1,720,979 research outputs found
Modelling and computation of drops and bubbles in turbulence
No full textExistence of drops and bubbles in turbulence is granted by their interface. Interfaces are a macroscopic perception of molecular properties, are not property of the drop or the carrier fluid and their role is enormously important in a number of environmental and industrial processes: it is across interfaces that momentum, heat and mass transfer fluxes occur. In this talk, We will briefly review the physics modelling and the current computational methodologies used to track interfaces and we will focus on the phase-field approach, in which the phase distribution is a field described by the order parameter φ. We will present several flow instances and phenomena in which surface tension, density and viscosity are varied, and we will also cover the role of surfactants in altering topological changes of drops (breakage and coalescence) in connection with the characteristics of turbulence. Finally, we will examine the heat transfer between a dispersed phase of large deformable drops and a carrier fluid focusing on the flow structure inside the drops
A Phase Field Method for surfactant-laden multiphase flows with different solubilities
No full textIn this work we present a new phase field model for multiphase flows with soluble surfactants that accounts for unmatched solubility. The advection-diffusion of the phase field and of the surfactant is described using two Cahn-Hilliard equations together with a two-order-parameter Ginzburg-Landau free energy functional. Here, an asymmetric term is introduced to penalize the presence of surfactant in one of the phases. This modification allows to circumvent existing limitations on phase field method applications to liquid-gas systems. The model has been tested on planar configurations characterized by two phases with different solubility using a pseudo-spectral code. The results demonstrate the ability to accurately reproduce the expected surfactant distribution. The simulation of a single droplet and of a droplet-droplet interaction in shear flow is then examined in order to understand how the difference in surfactant bulk concentration between the phases affects the overall interfacial behavior
Heat transfer in drop-laden low-Prandtl-number channel turbulence
Full text linkIn this work, we numerically investigate heat transfer in low-Prandtl-number drop-laden wall-bounded turbulence. These flows are characteristic of nuclear and fusion technologies, where liquid metals - known for their high thermal conductivity - are laden with drops or bubbles of another liquid or pressurised gas. To this end, we consider forced convection turbulence between two differentially heated parallel plates. The carrier phase (i.e. liquid metal) is characterised by a low Prandtl number, while for the dispersed phase, we explore a range of Prandtl numbers from (matched case) to (super-unitary Prandtl number in the dispersed phase). Simulations are conducted at constant friction Reynolds number, and for each dispersed phase Prandtl number, two volume fractions are examined: and. The simulation framework relies on direct numerical simulation of the Navier-Stokes equations, coupled with a phase-field method and the energy equation. Results show that an increase of the dispersed phase Prandtl number reduces heat transfer, leading to a lower Nusselt number for both volume fractions. To explain this behaviour, we analyse how the drops modify the temperature field, and demonstrate that the heat transfer reduction stems from a decreased diffusive heat flux within the dispersed phase. Finally, we propose a phenomenological model to predict the Nusselt number as a function of both the dispersed phase volume fraction and Prandtl number
INTERACTION BETWEEN CAPILLARY WAVES AND HYDRODYNAMIC TURBULENCE IN A TWO-LAYER OIL-WATER FLOW
No full textWe use pseudo-spectral Direct Numerical Simulation (DNS), coupled with a Phase Field Method (PFM), to investigate the turbulent Poiseuille flow of two immiscible liquid layers inside a channel. The two liquid layers, which have the same thickness (h1 = h2 = h), are characterized by the same density (ρ1 = ρ2 = ρ) but different viscosities (η1 ≠ η2), so to mimick a stratified oil-water flow. This setting gives the possibility to study the interplay between inertial, viscous and surface tension forces to be studied in the absence of gravity. We focus on the role of turbulence in initially deforming the interface and on the subsequent growth of capillary waves. After an initial transient, we observe the emergence of a stationary capillary wave regime. Capillary wave propagation and interaction is studied by means of a spatiotemporal spectral analysis and compared with previous theoretical and experimental results. The computed 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. At wave scales larger than the turbulent forcing range the observed scaling of the one-dimensional wavenumber spectrum suggests an energy equipartition regime (k-1), which is predicted by theory and has been recently observed in experiments with capillary wave turbulence in microgravity. At wave scales directly forced by hydrodynamic turbulence, an initially milder slope (k-4) of the wavenumber spectrum is followed by a sharper decay (k-6) of wave energy at larger wavenumbers, with the transition taking place near the Kolmogorov- Hinze critical scale, where surface tension forces and turbulent inertial forces are balanced
HEAT TRANSFER IN DROP-LADEN TURBULENT FLOWS
No full textWe study the heat transfer process in a multiphase turbulent system composed by a swarm of large and deformable drops and a continuous carrier phase. For a shear Reynolds number of Re = 300 and a constant drops volume fraction of Φ = 5.4%, we perform a campaign of direct numerical simulations (DNS) of turbulence coupled with a phase-field method and the energy equation; the Navier-Stokes equations are used to describe the flow field, while the phase-field method and the energy equation are used to describe the dispersed phase topology and the temperature field, respectively. Considering four Prandtl numbers, Pr = 1, 2, 4 and 8 and twoWeber numbers,We = 1.5 and 3.0, we investigate the heat transfer process from warm drops to a colder turbulent flow. Using detailed statistics, we first characterize the time evolution of the temperature field in both the dispersed and carrier phase. Then, we develop an analytic model able to accurately reproduce the behaviour of the dispersed and continuous phase temperature. We find that an increase of the Prandtl number, obtained via a decrease of the thermal diffusivity, leads to a slower heat transfer between the dispersed and carrier phase. Finally, we correlate the drop diameters and their average temperatures
Phase-field modeling of complex interface dynamics in drop-laden turbulence
