On the effect of buoyancy on lateral migration of bubbles in turbulent flows insights from Direct Numerical Simulations

International audienceBubble migration is a key concern in turbulent bubbly flows as it dramatically affects momentum and mass transfers between phases. Its prediction in steam-water conditions relevant to PWR applications is difficult to assess because experiments are often conducted with air/water flows that present substantially different properties. The effect of the deformability of bubbles on the lift force has been extensively studied experimentally, or numerically, and characterized based on the Eotvos and Reynolds numbers. Nonetheless, the effect of buoyancy is not well understood. The strength of gravity and the resultant enhancement of turbulence can have a significant impact on bubble migration in the cross-flow direction.In this work, we propose to use Direct Numerical Simulations (DNS) of turbulent bubbly flows to better understand the dominant physical mechanisms at play and cover ranges of conditions difficult to access experimentally. DNS offers a rich insight into the underlying physical phenomena and allows us to control the relative importance of different sub-physics. Starting from the flow conditions studied by Lu and Tryggvason [1], we perform four DNS of bubbly flows at a slightly higher Reynolds friction number, covering deformable and almost-spherical bubbles in weakly-buoyant or buoyant conditions. Separate effects of the Eotvos number and of an increasing gravitational force are assessed. Mean quantities, Reynolds stresses and higher-order statistics are computed to analyze the effect of bubbles on liquid turbulence levels, which influences the wall-normal void fraction profile. New insights on the way bubbles alters liquid turbulence levels and influence the lateral migration of bubbles are presented. Further experimental and numerical studies are required to support and extend this analysis

Contemporary Asian Artistic Expressions and Tourism – An Introduction

This introductory chapter presents and critically discusses the various themes underpinning this book. Firstly, it provides an examination of the notion of ‘contemporary art’, including an overview of the existing definitions and debates in the current literature. Secondly, this chapter discusses the nexus between tourism and contemporary art by providing an overview of the past studies conducted on cultural and heritage tourism. In this section, the various themes underpinning the different parts of the literature on art tourism (e.g. identity, authenticity, commoditisation and capitalism) are considered. Thirdly, a discussion on the relationship between tourism and Asian contemporary art is presented, which also includes a part problematising and questioning terms like ‘Asia’ and ‘Asian art’. Finally, an overview of the different chapters that constitute the backbone of this collection is offered alongside the four themes around which the book is structured

Analysis and modelling of Reynolds stresses in turbulent bubbly up-flows from direct numerical simulations

International audienceTwo-phase bubbly flows are found in many industrial applications. These flows involve complex local phenomena that are still poorly understood. For instance, two-phase turbulence modelling is still commonly based on single-phase flow analyses. A direct numerical simulation (DNS) database is described here to improve the understanding of two-phase turbulent channel flow at a parietal Reynolds number of 127. Based on DNS results, a physical interpretation of the Reynolds stress and momentum budgets is proposed. First, surface tension is found to be the strongest force in the direction of migration so that budgets of the momentum equations suggest a significant impact of surface tension in the migration process, whereas most modelling used in industrial application does not include it. Besides, the suitability of the design of our cases to study the interaction between bubble-induced fluctuations (BIF) and single-phase turbulence (SPT) is shown. Budgets of the Reynolds stress transport equation computed from DNS reveal an interaction between SPT and BIF, revealing weaknesses in the classical way in which pseudoturbulence and perturbations to standard single-phase turbulence are modelled. An SPT reduction is shown due to changes in the diffusion because of the presence of bubbles. An increase of the redistribution leading to a more isotropic SPT has been observed as well. BIF is comprised of a turbulent (wake-induced turbulence, WIT) and a non-turbulent (wake-induced fluctuations, WIF) part which are statistically independent. WIF is related to averaged wake and potential flow, whereas WIT appears when wakes become unstable or interact with each other for high-velocity bubbles. In the present low gravity conditions, BIF is reduced to WIF only. A thorough analysis of the transport equations of the Reynolds stresses is performed in order to propose an algebraic closure for the WIF towards an innovative two-phase turbulence model

On the effect of buoyancy on lateral migration of bubbles in turbulent flows insights from Direct Numerical Simulations

International audienceBubble migration is a key concern in turbulent bubbly flows as it dramatically affects momentum and mass transfers between phases. Its prediction in steam-water conditions relevant to PWR applications is difficult to assess because experiments are often conducted with air/water flows that present substantially different properties. The effect of the deformability of bubbles on the lift force has been extensively studied experimentally, or numerically, and characterized based on the Eotvos and Reynolds numbers. Nonetheless, the effect of buoyancy is not well understood. The strength of gravity and the resultant enhancement of turbulence can have a significant impact on bubble migration in the cross-flow direction.In this work, we propose to use Direct Numerical Simulations (DNS) of turbulent bubbly flows to better understand the dominant physical mechanisms at play and cover ranges of conditions difficult to access experimentally. DNS offers a rich insight into the underlying physical phenomena and allows us to control the relative importance of different sub-physics. Starting from the flow conditions studied by Lu and Tryggvason [1], we perform four DNS of bubbly flows at a slightly higher Reynolds friction number, covering deformable and almost-spherical bubbles in weakly-buoyant or buoyant conditions. Separate effects of the Eotvos number and of an increasing gravitational force are assessed. Mean quantities, Reynolds stresses and higher-order statistics are computed to analyze the effect of bubbles on liquid turbulence levels, which influences the wall-normal void fraction profile. New insights on the way bubbles alters liquid turbulence levels and influence the lateral migration of bubbles are presented. Further experimental and numerical studies are required to support and extend this analysis

Modelling of the laminar dispersion force in bubbly flows from direct numerical simulations

