Tomographic diffractive microscopy with a wavefront sensor

Tomographic diffractive microscopy is a recent imaging technique that reconstructs quantitatively the three-dimensional permittivity map of a sample with a resolution better than that of conventional wide-field microscopy. Its main drawbacks lie in the complexity of the setup and in the slowness of the image recording as both the amplitude and the phase of the field scattered by the sample need to be measured for hundreds of successive illumination angles. In this Letter, we show that, using a wavefront sensor, tomographic diffractive microscopy can be implemented easily on a conventional microscope. Moreover, the number of illuminations can be dramatically decreased if a constrained reconstruction algorithm is used to recover the sample map of permittivity.     

Hybrid passive/active absorbers for flow ducts

A new type of acoustic liner developed for broadband noise reduction in flow ducts is considered in this paper. It combines passive absorbent properties of a porous layer and active control at its rear face. The complete design procedure of this hybrid passive/active liner is developed here. The passive part is first considered with the determination of a suitable porous material and the cut-off frequency separating the active low frequency regime from the passive high frequency one. The control system is then presented: a digital adaptive feedback control is performed independently cell by cell, allowing an easy subsequent increase of the liner surface. The entire optimization process has been successfully applied to a laboratory flow duct: both predictions and measurements show the interest of the hybrid liner to reduce the noise radiation.     

On a boundary layer problem for the nonlinear Boltzmann equation

This article deals with a boundary-layer problem arising in the kinetic theory of gases when the mean free path of molecules tends to zero. The model considered here is the stationary, nonlinear Boltzmann equation in one dimension with a slightly perturbed reflection boundary condition. We restrict our attention to the case of hard spheres collisions, with Grad's cutoff assumption. Existence, uniqueness and asymptotic behavior are derived by means of energy estimates.     

Femtosecond electronic relaxation of excited metalloporphyrins in the gas phase

A systematic study of the ultrafast decay of metalloporphyrins containing various transition metals with partially filled 3d shells and zinc (3d filled) is reported here after excitation in the second excited state of the system (Soret band). Both time-of-flight mass spectrometry and velocity map imaging have been used for detection. A general biexponential decay with a short time constant tau1 approximately 100 fs is observed for the transition metal porphyrins, followed by a tau2 approximately 1 ps time decay. This evolution is interpreted as a porphyrin-to-metal charge transfer, tau1, followed by a back transfer, tau2, which leads to an excited state (d,d*) localized on the metal. These conclusions stem from the different behaviors of zinc and the transition metal porphyrins. A porphyrin-to-metal charge transfer model is chosen to describe the relaxation mechanism, based upon the fact that transition metalloporphyrins can accept electrons on the metal site, in contrast to zinc porphyrins.     

Homogeneous turbulence subjected to rotation-dominated plane distortion

A new configuration of distorted homogeneous turbulence is investigated in the domain of rotation-dominated elliptical flows. The experimental results are compared with exact numerical solutions obtained in Fourier space for the linear part of the problem. The inherently periodic character of the flow results in typical oscillatory variations of the anisotropy and the pressure-strain correlations. A new two-point closure approach of the EDQNM type is proposed for the nonlinear problem; it is based on a representation of the spectral tensor by scalar functions expanded in terms of spherical harmonics.     

Large Eddy Simulation of a Motored Single-Cylinder Piston Engine: Numerical Strategies and Validation

This paper describes a compressible Large Eddy Simulation (LES) used to investigate cyclic variations for nonreacting flow in an optical single cylinder engine setup. The simulated operating point is part of a large experimental database designed to validate LES for cycle-to-cycle prediction, and constitutes a first step towards the realization of fired operating points. The computational domain covers almost the whole experimental setup (intake and exhaust plenums, intake and exhaust ducts, cylinder) to account for acoustic phenomena. The assessment of the computation is performed in two regions of the domain: the intake and exhaust duct predictions are compared to the results of a Helmholtz solver and the experiment (pressure transducers and Particle Image Velocimetry (PIV)) while the in-cylinder dynamics are compared to PIV measurements. The ability of the developed methodology to capture the correct level of cycle-to-cycle variations is demonstrated considering in-cylinder pressure and velocity fields predictions. Cycle-to-cycle variations in velocity are highlighted and localized using a proper orthogonal decomposition analysis.     

Topography Influence on the Lake Equations in Bounded Domains

We investigate the influence of the topography on the lake equations which describe the two-dimensional horizontal velocity of a three-dimensional incompressible flow. We show that the lake equations are structurally stable under Hausdorff approximations of the fluid domain and L ^p perturbations of the depth. As a byproduct, we obtain the existence of a weak solution to the lake equations in the case of singular domains and rough bottoms. Our result thus extends earlier works by Bresch and Métivier treating the lake equations with a fixed topography and by Gérard-Varet and Lacave treating the Euler equations in singular domains.