New internship proposal at Saint-Gobain Recherche (possibility for a PhD thereafter)
Some highlights on my current research.
In the following are presented my current research activities. These are only briefly sketched, so feel free to contact me whenever you want some details.
Some details coming soon. In the meantime, why not take a look at this interesting movie illustrating the fragmentation of a stretched liquid ligament ? (here's a heavier but more compatible version of the movie)
In the context of plane shear flows, it is now known that two amplification mechanisms play a crucial role in the transition process. Recent studies (e.g. Hof et al., Science, 2004) have proven that at least one of these, the so-called lift-up effect, was intimately linked to the existence of nonlinear travelling wave in plane shear flows. Moreover, these mechanisms control the variability of the flow, i.e. are responsible for a process of selection and amplification leading the flow to display some specific response when excited with a random noise (e.g. Farrell & Ioannou, Phys. Fluids, 1993). For example, a boundary layer will have a natural "tendency" to exhibit streaks of high and low longitudinal speed in the presence of free-stream disturbances. The study of the amplification mechanisms provides with an insight in to this intrinsic behaviour of the flow.
During this PhD thesis, we have been able to isolate and identify some original amplification mechanisms active in swirling flows. These mechanisms are believed to be of fundamental importance in the understanding of the behaviour of vortices, just as their counterparts in plane shear flows are key features of the transition process in this kind of flows.
In plane shear flows, such as boundary layers, the systematic
formation of streaks of high and low velocity is usually
ascribed to an intrisic property of the flows referred to
as the "lift-up effect". This mechanism takes advantage
of the ambient shear to transform initially weak streamwise rolls
into strong streamwise streaks. So, as under white noise forcing
conditions, rolls are allways (even slightly) excited, streaks
allways emerge in response. Even if this schematic view does not
explain subtleties as streaks spacing for example, it certainly
points out some of the keys allowing the understanding of plane
shear flows behaviours.
Analogously, we recently identified in swirling flows a streamwise-independant
mechanism able of strongly amplify axisymmetric disturbances. But this
time, the mechanism acts in the opposite way, as it now transforms
weak streaks of high and low velocity into strong rolls ! This
"anti-lift-up" scenario is explained using the peculiarities
of the flow, namely shear and rotation, and the associated conservation
laws (angular momentum conservation), all generic to vortices.
This new mechanism may shed new light on the systematic formation of
vortex rings developing at the periphery of a vortex embedded
in an external turbulence (e.g. Melander & Hussain, 1993).
The anti-lift-up scenario is described in a forthcoming
article called "Amplification mechanisms in vortices".
To be written.
To be written.
