{"id":{"repo_id":"uic","oai_identifier":"oai:figshare.com:article/31451602"},"canonical_url":"https://search.dev.ndltd.org/etd/uic/oai:figshare.com:article/31451602","repository":{"repo_id":"uic","name":"University of Illinois - Chicago","base_url":"https://api.figshare.com/v2/oai"},"display":{"title":"Multiphase Flow Dynamics in Confined Domains—A DNS Study of Bubbly, Thermal and Riblet-Induced Turbulence","abstract":"This thesis presents a comprehensive investigation of multiphase and wall-bounded turbulent flows using high-fidelity direct numerical simulations (DNS), with an emphasis on the fundamental mechanisms governing heat transfer, turbulence modulation, and instability-driven transition. By systematically studying three canonical configurations—liquid–liquid emulsions in Rayleigh–Bénard convection, buoyancy-driven bubbly flows in vertical channels, and channel flows over riblet-structured walls—this work advances the physical understanding of multi-physics interactions across a broad spectrum of thermofluid systems relevant to energy, chemical, and aerospace applications. In the first part, DNS of multiphase Rayleigh–Bénard convection is performed using the VOF-MTHINC method to explore the coupled effects of dispersed-phase volume fraction, viscosity ratio, and thermal diffusivity ratio on turbulent convection. The study reveals that immiscible liquid–liquid emulsions profoundly alter heat transport and energy transfer across scales. At fixed Rayleigh and Prandtl numbers, increasing the dispersed-phase volume fraction enhances heat transfer by up to 10\\% due to droplet-induced energy transfer to smaller turbulent scales, even though global turbulence intensity is reduced. Varying viscosity ratios further amplifies mixing: at a volume fraction of 20\\% and a viscosity ratio of 10, heat transport rises by ~25\\% owing to intensified turbulence in the less viscous carrier phase. Conversely, introducing a dispersed phase with higher thermal diffusivity suppresses convective transport, reducing the Nusselt number by as much as 50\\% through accelerated conduction and the depletion of near-wall droplets. Analysis of droplet size distributions identifies distinct scaling regimes dominated by coalescence and breakup, confirming the multiscale nature of droplet dynamics in thermal convection. These results establish how droplet rheology and transport properties dictate the interplay between turbulence and thermal efficiency in emulsions. The second part investigates buoyancy-driven bubbly flows in a vertical channel, where interface-resolved DNS with a conservative diffuse-interface (CDI) method provides full spatiotemporal resolution of bubble–turbulence interactions. By systematically varying the Galilei (390–1100) and Eötvös (0.85–8.5) numbers at a constant void fraction of 2.7\\%, the study isolates the effects of inertia and deformability on pseudo-turbulence. The results demonstrate that bubble deformability critically dictates spatial distribution and turbulent regimes. At low Eötvös numbers, bubbles remain near the walls, producing stratified layers with suppressed core mixing. At higher Eötvös numbers, bubbles deform, break up, and redistribute toward the channel center, where they strongly enhance turbulence and velocity fluctuations. Increasing Galilei number further intensifies rise velocities, vorticity generation, and wall-bounded turbulence, leading to stronger shear-layer instabilities and near-wall fluctuations. Energy budget analyses confirm that pseudo-turbulence emerges from the coupling of interfacial deformation, inertial forces, and vorticity generation. These findings identify distinct dynamical regimes of bubble-induced turbulence, bridging experimental observations and providing predictive insights for optimizing bubbly flows in chemical reactors, nuclear thermal-hydraulics, and environmental systems. The third part examines single-phase channel flows modified by large riblet structures, focusing on laminar-to-turbulent transition. Using an immersed boundary method on Cartesian grids, DNS resolves riblet-induced near-wall instabilities in both two- and three-dimensional domains. The parametric study considers height blockage ratio (HBR) and length blockage ratio (LBR) as key geometric controls, systematically mapping transition thresholds. The results reveal that increasing riblet height or decreasing riblet spacing lowers the critical Reynolds number, with nonlinear destabilization observed for $0.2 \\leq HBR \\leq 0.35$. Dimensionality strongly influences stability: three-dimensional domains consistently transition earlier than two-dimensional counterparts, underscoring the role of spanwise instabilities, secondary flows, and vortex lodging. Flow diagnostics highlight Kelvin–Helmholtz instabilities over riblet crests as the dominant transition mechanism, with riblet-induced adverse pressure gradients driving shear-layer roll-up and vortex shedding. Pressure distributions exhibit strong stagnation–suction asymmetry scaling with riblet geometry. These results provide