Statistical and physical characterization of rainfall-driven overland flow morphologies

Environmental transport processes are highly coupled with the shape of landscapes. Modern catchment analysis with high-resolution data and huge computational powers demand more detailed within-cell metrics for surface evolution modeling. This dissertation involves experiments and numerical simulations of rainfall-driven sediment transport at laboratory scales of which areas were less than the common computational cell sizes at catchment scales. The objective was a stochastic and physical study of unchanneled overland flow morphologies. In the first step, a detailed laboratory study was conducted to highlight the effects of morphological changes on hysteretic sediment transport under a time-varying rainfall. A rainfall pattern composed of seven sequential stepwise varying rainfall intensities was applied to a 5-m×2-m soil erosion flume. Clockwise hysteresis loops in the sediment concentration versus discharge curves were measured for the total eroded soil and the finer particle sizes. However, for larger particle sizes, hysteresis effects decreased and suspended concentrations tended to vary linearly with discharge. Overall, it is found that hysteresis varies amongst particle sizes and that the predictions of the HR model are consistent with hysteretic behavior of different sediment size classes. After demonstrating the role of morphological changes (generation of a shield layer on the topsoil) on erosion patterns, overland flow morphologies were statistically characterized. To do so, the catchment-scale network analyses were applied on the micro-roughnesses of unchanneled surfaces at the laboratory scale (2-m×1-m flume). The scaling relation between the drainage area and stream length (Hack¿s law), along with exceedance probabilities of drainage area, discharge, and upstream flow network length, is well known for catchment-scale channelized fluvial regions. It was found that the relationships for the overland flow network were the same as those found for large-scale catchments and for laboratory experiments with observable channels. In addition, the scaling laws were temporally invariant, even though the network dynamically changed over the course of the experiment. In the next step, we tested the applicability of a physically-based catchment scale landscape evolution model (LEM) at laboratory scale and in absence of rills. We modeled the overland flow as a network that preserves the water flux for each cell in the discretized domain. This network represented the surface flow and determined the evolution direction. The spectral analysis confirmed that the model predictions capture the main characteristics of the measured morphology. However, the model could not reproduce the experimental scaling relation as the micro-roughnesses of the surface were not produced by the model. In order to modify the LEM, it was solved as a stochastic partial differential equation. The results showed that an extra term for roughness was necessary to simulate more details of the morphology. Furthermore, a new deterministic approach (diffusion coefficient as a step function of curvature) was proposed and tested to improve the model predictions in the statistical sense.\\ Besides the scientific contributions, useful modeling, optimization and data analysis tools (C++ and Python codes) were provided for future geomorphological studies

Quality Evaluation and Study of Ecological Toxicity of Heavy Metals in Shadegan Wetland

Wetlands hold a principal position in storing food for primary producers, so they are not able to bear the pressure. The slightest disturbance, hence, may harm wetlands and cause detrimental effects. The present study aims at monitoring heavy metals and evaluation of the sediment quality index of Shadegan wetland in Iran. Thus, a sampling of surface sediments of the wetland was performed at ten stations with three replications; after the preparation of samples with aqua regia, the concentrations of heavy metals were measured by atomic absorption spectroscopy. The quantification of sediment pollution using the contamination factor, contamination degree, pollution load index, ecological risk assessment index, and ecological toxicity of heavy metals in the region were all carried out. The results of Cf and Cd showed that the degree of zinc and copper contamination is low; however, the degree of lead contamination is moderate. Moreover, the obtained PLI was less than 1 indicating a lack of sediments contamination with heavy metals. The RI was less than 150 indicating a low risk of contamination. In addition, comparing the concentrations of elements with National Oceanic and Atmospheric Administration and Sediment Quality Guidelines showed slightly toxic and non-toxic sediments, respectively. Finally, based on a mixture of effect range median, all sediment samples are placed in the first category with less than 12% toxicity probability. Copyright © 2017 Journal of Contemporary Urban Affairs

Gas mixing enhancement in minichannels using a rotationally oscillatory circular cylinder

