Cherry flow: physical measures and perturbation theory

In this article we consider Cherry flows on the torus which have two singularities, a source and a saddle, and no periodic orbits. We show that every Cherry flow admits a unique physical measure, whose basin has full volume. This proves a conjecture given by Saghin and Vargas [Invariant measures for Cherry flows.Comm. Math. Phys.317(1) (2013), 55–67]. We also show that the perturbation of Cherry flows depends on the divergence at the saddle: when the divergence is negative, this flow admits a neighborhood, such that any flow in this neighborhood belongs to one of the following three cases: it has a saddle connection; it is a Cherry flow; it is a Morse–Smale flow whose non-wandering set consists of two singularities and one periodic sink. In contrast, when the divergence is non-negative, this flow can be approximated by a non-hyperbolic flow with an arbitrarily large number of periodic sinks.

Acoustic–vorticity coupling in linearly varying shear flows using the WKB method

The evolution of acoustic and vorticity perturbations in a two-dimensional incompressible linear flow is investigated. A weighted decomposition of the flow into a hyperbolic part and a rotation part allows continuous spanning of all linear flows such as hyperbolic flow, plane Couette flow and rigid rotation for instance. Using the Kelvin non-modal approach, the equations governing the time evolution of plane wave perturbations are reduced into a system of three first-order ordinary differential equations. This system is analysed using a WKB method where the small parameter ε is the ratio of the shear rate of the flow over the typical frequency of the perturbations. With this method, a basis of three modes naturally appears: two acoustic modes and one vorticity mode. At finite but small ε , couplings between the modes appear when the length of the wavenumber is minimal. For hyperbolic flow, incident vorticity mode generates the two acoustic modes, and an incident acoustic mode generates the other acoustic mode. More generally, for all flows, the hyperbolic part of the flow is responsible of the coupling between acoustic and vorticity modes, but also of the coupling between the two acoustic modes. These phenomena are illustrated by displaying wavepacket evolutions.

Stable topological transitivity properties of ℝn-extensions of hyperbolic transformations

We consider ℝn skew-products of a class of hyperbolic dynamical systems. It was proved by Niţică and Pollicott [Transitivity of Euclidean extensions of Anosov diffeomorphisms. Ergod. Th. & Dynam. Sys. 25 (2005), 257–269] that for an Anosov diffeomorphism ϕ of an infranilmanifold Λ there is (subject to avoiding natural obstructions) an open and dense set f:Λ→ℝN for which the skew-product ϕf(x,v)=(ϕ(x),v+f(x)) on Λ×ℝN has a dense orbit. We prove a similar result in the context of an Axiom A hyperbolic flow on an attractor.