DNS-based guidelines for tailoring riblet dimensions to either suppress transition (for drag reduction) or promote early turbulence (for mixing enhancement), with direct implications for aerospace, thermal management, and microfluidics. Taken together, this thesis establishes a unified framework for understanding turbulence modulation across three distinct yet interconnected domains: emulsions modulating thermal convection, bubbles driving pseudo-turbulence in buoyancy-driven flows, and riblets dictating transition dynamics in wall-bounded shear flows. By integrating high-resolution DNS, advanced interface-capturing techniques, and systematic parameter studies, the work identifies multiscale mechanisms—ranging from droplet coalescence and bubble deformability to riblet-induced shear instabilities—that control momentum and heat transfer in complex flows. Beyond advancing fundamental fluid dynamics, the findings provide actionable physical insights for the design of multiphase reactors, thermal systems, and surface-engineered flow-control technologies.","abstract_html":"This thesis presents a comprehensive investigation of multiphase and wall-bounded turbulent flows using high-fidelity direct numerical simulations (DNS), with an emphasis on the fundamental mechanisms governing heat transfer, turbulence modulation, and instability-driven transition. By systematically studying three canonical configurations—liquid–liquid emulsions in Rayleigh–Bénard convection, buoyancy-driven bubbly flows in vertical channels, and channel flows over riblet-structured walls—this work advances the physical understanding of multi-physics interactions across a broad spectrum of thermofluid systems relevant to energy, chemical, and aerospace applications. In the first part, DNS of multiphase Rayleigh–Bénard convection is performed using the VOF-MTHINC method to explore the coupled effects of dispersed-phase volume fraction, viscosity ratio, and thermal diffusivity ratio on turbulent convection. The study reveals that immiscible liquid–liquid emulsions profoundly alter heat transport and energy transfer across scales. At fixed Rayleigh and Prandtl numbers, increasing the dispersed-phase volume fraction enhances heat transfer by up to 10\\% due to droplet-induced energy transfer to smaller turbulent scales, even though global turbulence intensity is reduced. Varying viscosity ratios further amplifies mixing: at a volume fraction of 20\\% and a viscosity ratio of 10, heat transport rises by ~25\\% owing to intensified turbulence in the less viscous carrier phase. Conversely, introducing a dispersed phase with higher thermal diffusivity suppresses convective transport, reducing the Nusselt number by as much as 50\\% through accelerated conduction and the depletion of near-wall droplets. Analysis of droplet size distributions identifies distinct scaling regimes dominated by coalescence and breakup, confirming the multiscale nature of droplet dynamics in thermal convection. These results establish how droplet rheology and transport properties dictate the interplay between turbulence and thermal efficiency in emulsions. The second part investigates buoyancy-driven bubbly flows in a vertical channel, where interface-resolved DNS with a conservative diffuse-interface (CDI) method provides full spatiotemporal resolution of bubble–turbulence interactions. By systematically varying the Galilei (390–1100) and Eötvös (0.85–8.5) numbers at a constant void fraction of 2.7\\%, the study isolates the effects of inertia and deformability on pseudo-turbulence. The results demonstrate that bubble deformability critically dictates spatial distribution and turbulent regimes. At low Eötvös numbers, bubbles remain near the walls, producing stratified layers with suppressed core mixing. At higher Eötvös numbers, bubbles deform, break up, and redistribute toward the channel center, where they strongly enhance turbulence and velocity fluctuations. Increasing Galilei number further intensifies rise velocities, vorticity generation, and wall-bounded turbulence, leading to stronger shear-layer instabilities and near-wall fluctuations. Energy budget analyses confirm that pseudo-turbulence emerges from the coupling of interfacial deformation, inertial forces, and vorticity generation. These findings identify distinct dynamical regimes of bubble-induced turbulence, bridging experimental observations and providing predictive insights for optimizing bubbly flows in chemical reactors, nuclear thermal-hydraulics, and environmental systems. The third part examines single-phase channel flows modified by large riblet structures, focusing on laminar-to-turbulent transition. Using an immersed boundary method on Cartesian grids, DNS resolves riblet-induced near-wall instabilities in both two- and three-dimensional domains. The parametric study considers height blockage ratio (HBR) and length blockage ratio (LBR) as key geometric controls, systematically mapping transition thresholds. The results reveal that increasing riblet height or decreasing riblet spacing lowers