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Écoulements cisaillés en proche paroi : Effets macroscopiques de morphologies de surface microstructurées – Microsillon

Submission summary

Background: The proposed research takes place within the general context of wall-bounded shear flows and concerns both internal and external flows. This class of flows is found in a variety of practical configurations such as aircraft wings, ship hulls, turbomachinery elements, cooling fans... In all these applications, the global performance (viscous dissipation, transition to turbulence, thermal transfer rates, noise levels) closely depends on the near-wall flow structures. Innovative designs thus require a better understanding of the near-wall dynamics. Recent research has been carried out by the project members in this field. It has been shown that the boundary layer over a flat plate may be stabilized by a row of small wall-mounted obstacles, and that serigraphied roughness patterns may enhance the convective heat transfer. For the flow produced by a rotating disk, a full analysis of local stability properties has shown that transition may be delayed by applying specific perturbations. In all these investigations, the global dynamics of the system are modified by local application of small amplitude perturbations. Objectives: For flows in contact with a solid boundary, the objective of this proposal is to influence the global dynamics by using a microstructured surface morphology: "microgrooves" of sub-millimeter scale engraved according to well-defined patterns. This project will address both laminar boundary layers prior to transition, where microstructures may exploit specific instabilities, as well as turbulent boundary layers, exhibiting unstable dynamical structures, also influenced by the small details of the surface. A fully three-dimensional investigation of the boundary layer dynamics will allow the role played by shape, size, anisotropy, asymmetry... of the microstructured patterns to be identified. Description of project and methodology: By bringing together, in the same team, complementary expertise in different areas, this investigation will initiate a new research direction at LMFA, made possible by the recent creation of the Lyon Institute of Nanotechnologies (INL) and the associated Technology Platform located on the École Centrale de Lyon campus. The proposed research is a blend of theoretical analyses, numerical simulations and experimental realizations, in close interaction. Since the aim is the development of general methods applicable to a wide class of flows, particularly relevant flow configurations displaying a simple geometry will be considered: (1) Boundary layer over a flat plate This two-dimensional flow displays viscous instabilities and transient spatial amplification due to non-normal dynamics. When this two-dimensional flow is subjected to three-dimensional perturbations, its stability properties are deeply modified. (2) Boundary layer produced by a rotating disk This flow is the archetype of three-dimensional boundary layers. It is strongly unstable and its primary and secondary stability features begin to be well established. The participants in this project have a thorough understanding (both theoretical and experimental) of these two flow configurations, of which experimental realizations already exist at LMFA. In this project, the aim is to build on previously acquired expertise and to address the collective behaviour resulting from microstructured patterns engraved on the surface. The choice of the shape and structure of these patterns will be guided by the numerical computation of the relevant stability properties. The different surface morphologies will be manufactured by lithography on silicon wafers, by the technological platform of INL, next door to LMFA. To carry out this project, the existing experimental facilities must be adapted and improved. Investigations and measurements of the very near wall dynamics require a facility with a controlled environment (air flow, dust levels, temperature). These requirements are relatively uncommon in classical fluid mechanics laboratories. However, at LMFA there is the possibility to refurbish an existing room, no longer in use, so as to create the necessary controlled experimental environment. Expected results: A better understanding of the role played by surface morphology in heat transfer rates, viscous dissipation and transition to turbulence. Identification of optimal arrangements of microchannels, as a function of the desired global performance. Applications in the aeronautics and electronics (cooling of microchips) industries.

Project coordination

Benoît PIER (Organisme de recherche)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Help of the ANR 140,000 euros
Beginning and duration of the scientific project: - 36 Months

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