共查询到20条相似文献,搜索用时 31 毫秒
1.
A novel no-moving-part valve was developed mainly for applications in microchemistry, where Reynolds numbers tend to be extremely
low. The valve of very small size and working with viscous biological fluids was designed for operating at Re values between
10 and 100. In this range, the inertial jet flow effects used in standard fluidics could not be relied upon and a qualitatively
new operating principle was introduced: the control fluid forms a “plug” at the entrance of the collector, precluding an entry
of the main fluid flow. In particular, an array of new valves was used in a selector unit to allow a single reagent flow into
a reactor from several available sources. The flow visualization validated the assumptions on which the design is based and
compared well with numerical flowfield computations. 相似文献
2.
3.
The problem of transition prediction for hypersonic boundary layers over a sharp cone has been studied in this work. The Mach
number of the oncoming flow is 6, the cone half-angle is 5°, and the angle of attack is 1°. The conventional e
N
method is used, but the transition location so obtained is obviously incorrect. The reason is that in the conventional method,
only the amplifying waves are taken into account, while in fact, for different meridians the decay processes of the disturbances
before they begin to grow are different. Based on our own previous work, new interpretation and essential improvement for
the e
N
method are proposed. Not only the amplification process but also the decay process is considered. The location, where by
linear stability theory, the amplitude of disturbance wave is amplified from its initial small value to 1%, is considered
to be the transition location. The new result for transition prediction thus obtained is found to be fairly satisfactory.
It is also indicated that for the calculation of base flow, boundary layer equations can be used for a small angle of attack.
Its computational cost is much smaller than those for DNS.
Supported by the National Natural Science Foundation of China (Grant Nos. 10632050 and 90716007), the Special Foundation for
the Authors of National Excellent Doctoral Dissertations (Grant No. 200328), and the Foundation of Liu-Hui Center of Applied
Mathematics of Nankai University and Tianjin University 相似文献
4.
In vitro experiments are often unable to reproduce all the complexities of biological flows observed in vivo. The in vitro
models are often rigid, use Newtonian fluids, and/or some ideal geometry tested under ideal physiological parameters. The
study presented in this article describes the in vitro assessment of mitral prosthetic heart valves in a setup able to simulate
the pulsatile blood flow in a model of the left heart with moving walls. The specific laboratory mockup built for these experiments
consists in a Dual Activation Simulator (DAS) that provides a realistic simulation of the atrial and ventricular flow in anatomically
shaped silicone models cavities. This mockup, initially designed for ultrasonic velocity measurements took recently advantage
of the use of particle image velocimetry. We present here some aspects of flow visualization and phase averaged two-dimensional
PIV measurements which can provide new insight in the interaction between the flow dynamics and the heart valves. 相似文献
5.
In this paper, the method of dynamic flow birefringence (DFB) have been studied extensively under the consideration of an application to cardiovascular models. The method utilises the optical interference patterns observed in the birefringent flow for determination of the fluid shear stress and velocity distribution. In order to measure a flow in a cardiovascular model, an assumption of a simplified stress–optical relation in a pulsatile flow is suggested and special experimental techniques such as birefringent fluid for simulating blood and new experimental system have been developed. Application studies focus on pulsatile flows in typical models, namely arterial bifurcation and mechanical heart valves. Experimental results are discussed and compared with those of other researchers. 相似文献
6.
H. Sakakima M. Satomi Y. Sugita Y. Kawawake 《Journal of magnetism and magnetic materials》2000,210(1-3)
Spin valves having thin oxide layers in the pinned and/or free layers were prepared by sputtering. MR ratios of the spin valves were increased from 8.1 to 11.9% by inserting the oxide layer into the pinned layer in Ta/PtMn/CoFe/Cu/CoFe/Ta spin valves. MR ratio of 13.9% and considerably large sheet ΔR of 2.55 Ω were obtained in the PtMn-based spin valves having the oxide layer in the pinned and free layer. Larger MR ratio of 17.3% and the sheet ΔR of 1.3 Ω were obtained in the PtMn-based dual-type spin valves having the oxide layer in both pinned layers. α-Fe2O3 based spin valves having thin oxide layers were also prepared. MR ratios of the spin valves were increased from 11.9 to 14.3% by inserting the oxide layer into the free layer in α-Fe2O3/CoFe/Cu/CoFe/Ta spin valves. The enhancement of the MR ratios may be attributed to the specular scattering effect of the conduction electrons by the thin oxide layers. 相似文献
7.
This article presents the experimental results of an evaluation of the influence of internal flow patterns on the heat transfer performance of a closed-loop oscillating heat pipe with check valves. It was found that the internal flow patterns could be classified according to four flow patterns: dispersed bubble flow, bubble flow, slug flow, and annular flow, respectively. The main regime of each flow pattern can be determined from a flow pattern map. The map can be used to predict the trend of the heat transfer rate in the closed-loop oscillating heat pipe with check valves. 相似文献
8.
