Archives : Évènements

Résumé: Free-surface flows are subdued to hydrodynamic instabilities, one of them observed as a coherent structure propagating at the interface between the viscous fluid flow and air named roll waves. Those instabilities develop from spatial and/or time disturbances, reaching a stationary regime downstream. Then, one important feature to predict the evolution of such a phenomenon is the length required for disturbances to travel before reaching their stationary form. The present work brings a theoretical and numerical analysis of such length required for roll waves to become stationary in a free-surface laminar flow of a Newtonian fluid. Two types of stability analysis are brought to verify flow stability and obtain parameters for wave growth rate in a Saint-Venant-like system. Then, numerical simulations are performed of the free-surface laminar transient flow of glycerin. The Navier-Stokes equations were solved using the finite volumes method, Euler schemes and PIMPLE, and the VoF technique to solve the interface. Boundary conditions were specified to obtain a steady and uniform regime given a Froude number. Then, a sinusoidal perturbation with controllable properties was applied to the inlet velocity. From the numerical results, the spatial development of the roll waves was evaluated, focusing on the establishment length as a function of the Froude number and the perturbation amplitude. The analyses performed allowed the verification of the influence of the flow’s hydraulic regime over the establishment length, and it was possible to obtain a new equation as a function of the perturbation amplitude.

Emmanuelle Rio

The stability of soapy objects such as films, bubbles and foams has been studied widely because of the numerous applications concerning food industry, climate prediction or artistic utilization of giant bubbles.

It has been demonstrated that their stability is primarily affected by the thinning dynamics of the thin soap films. The drainage dynamics, which is the capillary or gravity driven flow in the liquid film has been widely investigated [1]. However, more recently, researchers also became interested in the influence of evaporation on this thinning dynamics [2,3].

In this seminar, I will show that, to describe bubble stability, evaporation must be taken into account as soon as the films are thin enough [4]. We will see that the bubble lifetime can be predicted by taking into account both the drainage and the evaporation to describe the thinning dynamics [2] and that this is all the more important concerning the stability of giant soap films. I will also esquisse some hypothesis concerning the influence of physical-chemistry on the film thinning and evaporation. This last result comes from a collaboration with bubbles artists, in which we developed a scientific approach to rationalize their best recipes.

[1] H. Lhuissier and E. Villermaux, J. Fluid Mech., 2012, 696, 5-44.
[2] J. Miguet, M. Pasquet, F. Rouyer, Y. Fang and E. Rio, Soft Matter, 2020, 16, 1082–1090.
[3] S. Poulain and L. Bourouiba, Phys. Rev. Lett., 2018, 121, 204502.
[3] A. Roux, A. Duchesne, M. Baudoin, Physical Review Fluids, 7(1), L011601,2022.
[4] Champougny, L., Miguet, J., Henaff, R., Restagno, F., Boulogne, F., Rio, E., Influence of evaporation on soap film rupture. Langmuir, 34(10), 3221-3227, 2018.

Yu-Chien (Alice)

Associate Project Scientist
Director of Lasers, Flames & Aerosols
UNIVERSITY OF CALIFORNIA
Irvine, CA USA

ABSTRACT:

Electric Field Effects on Laminar Diffusion Flames (E-FIELD Flames) is one of the Advanced Combustion via Microgravity Experiments (ACME) of the National Aeronautics and Space Administration (NASA). The E-FIELD Flames experiment studies a hydrocarbon flame jet to determine how an electric field leads to an electric body force and a resultant ion-driven wind when the normal 1-g buoyant force is not participating in the process. The E-FIELD Flames experiment was boarded onto the international space station on March 14th, 2018 (\pi day) and was accomplished in November of the same year. The goal of the study is to expose the physico-thermo-chemical processes when an electric field is applied without gravitational effect. The results show that the flame is most compact at saturation while the measured voltage to current (VCC) curve demonstrates parabolic behavior after saturation which differs from observations in 1 g on Earth. The second part of the talk will briefly survey the current research developing in the Keck Foundation Deep Ocean Power Science Laboratory (DOPSL) and the Lasers, Flames & Aerosols Research Group (LFA) at UCI, including high pressure combustion experiments, water addition combustion, methane hydrate combustion, carbon dioxide hydrate fire extinguishment, and hydrogen/methane blending for optical diagnostics measurement.

Abstract:

The advent of new powerful deep neural networks (DNNs) has fostered their application in a wide range of research areas, including more recently in fluid mechanics. In this presentation, we will cover some of the fundamentals of deep learning applied to computational fluid dynamics (CFD). Furthermore, we explore the capabilities of DNNs to perform various predictions in turbulent flows: we will use convolutional neural networks (CNNs) for non-intrusive sensing, i.e. to predict the flow in a turbulent open channel based on quantities measured at the wall. We show that it is possible to obtain very good flow predictions, outperforming traditional linear models, and we showcase the potential of transfer learning between friction Reynolds numbers of 180 and 550. We also discuss other modelling methods based on autoencoders (AEs) and generative adversarial networks (GANs), and we present results of deep-reinforcement-learning-based flow control.

Bio:
Dr. Ricardo Vinuesa is an Associate Professor at the Department of Engineering Mechanics, at KTH Royal Institute of Technology in Stockholm. He is also a Researcher at the KTH Climate Action Centre and Vice Director of the KTH Digitalization Platform. He studied Mechanical Engineering at the Polytechnic University of Valencia (Spain), and he received his PhD in Mechanical and Aerospace Engineering from the Illinois Institute of Technology in Chicago. His research combines numerical simulations and data-driven methods to understand and model complex wall-bounded turbulent flows, such as the boundary layers developing around wings and urban environments. Dr. Vinuesa has received, among others, an ERC Consolidator Grant and the Göran Gustafsson Award for Young Researchers.