Fluid-structure interaction of membrane aerofoils at low Reynolds numbers

Abstract

This thesis investigates the fluid-structure interaction (FSI) problem of elastic membrane aerofoils at low Reynolds numbers. The dynamics of the fluid and membrane coupled system is studied via direct numerical simulation (DNS) using a newly developed computational framework whose characteristics and validation are included in this report. A set of two-dimensional DNS were performed for varying Reynolds number, membrane elasticity and aerofoil geometry in order to investigate the effect of these relevant fluid and structural parameters on the behaviour of fluid-structure coupled system. Static and dynamic features of the system, and their effect in aerodynamic properties, are described and compared for the different parameter combinations. The case with highest Reynolds number, Re = 10; 000, and intermediate elasticity was chosen as a base case to further study the fluid-structure coupling mechanism, particularly at low angle of attack conditions. The dynamic behaviour was characterised via spectral analysis in the frequency and wavenumber-frequency domains, which allowed the propagating wave nature of the membrane vibrations and their effect on the surrounding pressure field fluctuations to be clarified. The membrane vibrations are found to introduce upstream-propagating pressure waves that seem to be responsible for a loss in aerodynamic efficiency compared to a rigid aerofoil. Stability aspects of the FSI problem are also investigated by performing numerical experiments to analyse the response of the system to initial flow perturbations. The solutions of the 2D DNS are used as initial conditions for three-dimensional simulations, upon which initial perturbations in spanwise velocity are added. As the simulation is advanced in time the evolution of the perturbations is studied to determine the stability characteristics of the flow. Amplifications of the perturbations are found for Re > 10; 000. The coupling of the fully three-dimensional developed flow and the elastic aerofoil is also analysed with spectral techniques. Comparison of two- and three-dimensional results reveals that the three-dimensional flow development causes a decrease in the amplitude of the system fluctuations, but the same coupling mechanism found in the two-dimensional approach is also present in the three-dimensional case.

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ORCID for Neil Sandham: orcid.org/0000-0002-5107-0944

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