共查询到20条相似文献,搜索用时 156 毫秒
1.
In this study, intermittent drying behavior of single layer rough rice with a moisture content of between 22 and 24% on the
dry basis was simulated by means of a liquid diffusion model based on a prolate spheroid geometry. For this purpose, solution
of the liquid diffusion equation was fitted to the experimental data for the drying air temperature 40°C, drying velocity
1.5 ms −1 and tempering periods ranging from 0 to 1 h. In order to make a comparison, solution of the liquid diffusion equation for
a finite cylindrical geometry was also fitted to the experimental data. The results show that the liquid diffusion model based
on a prolate spheroid geometry explains the drying behavior of rough rice more accurately. The results also show that greater
variations occur in diffusion coefficient with increasing tempering time for prolate spheroid geometry which is more realistic
geometry for a rough rice grain.
相似文献
2.
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.
相似文献
3.
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.
相似文献
4.
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.
相似文献
5.
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.
相似文献
6.
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.
相似文献
7.
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
8.
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.
相似文献
9.
Dynamic failure events such as armor penetration and explosive fragmentation are too complex to be treated by classical single-crack
continuum fracture mechanics. In such cases deformation and fracture result from multiple cracks, voids, and shear bands acting
simultaneously and influencing one another’s evolution. An alternative “meso” fracture mechanics is needed that treats microfailure
activity while permitting fast and inexpensive predictive computations. This paper discusses the approach and experiments
that elucidate and quantify failure physics on the micron level. “Rosetta Stone” experiments that isolate a damage mode, produce
statistical distributions of damage features, and “freeze in” damage at various stages of development are described and illustrated.
The observations and data lead to equations describing nucleation and growth of cracks, voids, and shear bands. The resulting
mesomechanical material failure models link the microworld with the macroworld and can be used in continuum hydrocodes for
fast, efficient simulations of dynamic fracture scenarios.
相似文献
10.
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.
相似文献
11.
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.
相似文献
12.
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.
相似文献
13.
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
14.
An anionic polyacrylamide solution was characterized in elongational flow by combining laser-Doppler velocimetry to determine
the strain rate in the flow direction and the two-color flow-induced birefringence method to measure the first normal stress
difference along the axial centerline of a hyperbolic die. The elongational rate was constant along the axial centerline of
the planar hyperbolic die as long as vortices at the die entrance did not occur. The transient elongational viscosity μ
+ was determined as a function of the elongational rate. The parameters varied are the Hencky strain rate and the polymer concentration.
μ
+ showed a pronounced increase over the linear viscoelastic behavior above critical Hencky strains of 1.2 to 1.5; that is, a significant strain hardening could be observed for polyacrylamide
solutions. This strain hardening is stronger the higher the elongational rate. A slight enhancement of strain hardening was
found by increasing the concentration from 0.5 to 1 g/l. The stress optical coefficient was determined as 1.8 × 10 −7 Pa −1 (0.5 g/l) and 1.2 × 10 −7 Pa −1 (1 g/l).
相似文献
15.
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.
相似文献
16.
A novel compact low-frequency oscillating hot-wire (OHW) anemometer is calibrated in a custom-built wind tunnel. Laser Doppler anemometry is used for reference velocity measurements, phase-locked with the oscillating wire. Three probe designs are calibrated, examining the influence of prong shape on the wake contamination. Results for two oscillation amplitudes and several frequencies are discussed. Through non-dimensional analysis, the optimum probe design and operating parameters are extracted. The OHW features a maximum measurable negative velocity of −1.0 m/s which is comparable to existing oscillating and flying hot-wire anemometers. The compact OHW can be applied to reversing flow in confined geometries such as flow in exhaust systems. 相似文献
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.
We study the free surface above a turbulent channel flow. We describe a laser scanning technique that can be used to measure the space–time turbulent surface gradient field along a line. A harmonically swiveling laser beam is focused on the surface and its angle of refraction is measured using a position sensing device. The registered signals can be converted easily to the desired gradient field, and spectra and correlations can be measured. Examples of measured spectra and correlation functions of the surface above a turbulent channel flow (Reynolds number R
λ ≈ 250) demonstrate the viability of the technique. We further assess the validity of Taylor’s frozen turbulence hypothesis that implies that time-dependent signals measured along a line that is oriented perpendicularly to the mean channel velocity can be interpreted as 2D measurements of the surface slope. While Taylor’s hypothesis works for a turbulent velocity field, it does not work for its free surface. 相似文献
19.
The performance of three implementations of pinhole-based camera models for use in common light-sheet imaging arrangements is investigated on the background of application to particle image velocimetry (PIV) and Doppler global velocimetry (DGV). Calibration data obtained from translated planar calibration targets was found to yield camera attitude within 0.1° on four different test cases with object distance varying as little as 2% depending on the choice of camera model. Camera calibration using data from a single image of coplanar points is considered a viable alternative to manual triangulation of camera positions but is restricted to off-normal viewing directions. 相似文献
20.
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
|