Effect of surfactant-laden droplets on turbulent flow topology

In this work, we investigate flow topology modifications produced by a swarm of large surfactant-laden droplets released in a turbulent channel flow. Droplets have same density and viscosity of the carrier fluid, so that only surface tension effects are considered. We run one single-phase flow simulation at $Re_\tau=\rho u_\tau h / \mu = 300$, and ten droplet-laden simulations at the same $Re_\tau$ with a constant volume fraction equal to $\Phi \simeq5.4\%$. For each simulation, we vary the Weber number ($We$, ratio between inertial and surface tension forces) and the elasticity number ($\beta_s$, parameter that quantifies the surface tension reduction). We use direct numerical simulations of turbulence coupled with a phase field method to investigate the role of capillary forces (normal to the interface) and Marangoni forces (tangential to the interface) on turbulence (inside and outside the droplets). As expected, due to the low volume fraction of droplets, we observe minor modifications in the macroscopic flow statistics. However, we observe major modifications of the vorticity at the interface and important changes in the local flow topology. We highlight the role of Marangoni forces in promoting an elongational type of flow in the dispersed phase and at the interface. We provide detailed statistical quantification of these local changes as a function of the Weber number and elasticity number, which may be useful for simplified models

Mass-conservation-improved phase field methods for turbulent multiphase flow simulation

The phase field method has emerged as a powerful tool for the simulation of multiphase flow. The method has great potential for further developments and applications: it has a sound physical basis, and when associated with a highly refined grid, physics is accurately rendered. However, in many cases, especially when dealing with turbulent flows, the available computational resources do not allow for a complete resolution of the interfacial phenomena and some undesired effects such as shrinkage, coarsening and misrepresentation of surface tension forces and thermo-physical properties can affect the accuracy of the simulations. In this paper, we present two improved phase field method formulations (profile-corrected and flux-corrected), specifically developed to overcome the previously mentioned drawbacks, and we benchmark their performance versus the classic one. The formulations are first tested considering the rise of a bubble in a quiescent fluid and the interaction of two droplets in laminar shear flow; then, their performances are compared in the simulation of a droplet-laden turbulent flow. The aim of this work is to review and benchmark the different phase field method formulations, with the final goal of laying down useful guidelines for the accurate simulation of turbulent multiphase flow with the phase field method

Gautrans sub-urban train station

No abstract available.Dissertation (MArch (Prof))--University of Pretoria, 2005.Architectureunrestricte

Heat transfer in drop-laden turbulence

Heat transfer by large deformable drops in a turbulent flow is a complex and rich-in-physics system, in which drop deformation, breakage and coalescence influence the transport of heat. We study this problem by coupling direct numerical simulation (DNS) of turbulence with a phase-field method for the interface description. Simulations are run at fixed-shear Reynolds and Weber numbers. To evaluate the influence of microscopic flow properties, like momentum/thermal diffusivity, on macroscopic flow properties, like mean temperature or heat transfer rates, we consider four different values of the Prandtl number, which is the momentum to thermal diffusivity ratio:, and. The drop volume fraction is for all cases. Drops are initially warmer than the turbulent carrier fluid and release heat at different rates depending on the value of, but also on their size and on their own dynamics (topology, breakage, drop-drop interaction). Computing the time behaviour of the drops and carrier fluid average temperatures, we clearly show that an increase of slows down the heat transfer process. We explain our results by a simplified phenomenological model: we show that the time behaviour of the drop average temperature is self-similar, and a universal behaviour can be found upon rescaling by. Accordingly, the heat transfer coefficient (respectively its dimensionless counterpart, the Nusselt number) scales as (respectively) at the beginning of the simulation, and tends to (respectively) at later times. These different scalings can be explained via the boundary layer theory and are consistent with previous theoretical/numerical predictions

Energy balance in lubricated drag-reduced turbulent channel flow

We use direct numerical simulation (DNS) to study drag reduction in a lubricated channel, a flow instance in which a thin layer of lubricating fluid is injected in the near-wall region so as to favour the transportation of a primary fluid. In the present configuration, the two fluids have equal density but different viscosity, so that a viscosity ratio can be defined. To cover a meaningful range of possible situations, we consider five different in the range. All DNS are run using the constant power input (CPI) approach, which prescribes that the flow rate is adjusted according to the actual pressure gradient so as to keep constant the power injected into the flow. The CPI approach has been purposely extended here for the first time to the case of multiphase flows. A phase-field method is used to describe the dynamics of the liquid-liquid interface. We unambiguously show that a significant drag reduction (DR) can be achieved for. Reportedly, the observed DR is a non-monotonic function of and, in the present case, is maximum for (flow-rate increase). Upon a detailed analysis of the energy budgets, we are able to show the existence of two different DR mechanisms. For and, DR is purely due to the effect of the surface tension-a localized elasticity element that separates the two fluids-which, decoupling the wall-normal momentum transfer mechanisms between the primary and the lubricating layer, suppresses turbulence in the lubricating layer (laminarization) and reduces the overall drag. For <[CDATA[\u3bb, turbulence can be sustained in the lubricating layer, because of the increased local Reynolds number. In this case, DR is simply due to the smaller viscosity of the lubricating layer that acts to decrease directly the corresponding wall friction. Finally, we show evidence that an upper bound for exists, for which DR cannot be observed: for, we report a slight drag enhancement, thereby indicating that the turbulence suppression observed in the lubricating layer cannot completely balance the increased friction due to the larger viscosity

