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排序方式: 共有70条查询结果,搜索用时 15 毫秒
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Andrea D. Beck David G. Flad Claudia Tonhäuser Gregor Gassner Claus-Dieter Munz 《Flow, Turbulence and Combustion》2016,97(2):475-511
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. 相似文献
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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. 相似文献
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Ross W. Mair David G. Cory Sharon Peled Ching-Hua Tseng Samuel Patz Ronald L. Walsworth 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,135(2):478-486
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. 相似文献
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ESPI solution for non-contacting MEMS-on-wafer testing 总被引:6,自引:0,他引:6
Petra Aswendt Claus-Dieter Schmidt Dirk Zielke Steffen Schubert 《Optics and Lasers in Engineering》2003,40(5-6):501-515
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. 相似文献
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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. 相似文献
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Huang MD Becker-Ross H Florek S Heitmann U Okruss M Patz CD 《Analytical and bioanalytical chemistry》2008,390(1):361-367
A new method for the determination of sulfur forms in wine, i.e., free SO2, total SO2, bound SO2, total S, and sulfate, is presented. The method is based on the measurement of the carbon monosulfide (CS) molecular absorption
produced in a conventional air–acetylene flame using high-resolution continuum source absorption spectrometry. Individual
sulfur forms can be distinguished because of the different sensitivities of the corresponding CS molecular absorption. The
sensitivity of free SO2 is about three times higher than the value for bound SO2 and sulfate. The method makes use of procedures similar to those used in classic reference methods. Its performance is verified
by analyzing six wine samples. Relative standard deviations are between 5 and 13% for free SO2 and between 1 and 3% for total SO2. For the validation of the accuracy of the new method, the results are compared with those of reference methods. The agreement
of the values for total SO2 with values of the classic method is satisfactory: five out of six samples show deviations less than 16%. Due to the instability
of free SO2 in wine and the known problems of the used reference method, serious deviations of the free SO2 results are found for three samples. The evaluation of the limits of detection focuses on the value for free SO2, which is the sulfur form having by far the lowest concentration in wine. Here, the achievable limit of detection is 1.8 mg L−1.
Figure Detection of non-metal elements using continuum source flame absorption spectrometry 相似文献
10.
Stueger H Mitterfellner T Fischer R Walkner C Patz M Wieber S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(25):7662-7664
Greetings from silicon valley: Alkali metal silanides (H(3)Si)(3)Si(-)M(+) were shown to be selectively accessible for the first time by the reaction of neopentasilane Si(SiH(3))(4) with tBuOM or iPr(2)NLi. The method allows the convenient derivatization of higher silicon hydrides and provides a simple access for unprecedented systematic studies on the chemical behavior of hydropolysilanes (see scheme). 相似文献