Shear-induced sedimentation in yield stress fluids

Stability of coarse particles against gravity is an important issue in dense suspensions (fresh concrete, foodstuff, etc.). On the one hand, it is known that they are stable at rest when the interstitial paste has a high enough yield stress; on the other hand, it is not yet possible to predict if a given material will remain homogeneous during a flow. Using MRI techniques, we study the time evolution of the particle volume fraction during the flows in a Couette geometry of model density-mismatched suspensions of noncolloidal particles in yield stress fluids. We observe that shear induces sedimentation of the particles in all systems, which are stable at rest. The sedimentation velocity is observed to increase with increasing shear rate and particle diameter, and to decrease with increasing yield stress of the interstitial fluid. At low shear rate ('plastic regime'), we show that this phenomenon can be modelled by considering that the interstitial fluid behaves like a viscous fluid -- of viscosity equal to the apparent viscosity of the sheared fluid -- in the direction orthogonal to shear. The behavior at higher shear rates, when viscous effects start to be important, is also discussed. We finally study the dependence of the sedimentation velocity on the particle volume fraction, and show that its modelling requires estimating the local shear rate in the interstitial fluid

A topology-based animation model for the description of 2D models with a dynamic structure.

International audienceThis paper presents a model that describes the temporal evolution of 2D-topological structures to represent and control dynamic natural phenomena. As input, the user provides the system with a list of actions that gives a high-level description of the evolution in terms of application-specific operations. As output, a complete representation of the evolution is computed. Our model is composed of three parts: A structural model allowing the temporal representation of both topology and geometry; an event model that aims at detecting topological modifications and ensures consistency between topology and geometry; and a semantic model that simultaneously describes the evolution as a sequence of elementary modifications and manages the history of the various entities of the model. We show the efficiency of the model in the geology field, by studying two well-known phenomena, namely sedimentation and erosion

Object Tracking with a pan-tilt-zoom camera : application to car driving assistance

International audienceIn this paper, visual perception in car driving assistance is considered. The work deals with the development of a system combining a pan-tilt-zoom (PTZ) camera and a standard camera, in order to track the front vehicles. The standard camera has a small focal length, and is devoted to the analyse of the whole frontal scene. Here, the PTZ camera is used to track the closest vehicle. Camera rotations and zoom are controlled by visual servoing and by an efficient real time target tracking algorithm. The aim of this work is to keep the rear view image of target vehicle stable in scale and position. The methods presented were tested on real road sequences within the VELAC demonstration vehicle. Experimental results show the effectiveness of such an approach

Tracking with a pan-tilt-zoom camera for an ACC system

International audienceIn this paper, visual perception of frontal view in intelligent cars is considered. A Pan-Tilt-Zoom (PTZ) camera is used to track preceding vehicles. The aim of this work is to keep the rear view image of the target vehicle stable in scale and position. An efficient real time tracking algorithm is integrated. It is a generic and robust approach, particularly well suited for the detection of scale changes. The camera rotations and zoom are controlled by visual servoing. The methods presented here were tested on real road sequences within the VELAC demonstration vehicle. Experimental results show the effectiveness of such an approach. The perspectives are in the development of a visual sensor combining a PTZ camera and a standard camera. The standard camera has small focal length and is devoted to an analysis of the whole frontal scene. The PTZ camera gives a local view of this scene to increase sensor range and precision