Pulsed-Field-Gradient Measurements of Time-Dependent Gas Diffusion

Pulsed-field-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O₂, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 × 10⁻⁶m²s⁻¹for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times.     

On the Influence of Polynomial De-aliasing on Subgrid Scale Models

In this work we investigate the interplay of polynomial de-aliasing and sub-grid scale models for large eddy simulations based on discontinuous Galerkin discretizations. It is known that stability is a major concern when simulating underresolved turbulent flows with high order nodal collocation type discretizations. By changing the interpolatory character of the nodal collocation type discretization to a projection based discretization by increasing the number of quadrature points (polynomial de-aliasing), one is able to remove the aliasing induced stability problems. We focus on this effect and on the consequence for large eddy simulations with explicit subgrid scale models. Often, subgrid scale models have to achieve two possibly conflicting tasks in a single simulation: firstly stabilizing the numerics and secondly modeling the physical effect of the missing scales. Within a discontinuous Galerkin approach, it is possible to use either a fast (but potentially aliasing afflicted) nodal collocation discretization or a projection-based (but computationally costly) variant in combination with an explicit subgrid scale model. We use this framework to investigate the effect on the appropriate model parameter of a standard Smagorinsky subgrid scale model and of a Variational Multiscale Smagorinsky formulation. For this we first consider the 3-D viscous Taylor-Green vortex example to investigate the impact on the stability of the method and second the turbulent flow past a circular cylinder to investigate and compare the accuracy of the results. We show that the aliasing instabilities of collocative discretizations severely limit the choice of the model constant, in particular for high order schemes, while for de-aliased DG schemes, the closure model parameters can be chosen independently from the numerical scheme. For the cylinder flow, we also find that for the same model settings, the projection-based results are in better agreement with the reference DNS than those of the collocative scheme.     

Motion of micrometer sized spherical particles exposed to a transient radial flow: attraction, repulsion, and rotation

It is now accepted that the physical forces in ultrasonic cleaning are due to strongly pulsating bubbles driven by the sound field. Here we have a detailed look at bubble induced cleaning flow by analyzing the transport of an individual particle near an expanding and collapsing bubble. The induced particulate transport is compared with a force balance model. We find two important properties of the flow which explain why bubbles are effectively cleaning: During bubble expansion a strong shear layer loosens the particle from the surface through particle spinning and secondly an unsteady boundary layer generates an attractive force, thus collecting the contamination in the bubble's close proximity.     

ESPI solution for non-contacting MEMS-on-wafer testing

Rapid progress in the field of micro-electro-mechanical systems (MEMS) makes the development of appropriate measuring and testing means timely. Characterizing the mechanical properties of MEMS structures at a very early stage of manufacturing is a challenging task for quality assurance in this field. The paper describes a new solution that is based upon the vibration analysis of the microparts. The nanometer amplitudes are detected by advanced electronic speckle pattern interferometry (ESPI). A specific signal processing technique has been applied to make the solution robust. Comprehensive numerical simulations provide the theoretical base for the HNDT concept. A laboratory system for 4″ wafer has been built, and extensive tests show that such key properties as e.g. the thickness of springs or membranes can be determined exactly. Automated frequency scanning and corresponding digital image processing open the way to reliable and fast industrial systems for MEMS testing on wafer level.     

The Peculiar Dynamics of Cavitation Bubbles

Details from cavitation bubble dynamics are reported: jet formation, counterjet formation, shock wave radiation and light emission. Multiple shock wave radiation from single bubble collapse with jet formation could be time resolved by high speed photography with 20 million frames per second. An explanation of counterjet formation is given. Pictures of the light emission (sonoluminescence) in acoustic cavitation are presented.     

Dielectric,electro-optical and thermodynamic investigation of binary mixtures formed by pyridine with hydrocarbons and chlorobenzene

The dielectric constant, Kerr constant, excess enthalpy, and excess Gibbs energy of binary systems formed by pyridine with cyclohexane, benzene, ethylbenzene and chlorobenzene were experimentally determined. Complex analysis of the data obtained was carried out.The results obtained exhibit a different degree of association of the pyridine molecules, depending on the second component present in the mixture investigated. The structure and properties of the mixtures are discussed in detail.