by a circulating flow going through an array of plastic dowels in an experimental... 

The three dimensional resonant interaction between a surface wave and harmonic interfacial waves in a horizontally infinite two-layer fluid was investigated theoretically and experimentally. Initially, both layers assumed to be inviscid, but after obtaining the final equations of the interfacial wave's amplitude, with perturbation, the viscous damping effect, was incorporated in to the interaction equations. It is shown that, surface wave can trigger four opposite- traveling interfacial waves whose amplitudes grow with time. But because of viscosity damping effect, these amplitudes will become constant and won't change with time. The frequencies of the interfacial waves are approximately... 

The theory of resonant wave interaction has found considerable applications in meteorology, oceanography and limnology. It is known that the mechanism serves as an effective means of energy transfer in the atmosphere, oceans and lakes. In this project, the three-dimensional resonant interaction of a surface wave with two pairs of interfacial waves in a stratified fluid is studied theoretically. Witham’s variational method is formulated in order to apply to weak resonant interaction among waves whose amplitudes and phase angles vary slowly with position and time. The method is applied to a surface-interfacial wave interaction in a horizontally infinite two layer fluid. Combination of two... 

In this thesis, we present the results of a laboratory and numerical model study of the exchange flow generated by the temperature gradient between an open water heated by sunlight and a canopy area made of floating vegetation. Particle Image Velocimetry is employed to obtain the structures of the warm flow intruding the canopy at the surface and the bottom cold return flow for different roots length and vegetation density. Following an initial, transient stage of flow build-up governed by a balance between inertia and buoyancy forces, the flow reaches a steady state in the canopy area with a force balance between buoyancy and vegetation drag dominating the flow within roots. At the steady... 

Stratified flows are common in natural systems such as the oceans and the atmosphere. The hydraulics of two-layer flow over an obstacle, which helps with understanding and control of these flows, has been studied by many researchers. However, a type of two-layer flow, called the Approach-controlled flow, has not yet been fully understood, and there are ambiguities about it. Having considerable nonhydrostatic pressure in the so-called supercritical leap region, this flow type cannot be explained even qualitatively without accounting for the nonhydrostatic pressure. Also, the composite Froude number which is defined based on the hydrostatic pressure assumption for two-layer flows, is not... 

This experimental study investigates the interaction between surface waves and flexible stems in aquatic environments, focusing on the case where the wave frequency is close to the natural frequency of the stems. Aquatic plants have significant structural and ecological effects on aquatic environments, inducing various changes in the flow passing through them. In this research, a polyurethane stem model is used, which is submerged and randomly placed in water. The oscillation amplitude and pollutant dispersion are investigated. The experiments were conducted at four frequencies of 0.7, 0.83, 1, and 1.125 Hz, two wave heights of 2 and 3 cm, and two densities of 0.8 and 1.2%. Image processing... 

Using Variational Method, two important hydrodynamic phenomena, “internal gravity waves” and “gravity currents”, are modelled theoretically in this research. Variational method as a robust technique for modelling complicated problems related to fluid mechanics gives us the ability of finding closed-form formula for predicting the behavior of the flow in an ideal fluid system. In the first part of this research, focusing on the nonlinear resonant interaction between surface and internal waves, we model the evolution of interfacial waves on the interface of a two-layer fluid system. This phenomenon play an important role in the mixture of the fluid and cascade of energy in the environment.... 

Exchange flows are common physical phenomena in calm river banks and lakes. The sunlight penetrating into water column gradually attenuates due to light absobtion by water column. The light converts to heat energy in water molecules, suspended particles and disssolved matters, and results in warming the water body. Aquatic vegetation enhances the spatial variability in shallower regions of an aquatic system. Heterogeny in light penetration caused by emergent vegetation shading results in a spatial heterogeneity in water temperature. The density of water is dependent on its temperature, but the relation is not linear. As water is heated, it expands to become less dense. Horizontal density... 

Aquatic canopy zones play an important role in maintenance of ecosystems. These areas are ubiquitous near rivers, lakes, lagoons, estuaries and shore lines. Governing dynamical systems through these areas have direct influence on, how nutrients and other vital materials for living things disperse. Also, they can control the amount of dissolved oxygen within the body of water in these regions. Nowadays, investigating the effects of vegetation grabbed the attention of many scientists and is a very important burgeoning field. In this study, the effects of surface cooling which resulting from mild wind blow have been investigated. In fact, two type of different currents form when wind blows... 

The results of a theoretical and experimental study of a resonant interaction between a surface wave and two internal waves are presented. It is shown that the motion of a surface wave in a horizontally infinite two-layer fluid can lead to generation of two oblique internal waves. The internal waves have short wave length compared to the surface wave and have nearly opposite propagation directions. The frequencies of the internal waves are approximately half of the frequency of the surface wave.
In present thesis, Higher-order effects in sub-harmonic resonant interaction of a surface wave with a pair of interfacial waves in a two-layer fluid are studied theoretically and experimentally.... 