the critical Reynolds number, with nonlinear destabilization observed for $0.2 \\leq HBR \\leq 0.35$. Dimensionality strongly influences stability: three-dimensional domains consistently transition earlier than two-dimensional counterparts, underscoring the role of spanwise instabilities, secondary flows, and vortex lodging. Flow diagnostics highlight Kelvin–Helmholtz instabilities over riblet crests as the dominant transition mechanism, with riblet-induced adverse pressure gradients driving shear-layer roll-up and vortex shedding. Pressure distributions exhibit strong stagnation–suction asymmetry scaling with riblet geometry. These results provide DNS-based guidelines for tailoring riblet dimensions to either suppress transition (for drag reduction) or promote early turbulence (for mixing enhancement), with direct implications for aerospace, thermal management, and microfluidics. Taken together, this thesis establishes a unified framework for understanding turbulence modulation across three distinct yet interconnected domains: emulsions modulating thermal convection, bubbles driving pseudo-turbulence in buoyancy-driven flows, and riblets dictating transition dynamics in wall-bounded shear flows. By integrating high-resolution DNS, advanced interface-capturing techniques, and systematic parameter studies, the work identifies multiscale mechanisms—ranging from droplet coalescence and bubble deformability to riblet-induced shear instabilities—that control momentum and heat transfer in complex flows. Beyond advancing fundamental fluid dynamics, the findings provide actionable physical insights for the design of multiphase reactors, thermal systems, and surface-engineered flow-control technologies.","abstract_has_math":true,"creators":["Abbas Moradi Bilondi (23291989)"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2025,"date_issued":"2025-12-01T00:00:00Z","date_published":"2025-12-01T00:00:00Z","updated_at":"2026-07-27T21:34:28Z","subjects":["Engineering","Mechanical"],"languages":[],"rights":["In Copyright","Open Access after 2028-01-01"],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"https://doi.org/10.25417/uic.31451602.v1","outbound_label":"DOI","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:creator","label":"Author","values":["Abbas Moradi Bilondi (23291989)"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2025-12-01T00:00:00Z"]},{"key":"dc:relation","label":"Dc Relation","values":["https://figshare.com/articles/thesis/Multiphase_Flow_Dynamics_in_Confined_Domains_A_DNS_Study_of_Bubbly_Thermal_and_Riblet-Induced_Turbulence/31451602"]},{"key":"dc:type","label":"Dc Type","values":["Text","Thesis"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Engineering","Mechanical"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:rights","label":"Dc Rights","values":["In Copyright","Open Access after 2028-01-01"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["10.25417/uic.31451602.v1"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["This thesis presents a comprehensive investigation of multiphase and wall-bounded turbulent flows using high-fidelity direct numerical simulations (DNS), with an emphasis on the fundamental mechanisms governing heat transfer, turbulence modulation, and instability-driven transition. By systematically studying three canonical configurations—liquid–liquid emulsions in Rayleigh–Bénard convection, buoyancy-driven bubbly flows in vertical channels, and channel flows over riblet-structured walls—this work advances the physical understanding of multi-physics interactions across a broad spectrum of thermofluid systems relevant to energy, chemical, and aerospace applications. In the first part, DNS of multiphase Rayleigh–Bénard convection is performed using the VOF-MTHINC method to explore the coupled effects of dispersed-phase volume fraction, viscosity ratio, and thermal diffusivity ratio on turbulent convection. The study reveals that immiscible liquid–liquid emulsions profoundly alter heat transport and energy transfer across scales. At fixed Rayleigh and Prandtl numbers, increasing the dispersed-phase volume fraction enhances heat transfer by up to 10\\% due to droplet-induced energy transfer to smaller turbulent scales, even though global turbulence intensity is reduced. Varying viscosity ratios further amplifies mixing: at a volume fraction of 20\\% and a viscosity ratio of 10, heat transport rises by ~25\\% owing to intensified turbulence in the less viscous carrier phase. Conversely, introducing a dispersed phase with higher thermal diffusivity suppresses convective transport, reducing the Nusselt number by as much as 50\\% through accelerated conduction and the depletion of near-wall droplets. Analysis of droplet size distributions identifies distinct scaling regimes dominated by coalescence and breakup, confirming the multiscale nature of droplet dynamics in thermal convection. These results establish how droplet rheology and transport properties dictate the interplay between turbulence and thermal efficiency in emulsions. The second