共查询到20条相似文献,搜索用时 31 毫秒
1.
Photoelastic materials develop colored fringes under white light when subjected to mechanical stresses, which can be viewed
through a polariscope. This technique has traditionally been used for stress analysis of loaded components, however, this
can also be potentially used in sensing applications where the requirement may be measurement of the stimulating forces causing
the generation of fringes. This leads to inverse photoelastic problem where the developed image can be analyzed for the input
forces. However, there could be infinite number of possible solutions which cannot be determined by conventional techniques.
This paper presents neural networks based approach to solve this problem. Experiments conducted to prove the principle have
been verified with theoretical results and finite element analysis of loaded specimens. The developed technique, if generalized,
can be implemented for whole-field analysis of the stress patterns involving complex fringes under different loading conditions.
This can also provide direct visualization of the stress field, which may find application in a variety of specialized areas
including biomedical engineering and robotics.
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2.
Recent experimental techniques used to investigate shear banding are reviewed. After recalling the rheological signature of
shear-banded flows, we summarize the various tools for measuring locally the microstructure and the velocity field under shear.
Local velocity measurements using dynamic light scattering and ultrasound are emphasized. A few results are extracted from
current works to illustrate open questions and directions for future research.
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3.
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
4.
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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5.
This paper presents a theoretical model and corresponding experimental results of the oblique-incidence response of a luminescent
photoelastic coating (LPC). LPCs use a luminescent dye that both partially preserves the stress-modified polarization state
and provides high emission signal strength at oblique surface orientations. These characteristics enable the technique to
acquire full-field strain separated measurements and principal strain directions, potentially on complex three-dimensional
geometries, without the use of supplemental experimental or analytical techniques. Results of a single-layer LPC on a disk
in diametral compression are presented to assess a theoretical model and evaluate the measurement sensitivity.
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6.
A new experimental technique has been developed to investigate the onset of fracture in metals at low and intermediate stress
triaxialities. The gage section of a flat specimen has been designed such that cracks are most likely to initiate within the
specimen center, remote from the specimen boundaries. Along with the specimen, a biaxial testing device has been built to
apply a well-defined displacement field to the specimen shoulders. The stress state within the specimen is adjusted by changing
the biaxial loading angle. Using this new experimental technique, the crack initiation in metals can be studied experimentally
for stress triaxialities ranging from 0.0 to 0.6. The stress and strain fields within the specimen gage section are determined
from finite element analysis. The reliability of the computational model of the test set-up has been verified by comparing
the simulation results with laser speckle-interferometric displacement measurements during testing. Sample experiments have
been performed on the Al-7Si-Mg gravity die casting alloy. A three-step hybrid experimental–numerical calibration procedure
has been proposed and applied to determine a phenomenological crack formation criterion for the Al-7Si-Mg alloy.
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7.
A new design of the shear compression specimen (SCS) for investigating the viscoelastic shear response of polymers is presented.
The specimen consists of a polymer gage section with two metal ends that remain essentially rigid during deformation. Two
closed-form analytic models are developed to predict the average stress and strain in the gage section from the deformation-load
histories. This new SCS design and its analytic models are thoroughly evaluated via laboratory measurements and numerical
simulations. These simulations show that the deformations in the gage section are more uniform than in the original design,
and the distribution of the average shear stress and strain are highly homogenous. The simulation results yield good agreement
with those of closed-form analytic results and the experiments demonstrate that the new SCS geometry and its analytic models
are as reliable as other commonly employed specimens. It can also generate higher strain rates under usual loading conditions
because of its smaller specimen gage length. The need for care in specimen preparation is also discussed in detail as illuminated
by the experimental and simulation results.
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8.
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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9.
An experimental device was set up for the synchronous measurement of velocities and stresses in polyisobutylenes using laser-Doppler
velocimetry (LDV) and the two-colour flow-induced birefringence method (FIB). The materials investigated are three low molecular
polyisobutylenes. Velocity (LDV) and stress (FIB) measurements are performed in the flow entrance region and inside a slit
die with a contraction ratio of 1:10. The behaviour of the polyisobutylenes is Newtonian under the flow conditions applied.
Therefore, the stresses inside the fluids can be calculated and compared to the stresses experimentally determined. A good
agreement in shear and elongational flows was found between the calculated (LDV) and directly measured stresses (FIB). This
result demonstrates the applicability of the experimental setup as an optical rheometer that can preferentially be used to
measure elongational properties of low viscous fluids.
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10.
The wall shear stress plays a key role in the interaction between blood flow and the surrounding tissue. To obtain quantitative
information about this parameter, velocity measurements are required with sufficient spatial (and temporal) resolution. We
present a methodology for the determination of the wall shear stress in vivo in the vitelline network of a chick embryo. Velocity
data is obtained by microscopic particle image velocimetry using correlation ensemble averaging; the latter is used to increase
the signal-to-noise ratio of the measurements. The temporal evolution of the pulsatile flow is reconstructed by sorting the
image pairs based on a phase estimate. From these flow measurements, the wall shear stress can be derived either directly
from the magnitude of the gradients or from fits to velocity profiles. Both methods give results that are in good agreement
with each other, while the former method is significantly easier to implement. For more accurate studies, the full three-dimensional
velocity field may be required. It is demonstrated how this velocity field can be obtained by scanning the measurement volume.
