Virtual MEchanics GAthering -MEGA- Seminar: Talk 1 - Macroscopic modelling of micro-structured porous surfaces; Talk 2 - Efficient flapping propulsion with stiffness-distributed structures

Talk 1: Macroscopic modelling of micro-structured porous surfaces, by Giuseppe Zampogna (LFMI, EPFL)

Abstract: The dynamics of a fluid flowing across a thin permeable interface (i.e. a membrane) is an intrinsically multiscale phenomenon, owing to very different scales at play, rendering its description complex, from both a physical and computational point of view. A clear explanation of the mechanisms at the basis of membrane processes is then needed. Thanks to a multi-scale homogenization technique we develop a reduced-order, intuitive, robust and computationally cheap model to simulate the hydrodynamic interactions between a rigid membrane and a surrounding incompressible fluid flow. The model is able to provide a description of the micro- and macroscopic fluid behavior and consists of a constraint to be satisfied by the fluid velocity components, imposed within the fluid domain, over a virtual smooth surface passing through the center of each membrane pore. It shows that the membrane produces a jump in fluid stresses whose intensity and direction, evaluated solving problems at the microscale, depend on the external flow and on the pore geometry. To assert the validity of the macroscopic model developed, its solution is compared with the solution of the full-scale problem.

Bio: Giuseppe Zampogna is a EuroTech PostDoc Fellow in the Laboratory of Fluid Mechanics and Instabilities at EPFL, under the scientific supervision of Prof. F. Gallaire. Before coming in Lausanne, he has been postdoc at the Institut de Mécanique des Fluides de Toulouse and he obtained a PhD in fluid dynamics at the University of Genova in March 2016. He is particularly interested in multi scale phenomena like fluid flows interacting with poroelastic media, rough surfaces and micro structured porous surfaces.

Efficient flapping propulsion with stiffness-distributed structures, by Pierre Leroy-Calatayud (fleXLab, EPFL)

Abstract: Flying insects and many aquatic animals rely on flapping for locomotion. A feature commonly observed is that wings or fins, respectively, usually appear to be stiffer at the root and more compliant at the tip. Recent computational works on idealized propulsors have shown that such distribution of flexibility may lead to better performances compared to the homogeneous case, both in thrust production and efficiency, with peaks related to fluid-structure resonances. In this talk, I will present our experimental results on tapered elastomeric flappers moving in silicon oil. Different sets of tapered flappers are carefully fabricated to fix some chosen parameters, such as the total mass or, more importantly, the mean stiffness. The control over the latter quantity has often been overlooked in recent experimental studies on flexible propulsor. Our method fills this gap and enables us to investigate the role of the distribution of stiffness accurately. Our results qualitatively concur with the above considered computational work, showing band-broadening for power and thrust production at the resonant peak, as well as an overall higher efficiency for samples with increasing tapering. These results simultaneously further our understanding of flapping propulsion and might orient the design of small aquatic or air vehicles in the future. Investigation on the wake structure, and more generally, on the physical mechanisms leading to such an increase are still ongoing.

Bio: Pierre Leroy-Calatayud obtained his B.Sc. and M.Sc. in Physics from the Swiss Federal Institute of Technology in Zurich (ETH Zürich) in 2017 and 2020, respectively. He initially joined the fleXLab at EPFL as an exchange student to perform his Master’s Thesis. Since July 2020, he is working there as a research assistant (within the EPFL’s Master Valorization program). During his studies, he got the chance to get some experience in various fields of experimental physics such as quantum magnetism at ETH, physics of exotic atoms at CERN, or acoustic metamaterials at Hokkaido University.