Turbulence and Interface Waves in Stratified Oil–Water Channel Flow at Large Viscosity Ratio

We investigate the dynamics of turbulence and interfacial waves in an oil–water channel flow. We consider a stratified configuration, in which a thin layer of oil flows on top of a thick layer of water. The oil–water interface that separates the two layers mutually interacts with the surrounding flow field, and is characterized by the formation and propagation of interfacial waves. We perform direct numerical simulation of the Navier-Stokes equations coupled with a phase field method to describe the interface dynamics. For a given shear Reynolds number, Reτ= 300 , and Weber number, We= 0.5 , we consider three different types of oils, characterized by different viscosities, and thus different oil-to-water viscosity ratios μr= μo/ μw (being μo and μw oil and water viscosities). Starting from a matched viscosity case, μr= 1 , we increase the oil-to-water viscosity ratio up to μr= 100 . By increasing μr , we observe significant changes both in turbulence and in the dynamics of the oil–water interface. In particular, the large viscosity of oil controls the flow regime in the thin oil layer, as well as the turbulence activity in the thick water layer, with direct consequences on the overall channel flow rate, which decreases when the oil viscosity is increased. Correspondingly, we observe remarkable changes in the dynamics of waves that propagate at the oil–water interface. In particular, increasing the viscosity ratio from μr= 1 to μr= 100 , waves change from a two-dimensional, nearly-isotropic pattern, to an almost monochromatic one

Propagation of capillary waves in two-layer oil-water turbulent flow

We study the dynamics of capillary waves at the interface of a two-layer stratified turbulent channel flow. We use a combined pseudo-spectral/phase field method to solve for the turbulent flow in the two liquid layers and to track the dynamics of the liquid-liquid interface. The two liquid layers have same thickness and same density, but different viscosity. We vary the viscosity of the upper layer (two different values) to mimic a stratified oil-water flow. This allows us to study the interplay between inertial, viscous and surface tension forces in the absence of gravity. In the present set-up, waves are naturally forced by turbulence over a broad range of scales, from the larger scales, whose size is of order of the system scale, down to the smaller dissipative scales. After an initial transient, we observe the emergence of a stationary capillary wave regime, which we study by means of temporal and spatial spectra. The computed frequency and wavenumber power spectra of wave elevation are in line with previous experimental findings and can be explained in the frame of the weak wave turbulence theory. Finally, we show that the dispersion relation, which gives the frequency as a function of the wavenumber , is in good agreement with the well-established theoretical prediction,

Heat transfer in drop-laden turbulence

Heat transfer by large deformable drops in a turbulent flow is a complex and rich in physics system, in which drops deformation, breakage and coalescence influence the transport of heat. We study this problem coupling direct numerical simulations (DNS) of turbulence, with a phase-field method for the interface description. Simulations are run at fixed shear Reynolds and Weber numbers. To evaluate the influence of microscopic flow properties, like momentum/thermal diffusivity, on macroscopic flow properties, like mean temperature or heat transfer rates, we consider four different values of the Prandtl number, which is the momentum to thermal diffusivity ratio: Pr=1, Pr=2, Pr=4 and Pr=8. The drops volume fraction is Phi=5.4% for all cases. Drops are initially warmer than the turbulent carrier fluid, and release heat at different rates, depending on the value of Pr, but also on their size and on their own dynamics (topology, breakage, drop-drop interaction). Computing the time behavior of the drops and carrier fluid average temperatures, we clearly show that an increase of Pr slows down the heat transfer process. We explain our results by a simplified phenomenological model: we show that the time behavior of the drops average temperature is self similar, and a universal behavior can be found upon rescaling by t/Pr^2/3

A construção do planejamento de ensino - aprendizagem na perspectiva histórico-crítica pelos docentes e pedagogos de 1ª a 4ª séries do ensino fundamental da rede pública de ensino no município de Araucária

Orientador : Sônia Guariza MirandaMonografia (especialização) - Universidade Federal do Paraná, Setor de Educação,Curso de Especializaçao em Organização do Trabalho PedagógicoInclui bibliografi

Modelling the direct virus exposure risk associated with respiratory events

The outbreak of the COVID-19 pandemic highlighted the importance of accurately modelling the pathogen transmission via droplets and aerosols emitted while speaking, coughing and sneezing. In this work, we present an effective model for assessing the direct contagion risk associated with these pathogen-laden droplets. In particular, using the most recent studies on multi-phase flow physics, we develop an effective yet simple framework capable of predicting the infection risk associated with different respiratory activities in different ambient conditions. We start by describing the math- ematical framework and benchmarking the model predictions against well-assessed literature results. Then, we provide a systematic assessment of the effects of physical distancing and face coverings on the direct infection risk. The present results indicate that the risk of infection is vastly impacted by the ambient conditions and the type of respiratory activity, suggesting the non-existence of a universal safe distance. Meanwhile, wearing face masks provides excellent protection, effectively limiting the transmission of pathogens even at short physical distances, i.e. 1 m