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11.
The National Institute of Standards and Technology (NIST) has developed an electrical pulse-heated Kolsky Bar technique for
measuring the constitutive response of metals at heating rates of up to 6,000 K/s and strain rates up to 10 4 s −1. Under these conditions, which are approaching those found in high speed machining, thermally activated microstructural processes
such as grain growth, solid state phase transformation and dislocation annealing can be bypassed, leading to unique non-equilibrium
superheated microstructural states. Flow stresses can thus differ significantly from equilibrium high temperature conditions.
This paper describes the NIST pulse-heated Kolsky bar technique in detail, including a thorough assessment of uncertainties
in temperature and flow stress measurement.
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12.
Flow within a large-aspect-ratio cylindrical vortex cell has been explored experimentally. The flow was driven by a shear
layer above an opening in the cylinder circumference. Reynolds numbers, based on the length of the opening and the velocity
just outside it, exceed 50,000. It is shown that the expected solid body rotation within the cell, with a constant velocity
gradient across most of the core, is qualitatively present, but is significantly distorted by three-dimensional effects. Nonetheless,
turbulence levels within the core are very low, only rising to levels similar to those in regular turbulent shear flows within
the driving mixing layer itself and the cell-wall boundary layers.
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13.
Traditionally, the components of the stress with respect to a relevant coordinate system are used for the purpose of stress
visualisation and interpretation. A case for using a flow dependent measure to interpret and visualise stress is made for
two dimensional flow, together with a suggestion for extending the idea to three dimensions. The method is illustrated for
Newtonian and Oldroyd B fluids in both the eccentrically rotating cylinder and flow past a cylinder benchmark problems. In
the context of a generalised Newtonian fluid, the relation between the flow-dependent stress measure to other field variables
under certain flow conditions, is examined and is indicative of its importance in complex flow.
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14.
We introduce a novel class of algorithms for evaluating PIV image pairs. The mathematical basis is a continuous variational formulation for globally estimating the optical flow vector fields over the whole image. This class of approaches has been known in the field of image processing and computer vision for more than two decades but apparently has not been applied to PIV image pairs so far. We pay particular attention to a multi-scale representation of the image data so as to cope with the quite specific signal structure of particle image pairs. The experimental evaluation shows that a prototypical variational approach competes in noisy real-world scenarios with three alternative approaches especially designed for PIV-sequence evaluation. We outline the potential of the variational method for further developments.The publications of the CVGPR Group are listed under . 相似文献
15.
Inertial waves generated by a small oscillating disk in a rotating water filled cylinder are observed by means of a corotating
particle image velocimetry system. The wave takes place in a stationary conical wavepacket, whose angle aperture depends on
the oscillation frequency. Direct visualisation of the velocity and vorticity fields in a plane normal to the rotation axis
are presented. The characteristic wavelength is found to be approximately equal to the disk diameter. The classical dispersion
relation for plane waves is verified from the radial location of the wavepacket, and from the ellipticity of the projected
velocity diagram.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
相似文献
16.
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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17.
We measure the flow above an array of randomly driven, upward-facing synthetic jets used to generate turbulence beneath a free surface. Compared to grid stirred tanks (GSTs), this system offers smaller mean flows at equivalent turbulent Reynolds numbers with fewer moving parts. 相似文献
18.
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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19.
A new technique for parallel rheological characterization of asphalt in a combinatorial squeeze-flow array is described. The
basis of the technique is a device that is capable of subjecting multiple samples simultaneously to constant volume (Type
B) squeeze flow with application of a temperature gradient. The time-dependent sample dimensions, which are calculated from
digital images taken through the transparent top plate, are used to derive the flow curves. The results obtained using the
combinatorial setup compared favorably with those obtained using conventional parallel-plate torsional flow in a commercial
rheometer. With the existing setup, the accessible shear rate range is limited to about one decade at a single temperature.
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20.
The molecular mass dependence of the zero shear-rate viscosity of low-density polyethylene (LDPE) melts is revisited for a series of LDPE samples of different molecular masses and densities. The long-chain branching structure and absolute weight average molecular masses are determined using a state-of-the-art size exclusion chromatography system coupled with multi-angle laser light-scattering. Creep experiments in a stress-controlled rotational rheometer are used to determine the zero shear-rate viscosity. The experimental results give evidence of an exponential molecular mass dependence of zero shear viscosity. It is demonstrated that these results can qualitatively be explained by comparison with recent theoretical predictions of the rheological properties of comb-branched model polymers. 相似文献
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