M. E. Brachet D. Meiron S. Orszag B. Nickel R. Morf U. Frisch 《Journal of statistical physics》1984,34(5-6):1049-1063
We here report results obtained from numerical simulations of the TaylorGreen three-dimensional vortex flow. This flow is perhaps the simplest system in which one can study the generation of small scales by three-dimensional vortex stretching and the resulting turbulence. The problem is studied by both direct spectral numerical solution of the Navier-Stokes equations (with up to 2563 modes) and by power series analysis in time.The inviscid dynamics are strongly influenced by symmetries which confine the flow to an impermeable box with stress-free boundaries. There is an early stage during which the flow is strongly anisotropic with well-organized (laminar) small-scale excitation. The flow is smooth but has complex-space singularities within a distance(t) of the real space which are manifest through an exponential tail in the energy spectrum. It is found that(t) decreases exponentially in time to the limit of our resolution. Indirect evidence is presented that more violent vortex stretching takes place at later times, possibly leading to a real singularity (=0) at a finite time. These direct integration results are consistent with new presented results extending the Morf, Orszag, and Frisch temporal power series analysis from order t40 to order t80. Still, convincing evidence for or against the existence of a real singularity will require even more sophisticated analysis.The viscous dynamics (decay) have been studied for Reynolds numbersR (based on integral scale) up to 3000 and beyond the time tmax at which the maximum energy dissipation is achieved. Early time, highR dynamics are essentially inviscid and laminar. Then, instabilities starting at small scales, which may be driven by viscosity, make the flow increasingly chaotic (turbulent) with extended high-vorticity patches appearing away from the impermeable walls. Neart
max the small scales of the flow are nearly isotropic providedR>1000. Various features characteristic of fully developed turbulence are observed neart
max whenR=3000.Work supported by the NSF under grants Nos. ATM-8017284 and DMR-77-10210; by the Office of Naval Research under Contracts NOOO14-77-C-0138 and NOOO14-79-C-0478; by the NSERC of Canada; and by the A. P. Sloan Foundation. 相似文献
9.
We analyze the effective diffusivity of a passive scalar in a two-dimensional, steady, incompressible random flow that has
mean zero and a stationary stream function. We show that in the limit of small diffusivity or large Peclet number, with convection
dominating, there is substantial enhancement of the effective diffusivity. Our analysis is based on some new variational principles
for convection diffusion problems and on some facts from continuum percolation theory, some of which are widely believed to
be correct but have not been proved yet. We show in detail how the variational principles convert information about the geometry
of the level lines of the random stream function into properties of the effective diffusivity and substantiate the result
of Isichenko and Kalda that the effective diffusivity behaves likeɛ
3/13 when the molecular diffusivityɛ is small, assuming some percolation-theoretic facts. We also analyze the effective diffusivity for a special class of convective
flows, random cellular flows, where the facts from percolation theory are well established and their use in the variational
principles is more direct than for general random flows. 相似文献
10.
Numerical analysis of high Mach flow and flow reversal in the experimental advanced superconducting tokamak divertor
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The B2-Eirene (SOLPS 4.0) code package is used to investigate the plasma parallel flow, i.e., the scrape-off layer (SOL) flow, in the experimental advanced superconducting tokamak (EAST) divertor. Simulation results show that the SOL flow in the divertor region can exhibit complex behaviour, such as a high Mach flow and flow reversal in different plasma regimes. When the divertor plasma is in the detachment state, the high Mach flow with approaching or exceeding sonic speed is observed away from the target plate in our simulation. When the divertor plasma is in the high recycling state, the flow reversal with a small Mach number (|M|< 0.2) is observed near the X-point along the separatrix region. The driving mechanisms for the high Mach flow and the reversed flow are analysed theoretically through momentum and continuity equations, respectively. The profile of the ionization sources is shown to be a possible formation condition causing the complex behaviour of the SOL flow. In addition, the effects of the high Mach flow and the flow reversal on the impurity transport are also discussed in this paper. 相似文献
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We propose a generalization of De Gennes' theory of flow birefringence [3] to the case of two distinct local anisotropy variables and also some extensions to the existing three variable theories of light scattering spectra. We show that the application of the theories with one or with two local anisotropy variables to analyse flow birefringence and light scattering spectra give numerically different results. We also show that it is possible to determinewhether the second local anisotropy variable is a primary or a secondary variable. The VH and HH light scattering spectra of a number of symmetric top molecules are analysed. They show an unambiguous ‘shear mode coupling’ down to very small values of (q 2η/ρΦ). These results are discussed and compared to flow birefringence experiments in the light of our theoretical considerations. The three variable correction is shown to be small and within experimental error for all liquids studied except one (pyridine). The flow birefringence coefficient for CCl4 is measured and found to be negative as predicted by our theory. Limitations to the formalism of generalized hydrodynamics are also discussed briefly. 相似文献
13.
