Measurements of the air flow field in the freezer compartment of a top-mount no-frost refrigerator: the effect of temperature

The energy consumption of a household refrigerator is closely related to the distributions of temperature inside its compartments. Since, such distributions are dependent on the chilled air circulating inside the refrigerator, its energy consumption can be reduced and its efficiency improved through optimization of the air flow fields inside the compartments. In this work, particle image velocimetry (PIV) was used to measure velocity fields inside the freezer compartment of a commercial top-mount no-frost refrigerator at two critical vertical sections at three different temperatures levels. To perform the tests, a commercial unit was equipped with two specially constructed insulated windows which enabled clear visualization of the flow field under real operating conditions. The results show that significant changes in the air flow distribution occur when the temperature is reduced.     

Using PIV to measure granular temperature in saturated unsteady polydisperse granular flows

The motion of debris flows, gravity-driven fast moving mixtures of rock, soil and water can be interpreted using the theories developed to describe the shearing motion of highly concentrated granular fluid flows. Frictional, collisional and viscous stress transfer between particles and fluid characterizes the mechanics of debris flows. To quantify the influence of collisional stress transfer, kinetic models have been proposed. Collisions among particles result in random fluctuations in their velocity that can be represented by their granular temperature, T. In this paper particle image velocimetry, PIV, is used to measure the instantaneous velocity field found internally to a physical model of an unsteady debris flow created by using “transparent soil”—i.e. a mixture of graded glass particles and a refractively matched fluid. The ensemble possesses bulk properties similar to that of real soil-pore fluid mixtures, but has the advantage of giving optical access to the interior of the flow by use of plane laser induced fluorescence, PLIF. The relationship between PIV patch size and particle size distribution for the front and tail of the flows is examined in order to assess their influences on the measured granular temperature of the system. We find that while PIV can be used to ascertain values of granular temperature in dense granular flows, due to increasing spatial correlation with widening gradation, a technique proposed to infer the true granular temperature may be limited to flows of relatively uniform particle size or large bulk.     

Particle tracking techniques for electrokinetic microchannel flows

We have applied particle tracking techniques to obtain spatially resolved velocity measurements in electrokinetic flow devices. Both micrometer-resolution particle image velocimetry (micro-PMV) and particle tracking velocimetry (PTV) techniques have been used to quantify and study flow phenomena in electrokinetic systems applicable to microfluidic bioanalytical devices. To make the flow measurements quantitative, we performed a series of seed particle calibration experiments. First, we measure the electroosmotic wall mobility of a borosilicate rectangular capillary (40 by 400 microm) using current monitoring. In addition to this wall mobility characterization, we apply PTV to determine the electrophoretic mobilities of more than 1,000 fluorescent microsphere particles in aqueous buffer solutions. Particles from this calibrated particle/ buffer mixture are then introduced into two electrokinetic flow systems for particle tracking flow experiments. In these experiments, we use micro-PIV, together with an electric field prediction, to obtain electroosmotic flow bulk fluid velocity measurements. The first example flow system is a microchannel intersection where we demonstrate a detailed documentation of the similitude between the electrical fields and the velocity fields in an electrokinetic system with uniform zeta potential, zeta. In the second system, we apply micro-PIV to a microchannel system with nonuniform zeta. The latter experiment provides a simultaneous measurement of two distinct wall mobilities within the microchannel.     

Measuring the velocity fields of granular flows – Employment of a multi-pass two-dimensional particle image velocimetry (2D-PIV) approach

