Spectral instability of characteristic boundary layer flows. (English) Zbl 1359.35129

Grenier, Emmanuel; Guo, Yan; Nguyen, Toan T.

Duke Math. J. 165, No. 16, 3085-3146 (2016).

Authors’ abstract: In this paper, we construct growing modes of the linearized Navier-Stokes equations about generic stationary shear flows of the boundary layer type in a regime of a sufficiently large Reynolds number: \( R \to \infty \). Notably, the shear profiles are allowed to be linearly stable at the infinite Reynolds number limit, and so the instability presented is purely due to the presence of viscosity. The formal construction of approximate modes is well documented in physics literature, going back to the work of Heisenberg, C. C. Lin, Tollmien, Drazin, and Reid, but a rigorous construction requires delicate mathematical details, involving, for instance, a treatment of primitive Airy functions and singular solutions. Our analysis gives exact unstable eigenvalues and eigenfunctions, showing that the solution could grow slowly at the rate \( e^{t/\sqrt{R}} \). The proof follows the general iterative approach introduced in our companion paper, avoiding having to deal with matching inner and outer asymptotic expansions, but instead involving a careful study of singularity in the critical layers by deriving pointwise bounds on the Green function of the corresponding Rayleigh and Airy operators. Unlike in the channel flows, the spatial domain in the boundary layers is unbounded and the iterative scheme is likely to diverge due to the linear growth in the vertical variable. We introduce a new iterative scheme to simultaneously treat the singularity near critical layers and the asymptotic behavior of solutions at infinity. The instability of generic boundary layers obtained in this paper is linked to the emergence of Tollmien-Schlichting waves in describing the early stage of the transition from laminar to turbulent flows.

Reviewer: Guido Schneider (Stuttgart)

MSC:

35Q30	Navier-Stokes equations
35B35	Stability in context of PDEs
35B25	Singular perturbations in context of PDEs
76D05	Navier-Stokes equations for incompressible viscous fluids
76E05	Parallel shear flows in hydrodynamic stability
35B40	Asymptotic behavior of solutions to PDEs

Keywords:

spectral instability; boundary layers; Navier-Stokes equations; Rayleigh equations; Airy equation; Tollmien-Schlichting waves