Avoiding moving obstacles

To successfully move our hand to a target, we must consider how to get there without hitting surrounding objects. In a dynamic environment this involves being able to respond quickly when our relationship with surrounding objects changes. People adjust their hand movements with a latency of about 120 ms when the visually perceived position of their hand or of the target suddenly changes. It is not known whether people can react as quickly when the position of an obstacle changes. Here we show that quick responses of the hand to changes in obstacle position are possible, but that these responses are direct reactions to the motion in the surrounding. True adjustments to the changed position of the obstacle appeared at much longer latencies (about 200 ms). This is even so when the possible change is predictable. Apparently, our brain uses certain information exceptionally quickly for guiding our movements, at the expense of not always responding adequately. For reaching a target that changes position, one must at some time move in the same direction as the target did. For avoiding obstacles that change position, moving in the same direction as the obstacle is not always an adequate response, not only because it may be easier to avoid the obstacle by moving the other way, but also because one wants to hit the target after passing the obstacle. Perhaps subjects nevertheless quickly respond in the direction of motion because this helps avoid collisions when pressed for time. © 2008 Springer-Verlag

Sur les possibilités d'application de la théorie du filtrage optimum à la conception de chaînes détectrices de précision pour spectrométrie nucléaire

The authors try to find a new method for deriving low noise detector amplifiers. Wiener optimum filtering allows them to compute the electrical characteristics of a " linear filtering stage ". The low theoretical noise level shows that the use of Wiener filtering can improve the accuracy of nuclear spectrometry. The main problem is the practical construction of a Wiener filter. Using a half sine wave as input signal, it appears that the filter synthesis is near that of a wide band with tuned circuit. An approach to a Wiener filter can be made using high frequency transformers.On recherche une nouvelle méthode de calcul des amplificateurs à faible bruit pour détecteurs. Le filtrage optimum de Wiener permet de calculer les caractéristiques électriques d'un étage « filtrage linéaire ». Le faible niveau de bruit, calculé théoriquement, montre que l'emploi du filtrage de Wiener peut améliorer la précision de la spectrométrie nucléaire. Le problème principal est la synthèse pratique d'un filtre de Wiener. En employant comme signal d'entrée une demi-sinusoïde, il apparaît que la synthèse est proche de celle d'un circuit accordé à large bande. L'emploi des transformateurs haute fréquence permet une synthèse approchée d'un tel filtre

The subcritical transition to turbulence of Faraday waves in miscible fluids

International audienceWe study the development and the breaking process of standing waves at the interface between two miscible fluids of small density contrast. In our experiment, a subharmonic wave is generated by a time-periodic vertical acceleration via the Faraday instability. It is shown that its wavelength may be selected not only by the linear process predicted by the Floquet theory and favouring the most unstable modes allowed by the tank geometry, but also by a nonlinear mode competition mechanism giving the preference to subcritical modes. Subsequently, as the standing wave amplitude grows, a secondary destabilization process occurs at smaller scales and produces turbulent mixing at the nodes. We explain this phenomenon as a subcritical parametric resonance instability. Different approaches derived from local and global stability analysis are proposed to predict the critical wave steepness. These theories are then assessed against various numerical and experimental data varying the frequencies and amplitudes of the forcing acceleration

Permanence des grandes échelles en turbulence homogène avec densité variable

The principle of permanence of large eddies is one of the central pillars onto which our understanding of decaying homogeneous turbulence is built. The validity conditions of this principle have been thoroughly discussed for constant density flows, but not for variable-density ones. In this work, we show that density non-uniformities modify the remote action of the pressure field. It results into distant velocity correlations being submitted to a stronger non-linear transfer of energy. A simple example is proposed to illustrate this property and a spectral analysis of non-linear transfer terms is undertook to further characterize it. From there, we derive that large eddies in variable density flows remain permanent for a smaller set of initial conditions than when density is constant. Permanence strictly applies to initial spectra having an infrared exponent smaller than 2 instead of 4. Implicit large-eddy simulations are performed to verify the main predictions of this work

Minimum enstrophy principle for two-dimensional inviscid flows around obstacles

International audienceLarge-scale coherent structures emerging in two-dimensional flows can be predicted from statistical physics inspired methods consisting in minimizing the global enstrophy while conserving the total energy and circulation in the Euler equations. In many situations, solid obstacles inside the domain may also constrain the flow and have to be accounted for via a minimum enstrophy principle. In this work, we detail this extended variational formulation and its numerical resolution. It is shown from applications to complex geometries containing multiple circular obstacles that the number of solutions is enhanced, allowing many possibilities of bifurcations for the large-scale structures. These phase change phenomena can explain the downstream recombinations of the flow in rod-bundle experiments and simulations