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1.
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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2.
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
3.
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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4.
The development of torque-induced shear stresses in the presence of slippage, and the residual stresses remaining after torque-induced
slippage, are analysed using frozen-stress photoelasticity. Shaft/ring specimens were manufactured from epoxy photoelastic
material and were assembled by shrink-fitting prior to being loaded under various regimes, notably the application and release
of a torque load. The interface pressure was predicted from Lamé thick cylinder theory, and was also estimated by fitting
the Lamé model to the measured stress distributions. The distributions of interface shear stress were calculated from averaged
photoelastic data, and compared with the results of a dislocation-based model and with a nonlinear finite element model. For
a torque loaded specimen there was good agreement between experimental, theoretical and FE data. Another specimen was loaded
in torque then unloaded, with results showing the expected features of slippage and residual stress.
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5.
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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6.
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
7.
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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8.
Simultaneous multi-point hotwire measurements are used to investigate the three-dimensional wake topology of a square cylinder at high Reynolds numbers. Wavelet techniques are applied to detect the flow structures and to inquire on the validity or extension of previously proposed low Reynolds number topological models to turbulent wakes. Our results suggest that a flow topological model similar to the horizontal perturbation model proposed by Meiburg and Lasheras (J Fluid Mech 190:1–37, 1988) but with alternate rib cuts in the horizontal plane is plausible for the intermediate wake topology. 相似文献
9.
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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10.
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
11.
Planar Raman imaging through a spectrograph is demonstrated as a diagnostic tool for quantitative flow visualisation of internal supersonic wedge flow. A dedicated Bayesian deconvolution filter is used to remove the spectral structure that is introduced by the spectrograph. The 2D density field is determined with ca. 10% precision using average images over 6,000 laser pulses, down to 0.5 mm from the surface of the wedge. Direct interpretations of Raman intensities provide more precise density data than indirect interpretations based on shock geometry in 2D inviscid flow. 相似文献
12.
The understanding of the physics of flapping flight has long been limited due to the obvious experimental difficulties in studying the flow field around real insects. In this study the time-dependent three-dimensional velocity field around a flapping wing was measured quantitatively for the first time. This was done using a dynamically-scaled wing moving in mineral oil in a pattern based on the kinematics obtained from real insects. The periodic flow is very reproducible, due to the relatively low Reynolds number and precise control of the wing. This repeatability was used to reconstruct the full evolving flow field around the wing from separate stereoscopic particle image velocimetry measurements for a number of spanwise planes and time steps. Typical results for two cases (an impulsive start and a simplified flapping pattern) are reported. Visualizations of the obtained data confirm the general picture of the leading-edge vortex that has been reported in recent publications, but allow a refinement of the detailed structure: rather than a single strand of vorticity, we find a stable pair of counter-rotating structures. We show that the data can also be used for quantitative studies, such as lift and drag prediction. 相似文献
13.
The electrorheological properties (ER) of some fluids containing particles change extensively under the external electrical
field. This phenomenon is applicable in many industries and equipments, such as clutches and motor driven rotor, which would
transfer the spin to a drive shaft through a thin layer of electrorheological fluid. In this investigation, the effects of
external electrical field on ER properties of non-Newtonian fluids (silicon oil) with the addition of SnO 2 and Y 2O 3 nanoparticles were studied. The ER properties were measured for a wide range of SnO 2 and Y 2O 3 nanoparticle concentrations and DC electrical voltages using concentric cylinder rotary rheometer. Based on the results,
ER properties of nanofluids, e.g., apparent viscosity, shear stress, and yield stress, were enhanced by applying electrical
field and increasing SnO 2 and Y 2O 3 concentrations.
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14.
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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15.
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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16.
An enclosed rotating disk apparatus (RDA) with rotational speed up to 5,500 rpm and with temperature control from −5 to 55°C
was designed to screen the turbulent drag reducing effectiveness of small samples of newly synthesized drag reducing additives.
First, the rotating disk was calibrated with water using both logarithmic and power law models. Then experiments were carried
out to measure the frictional torque reduction for a drag reducing aqueous cationic surfactant system (5 mM Ethoquad O12 with
12.5 mM sodium salicylate) over a range of Re. The maximum drag reduction at 30°C was about 47% at Re = 1.90 × 10 6. For the first time, results with the RDA were compared with those in a circular pipe flow system. They showed similar trends
indicating it is a useful screening device for small samples, giving conservative estimates of surfactant effectiveness compared
with pipe flow.
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17.
The present paper reports a thorough comparison of the turbulent flow characteristics exhibited by a cubic surface-mounted obstacle and a simple geometric variant (gable roof of 30° roof pitch). The measurements supporting this study were obtained by the use of a 2D-DPIV system. Significant differences in the large-scale vortical structures and turbulent kinetic energy fields implied drastic consequences with respect to the advective and turbulent dispersive characteristics of the flow at roof and ground levels. 相似文献
18.
This work aims to develop a process for controlling a cylinder wake, especially the von Karman vortex street, in such way so as to drastically reduce the drag coefficient. A new technique for influencing the cylinder wake is proposed in the present experimental study. The flow around a circular cylinder is perturbed by temporarily changing the cylinder diameter. Experiments have been performed for Reynolds numbers in the range Re=9,500 to Re=31,500. Three values of the controlling frequencies are considered: fs1=0.41, fs2=0.54 and fs3=0.73, in addition to the stationary case corresponding to a non-deformable cylinder, fs0=0. The visualisation flow shows that the pulsing motion of the cylinder walls greatly influences both the near and far wake dynamics. A decrease of the drag is expected. 相似文献
19.
A novel seeding method for microscale particle image velocimetry (micro-PIV) is presented. The method relies on selective seeding of a thin fluid layer within an otherwise particle-free flow. In analogy to the laser sheet in macroscale PIV, the generated particle sheet defines both the depth and the position of the measurement plane, independent of the details of the optical setup. Selectively seeded micro-PIV is applied to measure the instantaneous velocity field in a microchannel with a depth-wise resolution 20% below the estimated optical measurement depth of the micro-PIV system. In principle, a measurement depth corresponding to the diameter of the tracer particles may be achieved. 相似文献
20.
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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