Measuring velocity fields plays a crucial role in investigating the dynamics of granular flows, which can improve the modeling of hazardous geophysical flows (e.g. avalanches and debris flows) and the control of powder flows in industrial applications. Non-invasive optical methods are invaluable tools for estimating this physical quantity at the laboratory scale. Despite the recent improvements of particle image velocimetry (PIV) algorithms, the employment of PIV to granular flows is still a non-trivial application, where there are several specific aspects to be carefully addressed. Here, we address the main challenges of granular PIV applications and systematically test the open-source window deformation multi-pass code, PIVlab [Thielicke and Stamhuis, J. Open Res. Soft., 2014], for dry granular flows in rotating drum and chute flow experiments. Three granular media (glass spheres, Ottawa sand and acetalic resin beads) with different optical properties are used as a broad test bench for validating the PIV approach. As well, comparisons between the estimations by PIVlab and those obtained by the commercial code, IDT ProVision-XS, are reported, where the advantages of the multi-pass approach are highlighted. This extensive experimental investigation allowed the evaluation of the accuracy of PIVlab in granular flow applications and also helped to assess the reliability of measurements of second-order statistics, such as the granular temperature. Finally, a guideline for setting a reliable PIV arrangement is suggested.     

Velocity field measurement of a round jet using quantitative schlieren

This paper utilizes the background oriented schlieren (BOS) technique to measure the velocity field of a variable density round jet. The density field of the jet is computed based on the light deflection created during the passage of light through the understudy jet. The deflection vector estimation was carried out using phase-based optical flow algorithms. The density field is further exploited to extract the axial and radial velocity vectors with the aid of continuity and energy equations. The experiment is conducted at six different jet-exit temperature values. Additional turbulence parameters, such as velocity variance and power spectral density of the vector field, are also computed. Finally, the measured velocity parameters are compared with the hot wire anemometer measurements and their correlation is displayed.     

Nonstationary phenomena in the region of shock-wave interaction with a boundary layer at transonic flow velocities

Nonstationary characteristics of detached flow have been experimentally studied during interaction of the boundary layer with a shock wave that appears on a profiled bump in transonic flow. The experiments were performed with variable shock-wave intensity and position in a T-325 wind tunnel. The flow was studied using methods of schlieren imaging, measuring average pressure and its pulsations on the surface of a model, and determining velocity fields by particle image velocimetry. Analysis of the experimental data showed that the observed shock-wave oscillations and flow pulsations in the detachment zone were related to disturbances present in the oncoming boundary layer.     

Three-dimensional rainbow schlieren tomography of a temperature field in gas flows

We present quantitative rainbow schlieren deflectometry with tomography for measurements of temperature in three-dimensional gas flows. The schlieren apparatus with a continuously graded spectral filter of known transmissivity was used to create color schlieren images of the test media. These images at multiple viewing angles were used to infer beam deflection angles by the medium. The deflection data were used with a tomographic technique to reconstruct the refractive index and thus the temperature field. The temperature distributions obtained by the rainbow schlieren tomography agreed with those measured by a thermocouple probe. This research demonstrates that tomography can be used with full-field schlieren deflectometry to measure quantitatively temperature in asymmetric gas flows. The technique could be used to obtain related properties such as pressure, density, and gas composition.     

Novel particle image velocimetry system based on three-color pulsed lamps and image processing

A novel particle image velocimetry (PIV) measurement system based on flashed lamps as light sources and digital reconstruction of particle positions by image processing is presented. Three different colors are used in order to distinguish, on a same photograph, the corresponding positions of each moving particle at three different instants of time. This solution can give significant advantages in several aerodynamic and hydrodynamic situations. In particular, it allows high flexibility for velocity measurements and independent settings of different acquisition parameters, like pulse powers and timing. Furthermore, the developed PIV measurement system is fully transportable, eye-safe, practical, and economical. The first hydrodynamics measurements achieved by using the proposed velocimetry measurement system are reported.     

Eulerian-Lagrangian analysis for particle velocities and trajectories in a pure wave motion using particle image velocimetry

This paper investigates the velocity and the trajectory of water particles under surface waves, which propagate at a constant water depth, using particle image velocimetry (PIV). The vector fields and vertical distributions of velocities are presented at several phases in one wave cycle. The third-order Stokes wave theory was employed to express the physical quantities. The PIV technique's ability to measure both temporal and spatial variations of the velocity was proved after a series of attempts. This technique was applied to the prediction of particle trajectory in an Eulerian scheme. Furthermore, the measured particle path was compared with the positions found theoretically by integrating the Eulerian velocity to the higher order of a Taylor series expansion. The profile of average travelling distance is also presented with a solution of zero net mass flux in a closed wave flume.     