Oscillating structures and actuators can induce flow kinematics that enhances mixing. This approach is specifically effective for mixing enhancement in meso-scale channels, where the flow kinematics can be actively controlled using micro-electro-mechanical-systems (MEMS). In this paper, numerical results for mixing of two incompressible ideal gas (Schmidt number of 1.0) streams through a 2 D mi nichannel via a rotationally oscillating circular cylinder are presented and discussed. Simulations are performed for blockage ratio of D/H=1/3 and Reynolds number of 100 and oscillation amplitudes of , and for subharmonic (F 1) regimes. Numerical results indicate that mixing performance is improved by about 70% compare to the plane channel at oscillation amplitude of and excitation frequency of 25% higher than the natural frequency of vortex shedding of a stationary cylinder. It is shown that the mixing efficiency is increased by increasing of amplitude in all the cases except at very low excitation frequencies. This study also shows that when the excitation frequency is equal to the vortex shedding frequency the maximum power is required for mixing of two gases

Gas mixing enhancement in minichannels using a rotationally oscillatory circular cylinder

Oscillating structures and actuators can induce flow kinematics that enhances mixing. This approach is specifically effective for mixing enhancement in meso-scale channels, where the flow kinematics can be actively controlled using micro-electro-mechanical-systems (MEMS). In this paper, numerical results for mixing of two incompressible ideal gas (Schmidt number of 1.0) streams through a 2 D mi nichannel via a rotationally oscillating circular cylinder are presented and discussed. Simulations are performed for blockage ratio of D/H=1/3 and Reynolds number of 100 and oscillation amplitudes of , and for subharmonic (F 1) regimes. Numerical results indicate that mixing performance is improved by about 70% compare to the plane channel at oscillation amplitude of and excitation frequency of 25% higher than the natural frequency of vortex shedding of a stationary cylinder. It is shown that the mixing efficiency is increased by increasing of amplitude in all the cases except at very low excitation frequencies. This study also shows that when the excitation frequency is equal to the vortex shedding frequency the maximum power is required for mixing of two gases

Quality Evaluation and Study of Ecological Toxicity of Heavy Metals in Shadegan Wetland

Wetlands hold a principal position in storing food for primary producers, so they are not able to bear the pressure. The slightest disturbance, hence, may harm wetlands and cause detrimental effects. The present study aims at monitoring heavy metals and evaluation of the sediment quality index of Shadegan wetland in Iran. Thus, a sampling of surface sediments of the wetland was performed at ten stations with three replications; after the preparation of samples with aqua regia, the concentrations of heavy metals were measured by atomic absorption spectroscopy. The quantification of sediment pollution using the contamination factor, contamination degree, pollution load index, ecological risk assessment index, and ecological toxicity of heavy metals in the region were all carried out. The results of Cf and Cd showed that the degree of zinc and copper contamination is low; however, the degree of lead contamination is moderate. Moreover, the obtained PLI was less than 1 indicating a lack of sediments contamination with heavy metals. The RI was less than 150 indicating a low risk of contamination. In addition, comparing the concentrations of elements with National Oceanic and Atmospheric Administration and Sediment Quality Guidelines showed slightly toxic and non-toxic sediments, respectively. Finally, based on a mixture of effect range median, all sediment samples are placed in the first category with less than 12% toxicity probability

Raindrop Impact on Saturated Soil

Soil erosion is an important environmental phenomenon that causes many side effects such as reduction of soil productivity in the fields and transportation of pollutants to non-contaminated areas. Extensive well designed and controlled flume experiments in Ecological Engineering Laboratory at EPFL revealed that in order to understand some important aspects of erosion process such as morphological changes of the soil surface and crust effects, a detailed mechanistic study of erosion in small scale is needed. In this regard, this work is a start point of small scale simulation to get more interpretation of the soil particle rearrangement as the effect of raindrops. In this paper, particle rearrangement in saturated soil as a result of raindrop impact is studied. A raindrop with specified diameter falls into the container of saturated soil and causes agitation of dense particles inside the container. The aim is to visualize this process by testing the effect of raindrop diameter, particle size and particle physical properties. The numerical solution consists of two steps. At first, the raindrop impact on water surface is simulated using Large Eddy Simulation and VOF method so that the pressure change of the under layer is measured. Then, the profile of the pressure wave is applied on the mixture of water and particles in order to simulate the fluid-particle interaction using Particle-in-Cell (MP-PIC) method. This simulation reveals that the vertical rearrangement of particles has an important effect on crust and compaction process of the soil which needs further investigation. By presentation of this work in 9th OpenFOAM workshop, we intend to introduce the ability of the latest version of OpenFOAM (v 2.3) to simulate dense particle interaction and to discuss on the possibility of development of the solver to have a more robust two-way coupling between CFD and particle based methods such as Discrete Element Method (DEM)

