Honami and monami waves are caused by large‐scale coherent vortex structures which form in shear layers generated by canopies. In order to reach new insights on the onset of such waves, the instability of these shear layers is studied. Two different approaches are used. In the first approach, the presence of the canopy is modeled via a drag coefficient, taken to vary along the canopy as by experimental indications. The second approach considers the canopy as a porous medium and different governing equations for the fluid flow are deduced. In this second case, the anisotropy of the canopy, composed by rigid cylindrical elements, is accounted for via an apparent permeability tensor. The results obtained with the latter approach approximate better experimental correlations for the synchronous oscillations of the canopy.

16 pagesInternational audienceLocal linear stability of swept and unswept incompressible boundary layers developing over compliant, fluid-saturated, porous plates is considered in the limit of small permeability. The analysis is meant to yield preliminary indications on the possible stabilization induced on the flow's hydrodynamic and hydroelastic modes by poroelastic media, such as those occurring in many natural and technological settings. As far as hydrodynamic modes are concerned, the main stabilizing effect is that of compliance, which however couples weakly to low-frequency crossflow modes. Permeability plays a damping role on hydroelastic modes, which here take the form of travelling wave flutter instabilities. The passive control of instabilities through poroelastic coatings specifically designed to selectively exploit the effect of compliance and/or permeability is a subject worthy of future research efforts

The spatial, linear stability of an incompressible parallel boundary layer flow with uniform suction through the wall is studied for the case of rigid and flexible bounding surfaces. It is demonstrated, first, that the effect of the plate’s permeability is crucial in defining the disturbance boundary conditions at the wall, and that stability limits depend strongly on it. Next, the combined effect of plate’s permeability and compliance on the onset of Tollmien-Schlichting modes is assessed. Finally, the possible insurgence of an absolute instability mode is studied, as function of the Reynolds number, the modulus of elasticity of the plate, and its permeability