共查询到20条相似文献,搜索用时 31 毫秒
1.
An iterative procedure, based on the proper orthogonal decomposition (POD), first proposed by Everson and Sirovich (J Opt
Soc Am A 12(8):1657–1664, 1995) is applied to marred particle image velocimetry (PIV) data of shallow rectangular cavity flow at Mach 0.19, 0.28, 0.38,
and 0.55. The procedure estimates the POD modes while simultaneously estimating the missing vectors in the PIV data. The results
demonstrate that the absolute difference between the repaired vectors and the original PIV data approaches the experimental
uncertainty as the number of included POD modes is increased. The estimation of the dominant POD modes is also shown to converge
by examining the subspace spanned by the POD eigenfunctions.
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2.
A theoretical analysis of the spatial resolution in terms of modulation transfer function of the Stereo PIV technique with
and without the correction of the misalignment error is performed, and the results show that some wavelengths of the flow
field can be significantly dephased and modulated. A performance assessment has been conducted with both synthetic and real
images and shows a good agreement with the theoretical analysis. The reconstruction of the three-dimensional displacement
field is achieved using both the methods proposed by Soloff et al. (Meas Sci Technol 8:1441–1454, 1997) and by Willert (Meas Sci Technol 8:1465–1479, 1997).
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4.
Three quantitative flow classification parameters have been studied in the context of Tanner and Huilgol’s suggestion of strong and weak flows. Seen in this context, the different types of streamlines possible for general 3-D flows furnish no indication with respect to the flow strength. This is in total contrast to 2-D flows, where the type of the streamline and the strength of the flow go hand in hand. Astarita’s [ J Non-Newton Fluid Mech, 6:69–76, 1979] flow classification parameter takes care of this fact and, if properly generalized, can be applied to more general flows: Two other flow classification parameters also have their basis in homogeneous 2-D flows, but their generalization leads, for general flows, to nonuniqueness and other unacceptable results. For 3-D flows, none of the parameters can quantitatively be used in general, and additional parameters, with their basis outside the 2-D flow regime, seem to be called for. 相似文献
5.
We present an alternative method of producing density stratifications in the laboratory based on the ‘double-tank’ method
proposed by Oster (Sci Am 213:70–76, 1965). We refer to Oster’s method as the ‘forced-drain’ approach, as the volume flow rates between connecting tanks are controlled
by mechanical pumps. We first determine the range of density profiles that may be established with the forced-drain approach
other than the linear stratification predicted by Oster. The dimensionless density stratification is expressed analytically
as a function of three ratios: the volume flow rate ratio n, the ratio of the initial liquid volumes λ and the ratio of the initial densities ψ. We then propose a method which does
not require pumps to control the volume flow rates but instead allows the connecting tanks to drain freely under gravity.
This is referred to as the ‘free-drain’ approach. We derive an expression for the density stratification produced and compare
our predictions with saline stratifications established in the laboratory using the ‘free-drain’ extension of Oster’s method.
To assist in the practical application of our results we plot the region of parameter space that yield concave/convex or linear
density profiles for both forced-drain and free-drain approaches. The free-drain approach allows the experimentalist to produce
a broad range of density profiles by varying the initial liquid depths, cross-sectional and drain opening areas of the tanks.
One advantage over the original Oster approach is that density profiles with an inflexion point can now be established.
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6.
The MTR 25 is a multitask rheometer (for shear and squeeze flow) with 25 kg of normal force and a partitioned plate. Torque
and normal force are measured at both, the inner disk and the outer ring of the plate. The first and second normal stress
differences can be determined from a single test. The axial stiffness is high (10 7 N/m) by using rigid springs and strain gauges for the load cell. Monodisperse polystyrene ( M
w = 206 kg/mol, 180°C) has been sheared in the range from 0.05 to 47 s − 1. The viscosity and first normal stress difference are highly reproducible. The second normal stress difference scatters and
mirrors the instability at the rim. A critical comparison is made between the MTR 25 method and the single transducer evaluation
method (RMS 800 method, Schweizer, Rheol Acta 41:337–344, 2002): Both yield excellent and coinciding viscosity and first normal stress difference data. The RMS 800 method gives more stable
second normal stress difference data, since the normal force from the outer ring, which is influenced by edge fracture, is
not used. Data for the RMS 800 method can be acquired on the MTR 25. The high normal force capacity permits larger samples
and higher shear rates than on the RMS 800.
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7.
We study an unstable highly concentrated emulsion of water droplets in oil with a nonionic surfactant. A technique of light diffusion coupled to a rheometer allows simultaneous measurement of average droplet radius and emulsion shear elastic modulus during time. Over the studied range of volume fraction (from 71 to 95%), we show that Princen and Kiss’ (J Colloid Interface Sci 112:427–437, 1986) model does not apply. A dimensional analysis based on the hypothesis of dominant van der Waals forces is proposed for nonionic surfactants, which is in good agreement with experimental data. We also show that the measured average droplet volume increases linearly with time and that the coalescence rate strongly depends on the volume fraction in relation with different topological conformations of droplets. 相似文献
8.