No full textTurbulent flows laden with large, deformable drops are ubiquitous in nature and in a wide range of industrial processes. Prediction of the interactions between drops, which deform under the action of turbulence, exchange momentum via surface tension, and that can also exchange heat or mass, are complicated due to the wide range of scales involved: from the largest scales of the flow, down to the Kolmogorov scales of turbulence, and further down to the molecular scale of the interface. Due to this wide range of scales, the numerical description of these flows is challenging and requires robust and accurate numerical schemes that are able to capture both the turbulence characteristics and the dynamics of ever-moving and deforming interfaces including their topological changes (i.e., coalescence and breakage). In the past decades, various numerical methods have been proposed for simulating two-phase flows, from interface-tracking methods, where the interface is explicitly tracked with the use of marker points to interface-capturing methods, where the interface is identified as the isovalue of a color/marker function. Phase-field methods belong to the category of interface-capturing methods, and have emerged as promising approaches to simulate complex two-phase flows. In phase-field methods, the transport equation to describe the drop motion is obtained from first thermodynamics principles, and phenomena acting at the interface scale can be conveniently modeled. Although in realistic case scenarios, the physical thickness of the interface cannot be directly simulated, this family of methods offers desirable properties that have attracted the interest of researchers in recent years. In this work, we describe the fundamentals of the phase-field modeling associated with the direct numerical simulation of turbulence in the context of drop-laden flows. We discuss the potentials of the phase-field method with reference to breakage and coalescence phenomena, and to the corresponding drop size distribution; we examine how to model surface tension changes due to surfactant distribution, and we outline the framework to model heat and mass transfer fluxes. Finally, we present our perspectives for future developments of phase-field modeling of drop-laden turbulent flows in the context of the current available literature
Computationally Efficient and Interface Accurate Dual-Grid Phase-Field Simulation of Turbulent Drop-Laden Flows
No full textIn this work, we develop a dual-grid approach for the direct numerical simulations of turbulent multiphase flows in the framework of the phase-field method (PFM). With the dual-grid approach, the solution of the Navier-Stokes equations (flow-field) and of the Cahn-Hilliard equation (phase-field) are performed on two different computational grids. In particular, a base grid - fine enough to resolve the flow down to the Kolmogorov scale - is used for the solution of the Navier-Stokes equations, while a refined grid - required to improve the description of small interfacial structures - is used for the solution of the Cahn-Hilliard equation (phase-field method). The proposed approach is validated, and its computational efficiency is evaluated considering the deformation of a drop in a two-dimensional shear flow. Analyzing the computational time and memory usage, we observe a reduction between ≃30% and ≃40% (with respect to the single-grid approach), depending on the grid refinement factor employed for the phase-field variable. The applicability of the approach to a realistic three-dimensional case is also discussed, by focusing on the breakage of a thin liquid sheet inside a turbulent channel flow. Indications on the grid resolution representing a good compromise between accuracy and computational efficiency in drop-laden turbulence are also provided
Turbulence and Interface Waves in Stratified Oil–Water Channel Flow at Large Viscosity Ratio
Full text linkWe 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
FLOW36: A spectral solver for phase-field based multiphase turbulence simulations on heterogeneous computing architectures
Full text linkWe present FLOW36, a GPU-ready solver for interface-resolved simulations of multiphase turbulence. The simulation framework relies on the coupling of direct numerical simulation of turbulence, used to describe the flow field, with a phase-field method, used to describe the shape and deformation of a deformable interface and the presence of surfactants. An additional transport equation for a passive scalar can be solved to describe heat transfer in multiphase turbulence. The governing equations are solved in a cuboid domain bounded by two walls along the wall-normal direction where no-slip, free-slip or fixed/moving wall boundary conditions can be applied, while periodicity is applied along the streamwise and spanwise directions. The numerical method relies on a pseudo-spectral approach where Fourier series (periodic directions) and Chebyshev polynomials (wall-normal direction) are used to discretize the governing equations in space. Equations are advanced in time using an implicit-explicit scheme. From a computational perspective, FLOW36 relies on a multilevel parallelism. The first level of parallelism relies on the message-passing interface (MPI). A second level of parallelism uses OpenACC directives and cuFFT libraries; this second level is used to accelerate the code execution when heterogeneous computing infrastructures are targeted. In this work, we present the numerical method and we discuss the main implementation strategies, with particular reference to the MPI and OpenACC directives and code portability, performance and maintenance strategies. FLOW36 is released open source under the GPLv3 license
Interaction between capillary waves and hydrodynamic turbulence in a two-layer oil-water flow
No full textWe use pseudo-spectral Direct Numerical Simulation (DNS), coupled with a Phase Field Method (PFM), to investigate the turbulent flow of two immiscible liquid layers inside a channel. This setting, in which the two fluids have the same density but different viscosity (so to mimick the flow of oil and water), allows for the interplay between inertial, viscous and surface tension forces to be studied in the absence of gravity. We focus on the interaction between capillary waves and turbulence at the liquid-liquid interface. Spatiotemporal spectral analysis of the capillary wave field shows wave propagation that is in agreement with the theoretical dispersion relation. The one-dimensional wavenumber spectrum suggests an energy equipartition regime at larger wave scales and a transition to a sharp decay of wave energy at smaller scales taking place near the Kolmogorov-Hinze critical scale, where surface tension forces and turbulent inertial forces are balanced
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