3 research outputs found
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
Quantitative visualization of swirl and cloud bubbles in Taylor–Couette flow
We develop a novel method to study the gas phase features in a bubbly Taylor–Couette flow when bubbles are arranged as elevated toroidal strings. The flow is recorded in the front view plane with a high speed camera for a Reynolds number of 1500 and a global void fraction of 0.14 %. An image processing algorithm is developed to discriminate bubbles accumulated in clouds near the inner cylinder (cloud bubbles) from bubbles trapped in the bulk flow by vortices (swirl bubbles). The analysis of the preferential positions, azimuthal velocities, and equivalent void fraction of clouds and swirl bubbles separately provides a new insight into the dynamics of the bubble’s entrapment