Wall roughness induces asymptotic ultimate turbulence

Turbulence is omnipresent in Nature and technology, governing the transport of heat, mass, and momentum on multiple scales. For real-world applications of wall-bounded turbulence, the underlying surfaces are virtually always rough; yet characterizing and understanding the effects of wall roughness for turbulence remains a challenge, especially for rotating and thermally driven turbulence. By combining extensive experiments and numerical simulations, here, taking as example the paradigmatic Taylor-Couette system (the closed flow between two independently rotating coaxial cylinders), we show how wall roughness greatly enhances the overall transport properties and the corresponding scaling exponents. If only one of the walls is rough, we reveal that the bulk velocity is slaved to the rough side, due to the much stronger coupling to that wall by the detaching flow structures. If both walls are rough, the viscosity dependence is thoroughly eliminated in the boundary layers and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of transport, whose existence had been predicted by Robert Kraichnan in 1962 (Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be extrapolated to arbitrarily large Reynolds numbers

Wave Guide Imaging through Time Domain Topological Energy

AbstractTime Domain Topological Energy (TDTE), uses a measure of the reflected ultrasonic field on an array of transducers placed on the boundary of the imaged medium. Two numerical determinations (direct and adjoint problems) of the acoustical field inside a reference medium are then necessary to obtain the image by computing the topological energy. This technique comes from the field of shape optimisation and mathematical developments for Non Destructive Testing and have shown close links with Time Reversal (TR) concepts. TR mirrors have been employed for various applications in a wide number of situations including wave guides (WG) where very good refocalisation performances have been obtained with a reduced number of transducers instead of an array. Moreover recent works have enlighten that the reverberation properties of a WG allow to re-focalise using TR with only one transducer. For TDTE imaging we choose to model a single transducer placed at one end of a wave guide. The boundaries of the WG create virtual sources that can be understood as a virtual array of transducers. Results obtained numerically for imaging using both TDTE and one transducer in a wave guide with increasing complexity : a hard spherical object and a set of three identical objects placed at the angles of an equilateral triangle are presented and preliminary experimental results are discussed