Arnaud Antkowiak []

Latest news:

[November, 2015]

New internship proposal at Saint-Gobain Recherche (possibility for a PhD thereafter)

Research Activities

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.

Liquid fragmentation

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)

Amplication Mechanisms in Vortices

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.

Anti-lift-up scenario

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".

Transient Resonance

To be written.

Subcritical transition in swirling flows

Tripole formation

To be written.

Last Modified: November 05, 2015 - HTML 5 - css