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Surface tension energy in bubbly flows physical impact and modelling in RANS Eulerian-Eulerian two-fluid model

International audienceThe Eulerian-Eulerian two-fluid model for turbulent bubbly flows has been extensively studied. Such a model has by construction inherent limitations because it relies on a particle approach. This method considersthe bubble as a pointwise particle without finite-size effects, which should be meaningful only when the characteristic length scale of averaged quantities variations is bigger than the bubble length scale. However,several physical phenomena such as the surface tension energy do not satisfy this criteria. Physical analysis based on Direct Numerical Simulations (DNS) of bubbly turbulent plane channels are performed with TrioCFD. In order to feed the RANS\footnote{Reynolds Average Navier Stokes} modelling, the upscaling process extracts knowledge from DNS. Then, new models are tested on the RANS Eulerian code Neptune\_CFD. In this work, the impact of surface tension energy is investigated.\\Firstly, the significant contribution of the surface tension source term in the momentum balance is demonstrated. In a RANS model, the forces applied on bubbles should be modelled, as well as the Reynolds Stress tensor. It is known that surface tension energy impacts both the turbulence and the forces through the deformability of the interface, but to the best of our knowledge, the interfacial momentum source termis never taken into account in a RANS model. The interfacial force is traditionnally splitted into several contributions such as drag, lift, added mass or turbulent dispersion force. Secondly, we demonstrated that the surface tension force has a behaviour similar to an equivalent turbulent dispersion force. Moreover,in some weakly turbulent cases and for large surface tension, the interfacialenergy has a significant impact on the bubble migration

Towards an innovative R ij -epsilon model for turbulence in bubbly flows from DNS simulations

International audienceThe Euler-Euler two-fluid model for turbulent bubbly flows has been extensively studied and modelling of turbulence is necessary for many industrial applications. In addition to the single-phase flow mechanism of Shear Induced Turbulence (SIT), two-phase bubbly flows develop a pseudoturbulence linked to the wake interactions known as Bubble Induced Turbulence (BIT). In classical Rij-epsilon modelling, this added phenomenon is modeled as a source term of turbulent kinetic energy in the Rij transport equation. Nevertheless, it has been demonstrated that SIT and BIT are statistically independent and that their physical meanings are strongly different. These results allow us to propose a splitting of the Rij transport equation into a transport equation for the SIT Reynolds stresses and another for the BIT Reynolds stresses.These new equations contain a lot of terms which require modelling. In this study, they are closed through physical assertions and computational results.Physical analysis based on Direct Numerical Simulations (DNS) of bubbly turbulent plane channels are performed with TrioCFD. In order to feed the RANS (Reynolds Average Navier Stokes) modelling, the up-scaling process extracts knowledge from DNS. Indeed, a method which allows to calculate the SIT and the BIT separately from the DNS is proposed. In the form of statistical profiles, all terms of the new formulation are extracted from the DNS, are analysed in order to improve our understanding of BIT and are used in support for the modelling development of an innovative Rij-epsilon model

Analysis and modeling of bubble-induced agitation from direct numerical simulation of homogeneous bubbly flows

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DNS of turbulent bubbly flows in plane channels using the front-tracking algorithm of triocfd

International audienceTwo-phase turbulence is studied using DNS of upward turbulent bubbly flows in a plane channel. Fully deformable monodispersed bubbles are tracked by the Front-Tracking algorithm implemented in TrioCFD code on the TRUST platform. Two sets of fluid properties are used. Firstly, two simulations are performed with virtual fluids at a low void fraction of 3\% and for a Reynolds friction number of 127 to benchmark our code against [1]. Good agreements are obtained for both deformable and spherical cases. A third simulation closer to pressurized water reactor (PWR) conditions was performed at higher void fraction and Reynolds number to push the limits of DNS capabilities. DNS results are averaged (i) to provide reference profiles for an up-scaling methodology towards RANS two-fluid models and (ii) to analyze the equilibrium between buoyancy, surface tension, viscous and turbulent shear at statistically steady-state. Surface tension forces and turbulence are essentials to capture the equilibrium. Their accurate modeling is the key to velocities and void fraction predictions in averaged codes.Our analysis reveals the important role of surface tension, not only in the determination of the bubble shapes, but also as a source of local imbalance of the momentum transfer between phases. More advanced models considering interfacial energy are necessary to predict these flows

On bubble forces in turbulent channel flows from direct numerical simulations

International audienceThe prediction of void fraction, which relies on interfacial force models, is a major issue in the context of boiling. The two-fluid model requires the modelling of the momentum transfer between phases. When bubbles are small (particle hypothesis), the momentum transfer is related to interfacial forces acting on bubbles. However, the splitting of these forces into drag, lift, added mass, etc., is not straightforward from the local point of view, where only the total interfacial force is defined as an integral of the constraint over the interface. For large-size bubbles, the particle hypothesis can be questioned. The momentum transfer can then be connected to the forces acting on a fluid element of the vapour phase. Based on the local and averaged formulations of the Navier-Stokes equations, a new balance equation for forces enables us to define lift, drag, added-mass and dispersion forces acting on a fluid element of the vapour phase. This equation gives a local definition for all the forces responsible for spatial distribution of bubbles and reflects the meaning usually assigned to the interfacial forces in the particle approach. Through this means, the link between the local formulation and physical phenomena is established and a new way of modelling the lift force is proposed. Furthermore, a new laminar dispersion force which relies on surface tension and pressure effects is introduced. The analysis of the budget equation on our direct numerical simulation database brings into light the large influence of this laminar dispersion force in the migration process. Different well-known physical behaviours can be modelled via this new force: the horizontal clustering of spherical bubbles in laminar flows and the oscillating trajectories of deformable bubbles