Estelle Guyez

Turbulent mixing at a stable density interface : the variation of the buoyancy flux–gradient relation

Experiments conducted on mixing across a stable density interface in a turbulent Taylor–Couette flow show, for the first time, experimental evidence of an increase in mixing efficiency at large Richardson numbers. With increasing buoyancy gradient the buoyancy flux first passes a maximum, then decreases and at large values of the buoyancy gradient the flux increases again. Thus, the curve of buoyancy flux versus buoyancy gradient tends to be N-shaped (rather than simply bell shaped), a behaviour suggested by the model of Balmforth et al. (J. Fluid Mech. vol. 428, 1998, p. 349). The increase in mixing efficiency at large Richardson numbers is attributed to a scale separation of the eddies active in mixing at the interface; when the buoyancy gradient is large mean kinetic energy is injected at scales much smaller than the eddy size fixed by the gap width, thus decreasing the eddy turnover time. Observations show that there is no noticeable change in interface thickness when the mixing efficiency increases; it is the mixing mechanism that changes. The curves of buoyancy flux versus buoyancy gradient also show a large variability for identical experimental conditions. These variations occur at time scales one to two orders of magnitude larger than the eddy turnover time scale.

Effects of particle properties on segregation-band drift in particle-laden rimming flow

We experimentally study rimming flow of a particle-laden fluid. We begin to investigate the details of the spatiotemporal segregation-band dynamics that were first documented by us elsewhere [E. Guyez and P. J. Thomas, Phys. Rev. Lett. 100, 074501 (2008)]. There exist eight relevant nondimensional parameters that must be expected to affect the drift dynamics of segregation bands in particle-laden rimming flow. Here we summarize results from experiments investigating the effects of three of these parameters that involve the particle size and the particle density. It is shown that two of the parameters are crucial to the initiation of the band drift and that bands become stationary whenever either one of the two parameters adopts values below an associated critical threshold. Based on the physical relevance of the two parameters it is concluded that the initiation of band drift is strongly affected by a competition between capillary forces and gravitational forces. The third nondimensional parameter studied...

Positron emission tracking of individual particles in particle-laden rimming flow

The motion of a single tracer particle in particle-laden rimming flows is investigated experimentally by means of Positron Emission Particle Tracking (PEPT). Semi-dilute suspensions, with a volume fraction of 8% of heavy particles are considered. The trajectory of the tracer particle is monitored for several thousand cylinder revolutions and related to the optically recorded drift of the large-scale granular segregation bands developing in the cylinder. Results of the data analysis provide first insights into the relation between behaviour of individual particles and the spatiotemporal dynamics displayed by the macroscopic particle-segregation patterns.

Interfacial Mixing by Horizontal Vortices and Shear Turbulence

In this paper the entrainment rates of shear flows and turbulent Taylor vortices are considered in the light of the mixed-layer deepening in oceans, seas and lakes. The entrainment by Taylor vortices can be considered to represent a model for entrainment by continuously driven horizontal vortices such as Langmuir circulation. When the forcing is related to the surface wind stress, the interfacial entrainment by Langmuir vortices can be estimated from the experimental results on turbulent Taylor vortices, and compared with the entrainment rates in pure shear-flows. The results indicate that mixing by Langmuir vortices is important for all Richardson numbers, Ri ∗ (based on wind-induced surface stress velocity), and that for Ri ∗ > 80 Langmuir vortices dominate the mixed-layer deepening.