The most important rheological and mathematical features of the pom–pom model are presently used to compare and improve other
constitutive models such as the Giesekus and Phan-Thien–Tanner models. A pragmatic methodology is selected that allows derivation
of simple constitutive equations, which are suited to possible software implementation. Alterations to the double convected
pom–pom, Phan-Thien–Tanner and Giesekus models are proposed and assessed in rheometric flows by comparing model predictions
to experimental data.
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9.
We develop the axisymmetric Synthetic Schlieren technique to study the wake of a microscale sphere settling through a density
stratification. A video-microscope was used to magnify and image apparent displacements of a micron-sized random-dot pattern.
Due to the nature of the wake, density gradient perturbations in the horizontal greatly exceed those in the vertical, requiring
modification of previously developed axisymmetric techniques. We present results for 780 and 383 μm spheres, and describe
the limiting role of noise in the system for a 157 μm sphere. This technique can be instrumental in understanding a range
of ecological and environmental oceanic processes on the microscale.
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10.
This paper reports laser-Doppler measurements of the mean flow and turbulence stresses in a swirling pipe flow. Experiments were carried out under well-controlled laboratory conditions in a refractive index-matched pipe flow facility. The results show pronounced asymmetry in mean and fluctuating quantities during the downstream decay of the swirl. Experimental data reveal that the swirl significantly modifies the anisotropy of turbulence and that it can induce explosive growth of the turbulent kinetic energy during its decay. Anisotropy invariant mapping of the turbulent stresses shows that the additional flow deformation imposed by initially strong swirling motion forces turbulence in the core region to tend towards the isotropic two-component state. When turbulence reaches this limiting state it induces rapid production of turbulent kinetic energy during the swirl decay. 相似文献
11.
We propose to analyse power law shear stress relaxation modulus observed at the sol–gel transition (SGT) in many gelling systems
in terms of fractional calculus. We show that the critical gel (gel at SGT) can be associated to a single fractional element
and the gel in the post-SGT state to a fractional Kelvin–Voigt model. In this case, it is possible to give a physical interpretation
to the fractional derivative order. It is associated to the power law exponent of the shear modulus related to the fractal
dimension of the critical gel. A preliminary experimental application to silica alkoxide-based systems is given.
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12.
To develop a tool for predicting of heat and mass transfer in Joule–Thomson cryocoolers working at subcritical pressures,
we study a counter flow heat exchanger with condensation by employing the integral method. The effects of inlet pressure and
working fluid are predicted. We also show that there is an optimal value of the enthalpy difference along the heat exchanger
for which its length is minimal.
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13.
Two- and three-dimensional flows in nearly cuboidal cavities are investigated experimentally. A tight cavity is formed in
the gap between two long and parallel cylinders of large radii by adding rigid top, bottom, and end walls. The cross-section
perpendicular to the axes of the cylinders is nearly rectangular with aspect ratio Γ. The axial aspect ratio Λ > 10 is large
to suppress end-wall effects. The fluid motion is driven by independent and steady rotation of the cylinders about their axes
which defines two Reynolds numbers Re
1,2. Stability boundaries of the nearly two-dimensional steady flow have been determined as functions of Re
1,2 for Γ = 0.76 and Γ = 1. Up to six different three-dimensional supercritical modes have been identified. The critical thresholds
for the onset of most of the three-dimensional modes, three of which have been observed for the first time, agree well with
corresponding linear-stability calculations. Particular attention is paid to the flow for Γ = 1 under symmetric and parallel
wall motion. In that case the basic flow consists of two mirror symmetric counter-rotating parallel vortices. They become
modulated in span-wise direction as the driving increases. Detailed LDV measurements of the supercritical three-dimensional
velocity field and the bifurcation show an excellent agreement with numerical simulations.
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14.
We focus on the linear viscoelastic response of heterogeneous nematic polymers to small amplitude oscillatory shear, paying
special attention to the macroscopic influence of strong plate anchoring conditions. The model consists of the Stokes hydrodynamic
equations with viscous and nematic stresses, coupled to orientational dynamics and structure driven by the flow gradient,
an excluded-volume potential, and a two-constant distortional elasticity potential. We show that the dynamical response simplifies
when plate anchoring is either tangential or homeotropic, recovering explicitly solvable Leslie–Ericksen–Frank behavior together
with weakly varying order parameters across the plate gap. With these plate conditions, we establish “model consistency” so
that all experimental driving conditions (plate-controlled velocity [strain] or shear stress, imposed oscillatory pressure)
yield identical dynamic moduli for the same material parameters and anchoring conditions, eliminating the culpability of device
influence in scaling behavior. Two physical predictions emerge that imply significant macroscopic elastic and viscous effects
controlled by plate anchoring relative to flow geometry: (1) The storage modulus is enhanced by two to three orders of magnitude
for homeotropic relative to parallel anchoring, across all frequencies. (2) The loss modulus exhibits enhancement of a factor
of two to three for homeotropic over tangential anchoring, restricted to low frequencies. We further deduce a scaling law
for the dynamic moduli versus anisotropy of the distortional elasticity potential.