Nowadays, vegetations have an important role in protecting ecological systems. One of the impacts of vegetation in water system is pollution diffusion. The physical characteristics of plants, affect how vegetation influence the diffusion phenomenon. Plants’ flexibility may change the drag force and pollution diffusion. Also density variations of vegetation in depths have important affects on flow characteristics.
In this study we investigate experimentally the affects of flexible vegetations and their density changes in depths on vertical mass transport. Thus the current flume has modeled for crossing flow through the plants. To model the steep plants, inclined rigid cylinders have been... 

Flexible vegetation has a significant impact on the small-scale hydrodynamics of aquatic environments, which governs the dispersion of dissolved and particulate material. Flexible canopies bend and oscillate in both the in-line and cross-flow directions due to periodic forcing associated with vortex shedding. The resultant plant motion affects the vegetation wake structure and, hence, the rate of lateral dispersion in these environments. Despite a recently-developed understanding of dispersion in rigid canopies, a framework that can predict the effect of plant-induced vibrations on mixing in flexible canopy environments is still lacking. Here, we investigate the role of the flexibility of... 

The purpose of this research is to investigate the movement and diffusion of pollution transversely around a set of stems of flexible plants with different materials and to observe the effect of resonance on lateral dispersion. Two stem samples of polyurethane and nylon were used under different Reynolds numbers. Vertically uniform dye is injected into the flow, and the evolution of the dye plume along the flow is measured using image processing. By measuring the displacements of selected stems we study the effects of resonance on stem vibration and lateral dispersion. The experiments were also conducted for no-stem dispersion and the results were compared with those with plant samples. The... 

Aquatic plants play an important role in water environment. Increasing drag force, distorting the velocity and turbulence structures and other related parameters, are the results of the presences of plants. These changes affect the dispersion and mass transport in the aquatic areas. This thesis presents laboratory visualizations and measurements of wave motion, and passive solute surface dispersion (longitudinal and lateral) in arrays of cylinders, a model for emergent, rigid aquatic plants. In the range of (where is Keulegan–Carpenter number, is the maximum oscillatory velocity perpendicular to the cylinder axis, is the period of oscillation, and is the diameter of the cylinder), and (where... 

Aqueous environment with vegetation in coastal areas plays important role in ecologic systems. The plants in aqueous environment caused to vertical and lateral changes of flow velocity profile. When sunlight penetrates the water, it will turns into heat energy. The plants caused to blocking the light path, to make a difference in temperature and thus the density differences on the horizon. This density gradient will lead to the gravity flow. Due to the observations, in coastal wetlands, beside the plants the emerged from the water, there are some completely submerged plants that their root in the soil and shallow layer of fluid covered on them. This study to approaching the conditions, is... 

In nature, tidal currents are visible in gorges and in saltwater and freshwater collisions. In the study of behavior of different types of flow, it is usually assumed that the pressure is hydrostatic and the equations are solved based on this assumption. In the Approach Controlled regime observed during the double layer in 1993, the modeling performed using the equations derived from this assumption did not fit the results of the experiments. This demonstrates the inability of the hydrostatic pressure assumption to solve this particular regime. Hence, the equations were written on the basis of non-hydrostatic pressure and then the observed results were in accordance with the results of the... 

In this research, the dynamic of flow, dispersion of contaminant and characteristic of wake in the presence of two flexible stems of polyurethane is investigated in a hydraulic flume. The stream velocity of 2.28, 4.47 and 7.58 cm/s are considered, which are equivalent to Reynolds number in stem diameter scale 173, 340 and 576. The spacing ratio of two stems are one, two and three in both transverse and longitudinal direction. Velocity fields are measured using Particle Image Velocimetry (PIV) technique. The size of wake area behind stems is determined using the flow field measurement. The results show that the spacing ratio l/d affects the wake area size and interactions of wakes. The... 

In this study, by performing a series of experiments, the effect of the presence of cylinders on the formation of vortices and the interaction of cylinders on each other and, finally, transverse propagation in a steady flow are investigated. The cylinder cylinders that are in the direction of the motion of the water, have a significant effect on the hydrodynamic aspects and propagation in these streams. In the pattern of flow around cylindrical cylinders, with increasing Reynolds number, the vortex behind the stem begins to oscillate and when they reach, vortices flow into the stream. The flow velocity field, the formation and vortex fluctuation, affects the process of particle propagation... 

Numerical and laboratory models were used to investigate gravity exchange flow caused by spatial heterogeneity in water temperature, through emergent vegetation with varying diameter along their height (denoted by the parameter ) causing varying stem frontal area along vertical axis. Numerical model was used to anticipate flow characteristics and their alterations relative to non-dimensional vegetation diameter ratio. A set of Teflon dowels with various heights was used to simulate considered vegetative cross-section changes along vertical axis. Interface profile, toe velocity and flow rate were studied. Interface height and flow rate were inquired near the gate where expected to be... 

The motion of a surface wave in a two-layer fluid can lead to generation of sub-harmonic interfacial waves through a nonlinear resonant interaction mechanism. The interfacial waves grow exponentially in time until they reach a maximum amplitude and then oscillate down to a final amplitude, referred to as ultimate amplitude, in long term. This may lead to considerable mixing of the layers in miscible fluid and increase the thickness of the diffuse layer. Experiments show that the diffuse layer causes frequency shift and decreases the ultimate amplitude. In this study, long time behavior of interfacial waves in a two layer immiscible fluid was investigated. A two-layer immiscible system...