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Interactions of Fluid Flows and Porous Structures

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Event details

Date and time 27.09.2018 16:1517:30  
Place and room
Speaker Pier Giuseppe Ledda, EPFL STI IGM Laboratory of Fluid Mechanics and Instabilities (LFMI); Matteo Pezzulla, EPFL STI IGM  Flexible Structures Laboratory (FleXLab)  
Category Conferences - Seminars
Talk 1: On the stability of wake flows past porous bluff bodies
Abstract 
The characterization of wake flows is typically made in the context of solid bodies. However, often in Nature and in many practical applications, porous media are present. The present work aims at numerically investigating the effect of the porosity and permeability on the wake patterns of porous bluff bodies at low-moderate Reynolds numbers. This study is developed in the framework of direct numerical simulation and stability analysis. To describe the flow behavior inside the porous media, a modified Darcy-Brinkman formulation is employed, where also the convective terms are retained to correctly account for the inertia effects at high values of permeability. First, the two-dimensional flow past rectangular cylinders is investigated, considering thickness-to-height ratios, t/d, ranging from 0.01 (at plate) to 1.0 (square cylinder) (see Figure 1a). The cases of the flow past a sphere and a disk are then studied, both for the first steady and the second unsteady bifurcation. The results show that the permeability of the bodies has a strong effect in modifying the characteristics of the flow instabilities, while the porosity weakly affects the flow patterns. In particular, the fluid flows through the porous bodies and thus, increasing the permeability, the recirculation regions detach from the body first and then disappear in the near wakes. Moreover, for all the configurations here investigated, critical values of the permeability have been identified above which any instabilities are prevented, at least in the parameters' space considered. As an example, Figure 1b shows the stability neutral curve (red line) for the case of the rectangular cylinder with t/d=0.25, together with the iso-contour of recirculation length L = 0 (black line) that delimits the solutions with detached and no recirculation regions.

Talk 2: Deformation of perforated elastic sheets due to the hydrodynamic loading by a viscous fluid
Abstract 
From spider webs and insect wings, to wire fences and parachutes, Nature and technology provide us with vast examples of perforated flexible structures that undergo elastic deformation due to fluid flow. Whereas fluid flow through porous media has been studied extensively, the fluid-structure interactions of a perforated slender elastic object that undergoes large deformations due to the hydrodynamic loading of a surrounding viscous fluid has received much less attention. Here, we use precision desktop experiments to focus on the prototypical problem of a perforated elastic plate moving through a viscous fluid, at low to moderate Reynolds number. We seek to provide a predictive framework for the deformation of perforated plates due to hydrodynamic loading to rationalize our experimental findings. For this purpose, we use a reduced theoretical model based on Kirchhoff-Euler beam theory coupled with a low Reynolds number description for the fluid forcing. Specifically, we quantify the effect of the interplay between elasticity, permeability, and viscous loading on the deformed shape of the structure. We hope that our findings may lead to a better understanding of fluid-structure interactions between slender porous structures and viscous flows, across biological and technological applications.

Practical information

  • General public
  • Free

Organizer

  • MEGA.Seminar Organizing Committee

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Tags

Solids Structures Fluids

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