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15.
Failure behavior of composite materials in general and particulate composites in particular is intimately linked to interactions
between a matrix crack and a second phase inclusion. In this work, surface deformations are optically mapped in the vicinity
of a crack–inclusion pair using moiré interferometry. Edge cracked epoxy beams, each with a symmetrically positioned cylindrical
glass inclusion ahead of the tip, are used to simulate a compliant matrix crack interacting with a stiff inclusion. Processes
involving microelectronic fabrication techniques are developed for creating linear gratings in the crack–inclusion vicinity.
The debond evolution between the inclusion–matrix pair is successfully mapped by recording crack opening displacements under
quasi-static loading conditions. The surface deformations are analyzed to study evolution of strain fields due to crack–inclusion
interactions. A numerical model based on experimental observations is also developed to simulate debonding of the inclusion
from the matrix. An element stiffness deactivation method in conjunction with critical radial stress criterion is successfully
demonstrated using finite element method. The proposed methodology is shown to capture the experimentally observed debonding
process well.
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16.
A series of baseline displacement measurements have been obtained using 2D Digital Image Correlation (2D-DIC) and images from
Scanning Electron Microscopes (SEM). Direct correlation of subsets from a reference image to subsets in a series of uncorrected
images is used to identify the presence of non-stationary step-changes in the measured displacements. Using image time integration
and recently developed approaches to correct residual drift and spatial distortions in recorded images, results clearly indicate
that the corrected SEM images can be used to extract deformations with displacement accuracy of ±0.02 pixels (1 nm at magnification
of 10,000) and mean value strain measurements that are consistent with independent estimates and have point-to-point strain
variability of ±1.5 × 10 −4.
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17.
This work is an attempt to test the concept of the hydrodynamic charge (analogous to the electric charge in electromagnetism)
in the simple case of a coherent structure such as the Burgers vortex. We provide experimental measurements of both the so-called
Lamb vector and its divergence (the charge) by two-dimensional particles images velocimetry. In addition, we perform a Helmholtz–Hodge
decomposition of the Lamb vector in order to explore its topological features. We compare the charge with the well-known Q-criterion in order to assess its interest in detecting and characterizing coherent structure. Usefulness of this concept
in studies of vortex dynamics is demonstrated.
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18.
The long-chain branched thermoplastic tetrafluoroethylene–hexafluoropropylene–vinylidenefluoride terpolymers (LCB THV) investigated
in this paper are new polymers with a unique combination of properties like a high stability against aging or weathering and
a very good chemical resistance. But not much is known about the rheological behaviour of the LCB THV, yet. In this paper,
non-linear rheological properties like shear thinning and strain hardening are studied. Two different types of the THV with
different contents of comonomers and, therefore, different melting points are examined. The THV with the higher melting point
is insoluble. The other with the lower melting temperature is soluble and, therefore, was characterised by size exclusion
chromatography coupled with light scattering with respect to its molecular structure. The results of the rheological measurements
show a pronounced shear-thinning and strain-hardening behaviour for the long-chain branched materials. Both properties are
of great importance for processing operations governed by shear and elongational flows.
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19.
A numerical bifurcation analysis is carried out in order to determine the solution structure of radial fins subjected to multi-boiling
heat transfer mode. One-dimensional conduction is employed throughout the thermal analysis. The fluid heat transfer coefficient
is temperature dependent on the three regimes of phase-change of the fluid. Six fin profiles, defined in the text, are considered.
Multiplicity structure is obtained to determine different types of bifurcation diagrams, which describe the dependence of
a state variable of the system like the temperature or the heat dissipation on the fin design parameters, conduction–convection
parameter (CCP) or base temperature difference (Δ T). Specifically, the effects of Δ T, CCP and Biot number are analyzed. The results are presented graphically, showing the significant behavioral features of
the heat rejection mechanism.
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20.
Local strain data obtained throughout the entire weld region encompassing both the weld nugget and heat affected zones (HAZs)
are processed using two methodologies, uniform stress and virtual fields, to estimate specific heterogeneous material properties
throughout the weld zone. Results indicate that (a) the heterogeneous stress–strain behavior obtained by using a relatively
simple virtual fields model offers a theoretically sound approach for modeling stress–strain behavior in heterogeneous materials,
(b) the local stress–strain results obtained using both a uniform stress assumption and a simplified uniaxial virtual fields
model are in good agreement for strains ɛ
xx < 0.025, (c) the weld nugget region has a higher hardening coefficient, higher initial yield stress and a higher hardening
exponent, consistent with the fact that the steel weld is overmatched and (d) for ɛ
xx > 0.025, strain localization occurs in the HAZ region of the specimen, resulting in necking and structural effects that complicate
the extraction of local stress strain behavior using either of the relatively simple models.
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