We investigate the rheological properties of a cubic fcc phase of micelles obtained by aggregation of a triblock copolymer
(PEO)127(PPO)48(PEO)127 in water as selective solvent. The resulting soft solid is submitted to a range of stresses varying from 20 to 800Pa in Couette
geometry. Creep and flow behaviour can be distinguished and interpreted in terms of structural changes previously observed by SAXS under flow. Contrasting
with other systems, no discontinuity in the flow behaviour is associated with the structural changes. The strong shear thinning
is interpreted from the scattering data, as resulting from the nucleation of a new structure of hexagonal compact planes parallel
to the Couette walls. This creates a lubricating domain in the gap, whose size grows with the applied shear rate. We argue
moreover that the very existence of flow (as a steady state opposed to creep) is associated with this so-called layer-sliding
structure in a fraction, however small, of the sample.
Received on 4 June 1999 and Received in final form 6 September 1999 相似文献
14.
Combustion instabilities in annular combustors are of great interest because of their industrial relevance. Azimuthal acoustic modes, which involve transverse acoustic forcing to flames, have become a key process related to annular combustor instabilities. Transverse mean flow may be a factor that affects azimuthal oscillations. This paper provides an analytical model for a transversely forced two-dimensional Bunsen flame under transverse mean flow. The model is established using a low-amplitude perturbation assumption applied to a G-equation formulation. Forced flame displacement and flame transfer functions (FTFs) are calculated. The results are verified based on numerical solutions of the G-equation. Effects of frequency, transverse mean flow velocity and vertical mean flow velocity on the FTFs are discussed. The symmetric flame without transverse mean flow has a vanishing response to transverse acoustic forcing, while asymmetric flames, which are formed with transverse mean flow, have a bandpass response to transverse forcing. The response at very low and high forcing frequencies is small, with higher transfer function gains only in a certain frequency range. This bandpass response, which is inherently linked to the asymmetry of the flame, is an important factor to account for when considering the flame dynamics related to transverse acoustic effects. 相似文献
15.
Abstract Multi‐commutation, which refers to the use of solenoid valves to construct the flow network, has been widely used for providing automation in flow injection analysis. In this paper, the coupling of multi‐commutation and multi‐optosensing is developed for the analysis of two pesticides in environmental water samples, fuberidazole and o‐phenylphenol. In optosensing, the use of the solid support allows the discrimination between the analytes and other compounds that, if measuring in solution, would interfere in the analysis; in addition, the sensitivity needed when facing environmental samples is obtained. The two analytes are separated by using C18 silica gel as solid support, taking into account their different kinetics of sorption/desorption when interacting with the solid support microbeads; the separation is performed in the same flow‐cell where the sensing detection is carried out (by using an additional amount of solid support in the cell itself above the irradiated microzone), so both separation and determination are integrated in the cell. The native fluorescence of fuberidazole and o‐phenylphenol was simultaneously measured at 314/356 and 250/345 nm, respectively. The detection limits obtained were 0.18 and 6.1 ng mL?1 for fuberidazole and o‐phenylphenol respectively, with a sampling frequency of about 12 samples per hour. A recovery study was performed in waters obtained for wells and rivers, with satisfactory results. 相似文献
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An interactive method is proposed for the solution of two-dimensional, laminar flow fields with identifiable regions of recirculation, such as the shear layer driven cavity flow. The method treats the flow field as composed of two regions, with an appropriate mathematical model adopted for each region. The shear layer is computed by the compressible boundary layer equations, and the slowly recirculating flow by the incompressible Navier-Stokes equations. The flow field is solved iteratively by matching the local solutions in the two regions. For this purpose a new matching method utilizing an overlap between the two computational regions is developed, and shown to be most satisfactory. Matching of u, v, as well as u/y is amply accomplished using the present approach, and the stagnation points corresponding to separation and reattachment of the dividing streamline are computed as part of the interactive solution. The interactive method is applied to the test problem of shear layer driven cavity. The computational results are used to show the validity and applicability of the present approach. 相似文献
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19.
《Combustion Theory and Modelling》2013,17(2):99-105
A premixed flame within a turbulent flow exhibits a decreasing enhancement of fuel consumption rate with increasing turbulence intensity, an effect known as the bending effect. Denet has shown that flow time correlations may be one cause of the bending effect. Using a Damköhler-Huygens front propagation model, we illustrate that the removal of flow components with reduced frequencies greater than unity (ω >kS L) causes a small reduction in front area but a large reduction in the flow intensity, which is the bending effect (ω is the frequency and k is the wavenumber). To be effective in producing front area, a flow mode must have a phase velocity, ω/k, smaller than the laminar burning velocity, S L. 相似文献
20.
In this paper, we characterise the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence, restricted in a vertical plane, under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) coexists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate k?3 scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a k?3 power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano–Obukhov scaling that transitions from a shallow k?11/5 form at large scales, to a steeper approximate k?3 scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterised by an approximate k?1.64 scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a k?2.5 scaling, while PE exhibits a forward transfer and its spectrum shows an approximate k?1.6 power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indicies are well explained by accounting for the scale-dependent nature of the corresponding flux. 相似文献