part investigates buoyancy-driven bubbly flows in a vertical channel, where interface-resolved DNS with a conservative diffuse-interface (CDI) method provides full spatiotemporal resolution of bubble–turbulence interactions. By systematically varying the Galilei (390–1100) and Eötvös (0.85–8.5) numbers at a constant void fraction of 2.7\\%, the study isolates the effects of inertia and deformability on pseudo-turbulence. The results demonstrate that bubble deformability critically dictates spatial distribution and turbulent regimes. At low Eötvös numbers, bubbles remain near the walls, producing stratified layers with suppressed core mixing. At higher Eötvös numbers, bubbles deform, break up, and redistribute toward the channel center, where they strongly enhance turbulence and velocity fluctuations. Increasing Galilei number further intensifies rise velocities, vorticity generation, and wall-bounded turbulence, leading to stronger shear-layer instabilities and near-wall fluctuations. Energy budget analyses confirm that pseudo-turbulence emerges from the coupling of interfacial deformation, inertial forces, and vorticity generation. These findings identify distinct dynamical regimes of bubble-induced turbulence, bridging experimental observations and providing predictive insights for optimizing bubbly flows in chemical reactors, nuclear thermal-hydraulics, and environmental systems. The third part examines single-phase channel flows modified by large riblet structures, focusing on laminar-to-turbulent transition. Using an immersed boundary method on Cartesian grids, DNS resolves riblet-induced near-wall instabilities in both two- and three-dimensional domains. The parametric study considers height blockage ratio (HBR) and length blockage ratio (LBR) as key geometric controls, systematically mapping transition thresholds. The results reveal that increasing riblet height or decreasing riblet spacing lowers the critical Reynolds number, with nonlinear destabilization observed for $0.2 \\leq HBR \\leq 0.35$. Dimensionality strongly influences stability: three-dimensional domains consistently transition earlier than two-dimensional counterparts, underscoring the role of spanwise instabilities, secondary flows, and vortex lodging. Flow diagnostics highlight Kelvin–Helmholtz instabilities over riblet crests as the dominant transition mechanism, with riblet-induced adverse pressure gradients driving shear-layer roll-up and vortex shedding. Pressure distributions exhibit strong stagnation–suction asymmetry scaling with riblet geometry. These results provide DNS-based guidelines for tailoring riblet dimensions to either suppress transition (for drag reduction) or promote early turbulence (for mixing enhancement), with direct implications for aerospace, thermal management, and microfluidics. Taken together, this thesis establishes a unified framework for understanding turbulence modulation across three distinct yet interconnected domains: emulsions modulating thermal convection, bubbles driving pseudo-turbulence in buoyancy-driven flows, and riblets dictating transition dynamics in wall-bounded shear flows. By integrating high-resolution DNS, advanced interface-capturing techniques, and systematic parameter studies, the work identifies multiscale mechanisms—ranging from droplet coalescence and bubble deformability to riblet-induced shear instabilities—that control momentum and heat transfer in complex flows. Beyond advancing fundamental fluid dynamics, the findings provide actionable physical insights for the design of multiphase reactors, thermal systems, and surface-engineered flow-control technologies."]},{"key":"dc:title","label":"Title","values":["Multiphase Flow Dynamics in Confined Domains—A DNS Study of Bubbly, Thermal and Riblet-Induced Turbulence"]}]}],"canonical_facts":{"dc:creator":["Abbas Moradi Bilondi (23291989)"],"dc:date":["2025-12-01T00:00:00Z"],"dc:description":["This thesis presents a comprehensive investigation of multiphase and wall-bounded turbulent flows using high-fidelity direct numerical simulations (DNS), with an emphasis on the fundamental mechanisms governing heat transfer, turbulence modulation, and instability-driven transition. By systematically studying three canonical configurations—liquid–liquid emulsions in Rayleigh–Bénard convection, buoyancy-driven bubbly flows in vertical channels, and channel flows over riblet-structured walls—this work advances the physical understanding of multi-physics interactions across a broad spectrum of thermofluid systems relevant to energy, chemical, and aerospace applications. In the first part, DNS of multiphase Rayleigh–Bénard convection is performed using the VOF-MTHINC method to explore the coupled effects of dispersed-phase volume fraction, viscosity ratio, and thermal diffusivity ratio on turbulent convection. The study reveals that immiscible liquid–liquid emulsions profoundly alter heat transport and energy transfer across scales. At fixed Rayleigh and Prandtl numbers, increasing the dispersed-phase volume fraction enhances heat