Muriel Amielh

Erosion threshold of saturated natural cohesive sediments : Modeling and experiments

[1]xa0Predicting the erosion resistance of saturated natural sediments requires taking into account cohesion, which results from interactions between clay particles. The current paper describes a combined experimental and theoretical examination of the threshold conditions for a mixture of clays and sands. Erosion threshold measured values are larger than those predicted from noncohesive models. Beyond the usual dependence on grain size, a significant correlation between erosion threshold and porosity measurements is confirmed for heterogenous mixtures of grains with particle Reynolds number lower than 5, for pH values in the range of 6–8, and under freshwater conditions. A model of the erosion criterion is proposed. First, a cohesion force between two spherical particles is introduced into the usual erosion criterion. This reveals a specific function of the cohesion force, called cohesion function. The force we consider is the long-range van der Waals interaction. Then, multiple interactions between one particle and those surrounding it are counted and modeled on the basis of both the coordination number and the porosity. Finally, the overall erosion threshold for the sediment bed is inferred from the average of the multiple interactions over the grain size distribution. The model highlights that cohesion comes from clay particles (about 2 10−6 m) and can affect the entire grain size range (from clays to sands) by means of coordination. The model relevance is assessed by comparison with experimental thresholds obtained from resuspension campaigns. The results show that the proposed cohesion model offers good agreement with experimental data.

Investigation of Axisymmetric Underexpanded Air and Helium Jets by Background Oriented Schlieren

Investigation of Axisymmetric Underexpanded Air and Helium Jets by Background Oriented Schlieren Dubois, J.*, Amielh, M.*, Anselmet, F.* and Gentilhomme, O.* *1 Institut de Recherche sur les Phénomènes Hors Equilibre (IRPHE), CNRS-Universités AixMarseille, Technopôle de Château-Gombert, 49 rue F. Joliot Curie, B.P. 146, 13384 Marseille Cedex 13, France *2 Institut National de lEnvironnement industriel et des RISques (INERIS), Parc Technologique ALATA B.P. N°2, 60550 Verneuil-en-Halatte, France Received 10 February 2009 and Revised 31 March 2009

Thorough analysis of vortical structures in the flow around a yawed bluff body

In the present study numerical investigations are used as a complementary approach to analyse large coherent structures in the flow around a bluff-body under cross-wind. The results from the numerical simulation are validated through wind-tunnel experiments and used to analyse unresolved observations from these experimental investigations. This underlines the duality of these two approaches in the analysis of the aerodynamics of a bluff-body. Vortical structures in a three-dimensional, asymmetric and turbulent wake are detected through the use of different vortex detection schemes (vorticity, Q, λ2 and Γ3), permitting to isolate flow structures. The vortex properties of two main lateral vortices are described and discussed on the basis of envelopes detected by the λ2 and the Γ3 criterion. Although the flow is highly three-dimensional and complex, the vortex properties are quite simple and show mechanisms like vortex stretching and bursting. These two criteria lead qualitatively to the same results; quantitative differences are due to the envelope diameter and are therefore observed only on integral flow quantities.

Effect of unsteady separation on an automotive bluff-body in cross-wind

This paper is treating the dynamic effects of cross-wind on a 3D bluff-body. It is shown that contrary to the mean flow, it is not the upper vortex that predominates the forces, but the flow generated and piloted by the front lee-strut wake. It is made responsible for the appearance of a frequency peak in the bluff-body force spectra. Furthermore it is shown that the intensity as well as the frequency of this peak depend on the yawing angle. Experimental and numerical results are used to analyse the flow dynamics.

Controlling the Acoustic Resonance in a Corrugated Flow Pipe

A new experimental study, aimed at investigating the coupling between the flow in a corrugated pipe, the acoustically generated flow oscillations, and the emitted resulting noise is carried out. Hot-wire anemometry, Particle Image Velocimetry, and microphone measurements are associated to characterize the flow. The flow response to the corrugation is shown to fit to the sixth to ninth acoustic modes of the pipe according to the flow rate. When low frequency acoustically generated oscillations interfere with this, one checks that they either significantly reduce the noise level or modify the peak frequencies. In addition, theoretical/numerical works are also performed, in order to provide an analytical framework describing the acoustical properties of such corrugated pipe flows.