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Séminaire de l’équipe TriboLub

Deux présentations sont au programme :

  • Anthony Voitus : « Modélisation d’un écoulement diphasique dans un palier hydrostatique ».
  • Francisca Husanu : « Triboemission of nanoparticle aerosols from nanofunctionalized materials: towards developing a standard quantitative method. »

Onset of large-scale convection in turbulent shear flows

Large-scale coherent rolls are observed frequently in unstably stratified turbulent wall-bounded flows where they strongly influence the turbulent transport and the mean flow properties. I will address the question of their genesis by means of a linear stability analysis of the turbulent mean flow where a model of turbulent Reynolds stresses is embedded in the linear stability operator. I will show that the onset of large-scale convection is associated to the linear instability of the mean flow to large-scale streamwise-uniform coherent rolls consistently with the results of recent direct numerical simulations performed in the same setting. I will also discuss the importance of including a model of the Reynolds stresses in the linear operator both in this analysis in resolvent analyses of turbulent shear flows.

In situ Raman spectroscopy of plasma electrochemical and plasma catalytic reactors

We will focus on transient changes to Raman spectra that appear in the presence of plasma, then disappear when the plasma is switched off. Such changes are observable in situ, not ex situ, and indicate the energy exchange from the plasma to water or the catalyst surface. The general approach will be to establish relationships between plasma properties and changes to the Raman spectra. Two primary cases will be studied. First, we will study PEC in air plasma-water systems at atmospheric pressure, using both batch reactor and electrospray configurations. We will focus on the spectral profile of the –OH stretch band of water and of probe molecules such as NO3 – . Analysis of the –OH stretch band reveals that the plasma weakens the hydrogen bonding network of water. To help pinpoint the cause, we will track the broadening of the N-O symmetric stretch mode (v1) of NO3 – at less than 20 µm depth from the plasma-liquid interface. Second, we will investigate a PAC reactor consisting of a low- to mediumpressure CO2 plasma in contact with CeO2 as a catalyst. In this case, the catalyst temperature will be tracked using the Stokes-to-anti-Stokes ratio of the Raman intensities of the first-order optical phonon of CeO2, as well as the spectral shift of this spectral feature.

Investigation of unsteady secondary flows and large-scale turbulence in heterogeneous turbulent boundary layers

The nature of wall-bounded turbulent flows over rough surfaces, whose roughness distribution is homogeneous, has been studied extensively and is relatively well defined. Most surfaces in nature and engineering applications, however, are heterogeneous and the heterogeneity can be arranged in an infinite number of ways. Examples include: rivets on aircraft, bio-fouling on ships, sedimentation on riverbeds, and forest and crop boundaries in the atmospheric surface layer. This talk will focus on a specific roughness arrangement composed of spanwise-alternating smooth and rough strips. Embedded within the turbulent boundary layer developing over such surface are secondary flows in the form of counter-rotating streamwise vortices. Instantaneously, these secondary flows are visually similar to the large-scale motions (LSMs/VLSMs) that occur naturally over smooth walls – both appear as elongated high- and low-momentum streaks. In this talk, I will investigate parameters affecting the formation of the secondary flows and take a closer look at the structures of turbulence in roughness-induced secondary flows, and how these structures compare to the naturally occurring LSMs/VLSMs.

The physics of birds: a story of feathers and tail

Birds are intrinsically complex physical objects, ultimately tuned for flight with light-weight highly resistant structure and optimized aerodynamic forms. From a physical point of view, birds thus offer a diversity of research interests: aerodynamic, biomechanical or even energetic. At the Biomimetics group of the University of Groningen, we investigate such aspects with follow-up developments in Engineering through drones or bio-inspired turbines. In this presentation, I will develop two ongoing projects around bird physics. First, I will dive into the biomechanical and aerodynamic properties of feathers. Then I will present how tail control in bird landing might be assisted by passive aerodynamic balance through the development of a bio-hybrid pigeon tail.

Cavitation Bubble as Microreactor: Performing Chemistry in a Bubble

The use of unconventional activation techniques, such as low and high frequency ultrasound (US), in combination with heterogeneous catalysts offers a powerful synergistic approach to transform renewable resources to value added chemicals. Taking advantage of the cavitation bubbles generated during ultrasound irradiation which often acts as a micro-reactor and the localized extreme conditions of temperature and pressure, small molecules can be activated to yield highly reactive radicals that can in synergy with catalysis promote the selective conversion bio-based substrates into valuable products which are hitherto difficult to obtained under conventional routes and at mild reaction conditions. Through selected examples, we demonstrate the potential of high frequency ultrasound working in concert with catalysis in promoting the formation of relevant industrially valuable chemicals

Bruit d’écoulement à basse vitesse en conduit : mesure et simulations numériques

L’aéroacoustique en conduit examine la génération, la propagation et la réception du bruit généré par les fluides, en particulier dans le contexte des systèmes de ventilation, de chauffage, de climatisation et d’autres systèmes similaires. Dans de nombreuses situations, la présence d’obstacles (volet, clapet, coude…) dans la conduite rend l’écoulement turbulent. Le bruit d’obstacle occasionné se transmet au réseau aéraulique et peut constituer une gêne sonore.
La prévision ainsi que la mesure des niveaux de bruits engendrés par des écoulements turbulents en conduit est un thème de recherche qui fut initié lors du projet CEVAS (Conception d’Equipement de Ventilation d’Air Silencieux, 2013-2016) portant sur le développement d’outils de simulation et/ou moyens d’essais qui permettent de prendre en considération de manière efficace les aspects bruits en amont de la phase de conception des systèmes de ventilation d’air automobile. Le projet a permis le développement d’une formulation intégrale originale basée sur l’analogie de Ribner. La méthode ne nécessite que la connaissance du champ de pression issu d’une simulation LES incompressible sur les bords du domaine de calcul et permet de s’affranchir d’un calcul volumique coûteux. Les comparaisons de la puissance acoustique rayonnée avec les mesures, via la méthode dite ‘2N-ports’, ainsi que des mesures du champ de vitesse turbulentes au voisinage d’un obstacle inséré dans le conduit montrent de bons accords. Le cas spécifique de deux obstructions identiques séparées par une distance comparable au diamètre hydraulique montre des mécanismes de rétroaction fluide-acoustique de forte intensité. On montre que ces effets ne peuvent être capturés via la simulation numérique qu’en prenant en compte la compressibilité du fluide.

Simulation des plasmas de tokamak : états stationnaires axisymétriques à flot non nul

La détermination classique des équilibres des plasmas de tokamaks repose sur la résolution de l’équation de Grad-Shafranov. On y suppose que le plasma, vu comme un fluide conducteur, a une vitesse nulle. Expérimentalement, il existe de nombreuses observations d’une rotation spontanée dans les tokamaks notamment dans la direction toroïdale. Cette rotation du plasma s’avère avoir des effets très importants sur l’amélioration du confinement et le passage dans le mode H de confinement amélioré. Pour éclaircir ces observations expérimentales, nous nous proposons de déterminer numériquement les états stationnaires axisymétriques des équations non-linéaires de la magnétohydrodynamique visco-résistive obtenues en réintroduisant le terme convectif. Cette étude doit prendre en compte le forçage dû à la présence d’un champ électrique extérieur dans la direction toroïdale servant à créer le courant toroïdal nécessaire dans un tokamak à la création de la composante poloïdale du champ magnétique.