Non-linear effects and shock formation in the focusing of a spherical acoustic wave : Numerical simulations and experiments in liquid helium

The focusing of acoustic waves is used to study nucleation phenomena in liquids. At large amplitude, non-linear effects are important so that the magnitude of pressure or density oscillations is difficult to predict. We present a calculation of these oscillations in a spherical geometry. We show that the main source of non-linearities is the shape of the equation of state of the liquid, enhanced by the spherical geometry. We also show that the formation of shocks cannot be ignored beyond a certain oscillation amplitude. The shock length is estimated by an analytic calculation based on the characteristics method. In our numerical simulations, we have treated the shocks with a WENO scheme. We obtain a very good agreement with experimental measurements which were recently performed in liquid helium. The comparison between numerical and experimental results allows in particular to calibrate the vibration of the ceramics used to produce the wave, as a function of the applied voltage.Comment: 20 pages, 26 figures. Submitted to The European Physical Journal

On wall pressure fluctuations and their coupling with vortex dynamicsin a separate d–reattache d turbulent flow over a blunt flat plate

This study deals with the numerical predictions through Large-Eddy Simulation ( LES ) of the separated–reattached turbulent flow over a blunt flat plate for analyzing main coherent structure features and theirrelation to the unsteady pressure field. A compressible approach that inherently includes acoustic prop- agation is here followed to describe the relationship between pressure fluctuations and vortex dynam- ics around the separation bubble. The objective of the present work is then to contribute to a betterunderstanding of the coupling between the vortex dynamics and the wall pressure fluctuations. The fil- tered compressible Navier–Stokes equations are then solved with a numerical method that follows a Lax–Wendroffapproach to recover a high accuracy in both time and space. For validations, the present numer- ical results are compared to experimental measurements, coming from both the Pprime laboratory (Sicotel al., 2012) and the literature (Cherry et al., 1984; Kiya and Sasaki, 1985; Tafti and Vanka,1991; Sicotet al., 2012). Our numerical results very well predict mean and fluctuating pressure and velocity fields.Flapping, shedding as well as Kelvin–Helmholtz characteristic frequencies educed by present simulationsare in very good agreement with the experimental values generally admitted. These characteristic modesare also visible on unsteady pressure signatures even far away from the separation. Spectral, POD andEPOD (extended POD) analyses are then applied to these numerical data to enhance the salient featuresof the pressure and velocity fields, especially the unsteady wall pressure in connection with either thevortex shedding or the low frequency shear-layer flapping. A contribution to the understanding of thecoupling between wall pressure fluctuations and eddy vortices is finally proposed

A conservative coupling algorithm between a compressible flow and a rigid body using an Embedded Boundary method

This paper deals with a new solid-fluid coupling algorithm between a rigid body and an unsteady compressible fluid flow, using an Embedded Boundary method. The coupling with a rigid body is a first step towards the coupling with a Discrete Element method. The flow is computed using a Finite Volume approach on a Cartesian grid. The expression of numerical fluxes does not affect the general coupling algorithm and we use a one-step high-order scheme proposed by Daru and Tenaud [Daru V,Tenaud C., J. Comput. Phys. 2004]. The Embedded Boundary method is used to integrate the presence of a solid boundary in the fluid. The coupling algorithm is totally explicit and ensures exact mass conservation and a balance of momentum and energy between the fluid and the solid. It is shown that the scheme preserves uniform movement of both fluid and solid and introduces no numerical boundary roughness. The effciency of the method is demonstrated on challenging one- and two-dimensional benchmarks

Shedding Intermittency in a Shock Wave-Laminar Boundary Layer Interaction

International audienc

Intercellular trafficking and enhanced in vivo antitumour activity of a non-virally delivered P27-VP22 fusion protein

International audienceVP22, a structural protein from herpes simplex virus type I, exhibits the unique property of intercellular trafficking. This protein is exported from primary expressing cells and subsequently imported into neighbouring cells. This property is conserved when VP22 is genetically fused to a protein, making it a promising tool to enhance the delivery of a gene product. We chose to study the intercellular transport and biological effect of a fusion protein between the putative tumour suppressor gene p27(Kip1) and VP22. We show that in vitro, P27VP22 is able to spread as efficiently as VP22. Functionality of the P27VP22 protein was demonstrated by its ability to inhibit cyclin/CDK2 complexes activity. In proliferation and clonogenicity assays, transfection with the P27VP22 plasmid resulted in a stronger cell growth inhibition when compared to transfection with the p27(Kip1) vector. In vivo, sub cutaneous tumours established in nude mice were injected with naked DNA encoding P27 or P27VP22. Our results show that P27VP22 can spread in vivo and that injections of the P27VP22 plasmid resulted in a significantly greater antitumour activity than injections of the P27 plasmid. This study confirms the usefulness of VP22-mediated delivery and suggests that P27VP22 may have applications in cancer gene therapy

sexhy experimental results on pressure dynamics from head-on reflections of hydrogen flames

International audienceIn the past few years, CEA has been fully involved at both experimental and modeling levels in projects related to hydrogen safety in nuclear and chemical industries, and has carried out a test program using the experimental bench SSEXHY (Structure Submitted to an EXplosion of HYdrogen) in order to build a database of the deformations of simple structures following an internal hydrogen explosion. Different propagation regimes of explosions were studied, varying from detonations to slow deflagrations. During the experimental campaign, it was found that high-speed deflagrations, corresponding to relatively poor hydrogen-air mixtures, resulted in higher specimen deformation compared to those related to detonations of nearly stoichiometric mixtures. This paper explains this counter-intuitive result from qualitative and quantitative points of view. It is shown that the overpressure and impulse from head-on reflections of hydrogen flames corresponding to poor mixtures of specific concentrations could have very high values at the tube end

sexhy experimental results on pressure dynamics from head-on reflections of hydrogen flames

International audienceIn the past few years, CEA has been fully involved at both experimental and modeling levels in projects related to hydrogen safety in nuclear and chemical industries, and has carried out a test program using the experimental bench SSEXHY (Structure Submitted to an EXplosion of HYdrogen) in order to build a database of the deformations of simple structures following an internal hydrogen explosion. Different propagation regimes of explosions were studied, varying from detonations to slow deflagrations. During the experimental campaign, it was found that high-speed deflagrations, corresponding to relatively poor hydrogen-air mixtures, resulted in higher specimen deformation compared to those related to detonations of nearly stoichiometric mixtures. This paper explains this counter-intuitive result from qualitative and quantitative points of view. It is shown that the overpressure and impulse from head-on reflections of hydrogen flames corresponding to poor mixtures of specific concentrations could have very high values at the tube end