transfer by up to 10\\% due to droplet-induced energy transfer to smaller turbulent scales, even though global turbulence intensity is reduced. Varying viscosity ratios further amplifies mixing: at a volume fraction of 20\\% and a viscosity ratio of 10, heat transport rises by ~25\\% owing to intensified turbulence in the less viscous carrier phase. Conversely, introducing a dispersed phase with higher thermal diffusivity suppresses convective transport, reducing the Nusselt number by as much as 50\\% through accelerated conduction and the depletion of near-wall droplets. Analysis of droplet size distributions identifies distinct scaling regimes dominated by coalescence and breakup, confirming the multiscale nature of droplet dynamics in thermal convection. These results establish how droplet rheology and transport properties dictate the interplay between turbulence and thermal efficiency in emulsions. The second part investigates buoyancy-driven bubbly flows in a vertical channel, where interface-resolved DNS with a conservative diffuse-interface (CDI) method provides full spatiotemporal resolution of bubble–turbulence interactions. By systematically varying the Galilei (390–1100) and Eötvös (0.85–8.5) numbers at a constant void fraction of 2.7\\%, the study isolates the effects of inertia and deformability on pseudo-turbulence. The results demonstrate that bubble deformability critically dictates spatial distribution and turbulent regimes. At low Eötvös numbers, bubbles remain near the walls, producing stratified layers with suppressed core mixing. At higher Eötvös numbers, bubbles deform, break up, and redistribute toward the channel center, where they strongly enhance turbulence and velocity fluctuations. Increasing Galilei number further intensifies rise velocities, vorticity generation, and wall-bounded turbulence, leading to stronger shear-layer instabilities and near-wall fluctuations. Energy budget analyses confirm that pseudo-turbulence emerges from the coupling of interfacial deformation, inertial forces, and vorticity generation. These findings identify distinct dynamical regimes of bubble-induced turbulence, bridging experimental observations and providing predictive insights for optimizing bubbly flows in chemical reactors, nuclear thermal-hydraulics, and environmental systems. The third part examines single-phase channel flows modified by large riblet structures, focusing on laminar-to-turbulent transition. Using an immersed boundary method on Cartesian grids, DNS resolves riblet-induced near-wall instabilities in both two- and three-dimensional domains. The parametric study considers height blockage ratio (HBR) and length blockage ratio (LBR) as key geometric controls, systematically mapping transition thresholds. The results reveal that increasing riblet height or decreasing riblet spacing lowers the critical Reynolds number, with nonlinear destabilization observed for $0.2 \\leq HBR \\leq 0.35$. Dimensionality strongly influences stability: three-dimensional domains consistently transition earlier than two-dimensional counterparts, underscoring the role of spanwise instabilities, secondary flows, and vortex lodging. Flow diagnostics highlight Kelvin–Helmholtz instabilities over riblet crests as the dominant transition mechanism, with riblet-induced adverse pressure gradients driving shear-layer roll-up and vortex shedding. Pressure distributions exhibit strong stagnation–suction asymmetry scaling with riblet geometry. These results provide DNS-based guidelines for tailoring riblet dimensions to either suppress transition (for drag reduction) or promote early turbulence (for mixing enhancement), with direct implications for aerospace, thermal management, and microfluidics. Taken together, this thesis establishes a unified framework for understanding turbulence modulation across three distinct yet interconnected domains: emulsions modulating thermal convection, bubbles driving pseudo-turbulence in buoyancy-driven flows, and riblets dictating transition dynamics in wall-bounded shear flows. By integrating high-resolution DNS, advanced interface-capturing techniques, and systematic parameter studies, the work identifies multiscale mechanisms—ranging from droplet coalescence and bubble deformability to riblet-induced shear instabilities—that control momentum and heat transfer in complex flows. Beyond advancing fundamental fluid dynamics, the findings provide actionable physical insights for the design of multiphase reactors, thermal systems, and surface-engineered flow-control technologies."],"dc:identifier":["10.25417/uic.31451602.v1"],"dc:relation":["https://figshare.com/articles/thesis/Multiphase_Flow_Dynamics_in_Confined_Domains_A_DNS_Study_of_Bubbly_Thermal_and_Riblet-Induced_Turbulence/31451602"],"dc:rights":["In Copyright","Open Access after 2028-01-01"],"dc:subject":["Engineering","Mechanical"],"dc:title":["Multiphase Flow Dynamics in Confined Domains—A DNS Study of Bubbly, Thermal and Riblet-Induced Turbulence"],"dc:type":["Text","Thesis"]},"updated_at":"2026-07-27T21:34:28Z"}