Date of Award
Doctor of Philosophy (PhD)
Dr. Abdul A. Khan, Committee Chair
Dr. Nadarajah Ravichandran
Dr. Ashok Mishra
Dr. Kalyan Piratla
Shallow flows are one of the most intrigued and complicated to understand in the field of hydraulics. In this research, three separate researches (turbulent jets in shallow flows, rain drop laboratory simulations, and characterization of shallow flows) concentrating on shallow turbulent flows were investigated. Knowledge of turbulent jets in shallow water depths is of utmost importance to adequately characterize the municipal and industrial discharges. Propagation and dilution of such discharges are influenced by the presence of channel bed and water surface at close proximity. In this study, the bed/water surface confinement effects on circular turbulent jets were numerically investigated by analyzing the key parameters such as decay of maximum velocity, velocity profiles, growth of jet, locus of maximum velocity, and turbulent properties. Results reveal that the confinement has profound impact on the entrainment and mixing characteristics of a jet. Entrainment is suppressed at the confining surface and as a consequence velocity decays at a lower rate. The locus of maximum velocity shifts toward the closest confining surface for asymmetrical cases, while it shifts toward the bed for symmetrical cases. Mixing of a jet both in the horizontal and the vertical planes are affected significantly by the confinement. Turbulence characteristics differ significantly in the vertical plane only. Findings from this study will be useful for characterizing properties of circular jets discharging in a confined ambient conditions. Motivated by various meteorological and hydrological applications, this research investigates the free fall of water drops to provide guidance in laboratory simulations of natural rainfall and to elucidate drop morphodynamics. Drop fall velocity and shape parameters such as axis ratio (ratio of the maximum vertical and horizontal chords of the drop), chord ratio (ratio of the shortest chord and longest chord of the drop), orientation angle (angle between the longest chord of the drop and the horizontal axis), and relative fluctuation of chords (difference between vertical and horizontal chord fluctuations) were investigated for three selected water drop sizes (2.6, 3.7, and 5.1 mm spherical volume equivalent diameter) using high speed imaging. Based upon experimental observations, three distinct fall zones were identified: Zone I, in which source induced oscillations and shape adjustment take place; Zone II, in which equilibrium-shaped drops accelerate to achieve terminal velocity; and Zone III, in which equilibrium-shaped drops fall at terminal velocity. Our results revealed that the fall distance values of approximately 6 m and 12 m can be used as conservative reference values for rainfall experiments with oscillation-free fall of drops (i.e. end of Zone I and onset of Zone II) and with equilibrium-shaped drops falling at terminal velocities (i.e. end of Zone II and onset of Zone III), respectively, for the entire raindrop size spectrum in natural rainfall. These required fall distance values are smaller than the distances discussed in the literature. Methodology and results presented here will facilitate optimum experimental laboratory simulations of natural rainfall. The hydraulic characteristics of the very shallow flows in a coarse grain bed, such as velocity profiles, near bed velocity profile, bed shear stress, resistance coefficients, were investigated using numerical simulations. Characteristics such as velocity profiles, near bed velocity profile, bed shear stress, resistance coefficients were analyzed. Two different approaches are used to represent grains on the bed. The first approach uses undular bed depicting grains topography with surface roughness . In the second approach, the bed is considered flat with equivalent sand roughness height representing the total roughness effects. Results reveal that the undular bed model performs better than the flat bed model in representing the velocity profile, turbulence characteristics, shear stress, and uniform flow depth. Two flow regions are identified from the undular bed simulations. Bed shear stresses are represented more accurately using the turbulent shear stress profiles. Resistance coefficients (friction factor or Manning's roughness coefficient) varies with Froude number and submergence ratio (depth divided by roughness height). In addition, undular bed contribution to the total flow resistance increases with decrease in flow depth.
Chowdhury, Md Nasimul Hoque, "Investigations of Shallow Flows and Laboratory Raindrop Morphodynamics" (2017). All Dissertations. 1887.