Heat Transfer Enhancement in a Straight Channel via a Rotationally Oscillating Adiabatic Cylinder

Heat convection from the uniformly heated walls of a straight channel in presence of a rotationally oscillating cylinder (ROC) is simulated at Re = 100. Heat transfer enhancement due to vortex shedding from the ROC is investigated. Systematic studies are performed to explore the rotation angle and frequency influences on heat transfer by varying the latter in range of the lock-in regime and the former from 0 to 2 π/3. All simulation results are based on the numerical solutions of two-dimensional, unsteady, incompressible Navier-Stokes and energy equations using an h/p type finite element algorithm. Considering time periodicity of the resulting flow and temperature fields, time averaged wall Nusselt number is reported to quantify the heat transfer enhancement for Pr = 0.1, 1.0, 5.0 and 10.0 fluids. Performance analyses of the ROC device based on its total power consumption and heat transfer enhancement are also presented

Effect of cylinder proximity to the wall on channel flow heat transfer enhancement

Heat-transfer enhancement in a uniformly heated slot mini-channel due to vortices shed from an adiabatic circular cylinder is numerically investigated. The effects of gap spacing between the cylinder and bottom wall on wall heat transfer and pressure drop are systemically studied. Numerical simulations are performed at Re=100, 0.1⩽Pr⩽10 and a blockage ratio of D/H=1/3. Results within the thermally developing flow region show heat transfer augmentation compared to the plane channel. It was found that when the obstacle is placed in the middle of the duct, maximum heat transfer enhancement from channel walls is achieved. Displacement of circular cylinder towards the bottom wall leads to the suppression of the vortex shedding, the establishment of a steady flow and a reduction of both wall heat transfer and pressure drop. Performance analysis indicates that the proposed heat transfer enhancement mechanism is beneficial for low-Prandtl-number fluids

Heat Transfer Augmentation in a Straight Channel via Two Oscillating Circular Cylinders

We consider flow-through systems in which the characteristic length is limited such that turbulent flow is not reached even at high fluid velocities, i.e., the flow remains laminar. Inthese flow regimes, inducing circulation or vortices in the flow enhances mixing and heat transfer. These can be created by placing obstacles in the flow path, for example. Heat transfer enhancement in a channel via a single stationary and oscillating cylinder was considered previously. Here, heat transfer augmentation by using two oscillating cylinders is investigated systematically and the results compared with the results for a single cylinder. Fully developed fluid flow with a parabolic velocity profile enters the channel in which two oscillating cylinders (blocking ratio of three) are placed a distance of 8D from the inlet. In the simulation, the Reynolds number was fixed at 900 (based on the channel hydraulic diameter) and the Prandtl number at 1. The optimal frequency for each condition was identified by measurement of the average Nusselt number curve over a period of oscillation. In comparison with a straight channel, using this mechanism improves heat transfer considerably, but placing a single cylinder with diameter of at the middle of channel is more efficient. Because the cylinders are offset, the results showed that the generated vortices are suppressed as a result of interaction with the walls. On the other hand, the vortices generated at the channel center are restricted to the middle of the channel and cannot move toward the walls in order to agitate the thermal boundary layer and increase heat transfer. Therefore, the generated vortices are not as effective in enhancing heat transfer as placing one cylinder with diameter of along the channel centerline, as considered previously [1, 2]

Rock Slope Stability Analysis in the Left Abutment of Bakhtiary Dam, Iran

In this research, directions of in-situ stresses in the rock slope in the left abutment of Bakhtiary dam (Center of Iran) are defined taking advantage of geological history, tectonic evolution of the area, and in-situ tests. To that end, the study draws on the kinematic analysis, limit equilibrium and numerical methods. It is of note that there is no possibility for toppling failure if kinematic analysis is used to study the stability in left abutment of Bakhtiary dam. The plane failure analysis indicated that there is a possibility of failure in the middle and upper walls based on joint set J1. Also, from geological perspective, wedge failure in the middle and upper walls is possible due to the intersection of bedding planes and Joint set J1. In the analysis of the slope stability using limit equilibrium, the least value of the safety factor obtained for plane failure belongs to joint set J1 in the upper wall, indicating that the left abutment is stable. Numerical analysis indicated that this slope needs support requirements