Contrast-enhancement techniques for particle-image velocimetry

In video-based particle-image velocimetry (PIV) systems for fluid mechanics research, it is sometimes desirable to image seed particles to be smaller than a camera pixel. However, imaging to this size can lead to marginal image contrast such that significant numbers of erroneous velocity vectors can be computed, even for simple flow fields. A variety of image-enhancement techniques suitable for a low-cost PIV system that uses video cameras are examined and tested on three representative flows. Techniques such as linear contrast enhancement and histogram hyperbolization are shown to have good potential for improving the image contrast and hence the accuracy of the data-reduction process with only a 15% increase in the computational time. Some other schemes that were examined appear to be of little practical value in PIV applications. An automated shifting algorithm based on mass conservation is shown to be useful for displacing the second interrogation region in the direction of flow, which minimizes the number of uncorrelated particle images that contribute noise to the data-reduction process.     

稀薄气体流动的粒子图像测速实验研究

建立了一套可对真空度进行调节的实验装置,从大气压开始逐渐降低系统压力,进行系列稀薄气体流动的PIV流场实验.系统压力从101kPa、90kPa逐次降至10kPa,使用所选粒子在不同系统压力下分别进行PIV实验获得测量区域流场分布情况.运用计算流体力学的方法模拟大气压条件下实验区域的内部流动,对比结果发现该种条件下数值模...     

Phase-conjugate holographic system for high-resolution particle-image velocimetry

A novel holographic particle-image velocimeter system has been developed for the study of threedimensional (3-D) fluid velocity fields. The recording system produces 3-D particle images with a resolution, a signal-to-noise ratio, an accuracy, and derived velocity fields that are comparable to high-quality two-dimensional photographic particle-image velocimetry (PIV). The high image resolution is accomplished through the use of low f-number optics, a fringe-stabilized processing chemistry, and a phase conjugate play-back geometry that compensates for aberrations in the imaging system. In addition, the system employs a reference multiplexed, off-axis geometry for the determination of velocity directions with the cross-correlation technique, and a stereo camera geometry for the determination of the three velocity components. The combination of the imaging and reconstruction subsystems makes the analysis of volumetric PIV domains feasible.     

Volume fraction and velocity fields of nearly uniform granular flows in a narrow channel geometry with smooth bed

The combined knowledge of the velocity and volume fraction fields is crucial for investigating the dynamics of granular flows, especially in the dense-collisional regime where both frictional and collisional dissipation mechanisms are significant. A laboratory investigation on steady dry granular flows in a straight channel is reported, where slip conditions are allowed at the basal surface and side walls. The stochastic-optical method (SOM), proposed by Sarno et al. (2016) for estimating the volume fraction in granular mixtures, is applied for the first time to granular flows. The velocity at the free surface and at the flume sidewall is measured by using a multi-pass particle image velocimetry (PIV) approach. The measurements of the velocity and volume fraction reveal a superimposition of different dynamic structures, which can be distinguished by means of a volume fraction threshold. Additionally, the profiles of measured volume fraction are exploited to estimate the pressure distribution, so as to numerically describe the velocity profiles by using the μ(I) rheology. It is found that the employment of the experimental volume fraction is superior in describing the flow dynamics, especially near the free surface.     

PIV血流场显示测速技术

通过分析多普勒测速技术与粒子图像测速技术的区别,从一个新角度把PIV全流场测速技术应用于血液流场的研究中。用激光片光源照亮血流粒子场,再计算确定实验系统光学参数,以获得最佳流场图片。对流场分析常用的互相关算法进行改进,辅以曲面拟合和误差修正,获得了亚像素级的全流场速度的大小和方向,并进一步计算出血流场的涡量分布和剪切率分布。为了验证改进的算法,对日本视频协会提供的PIV-STD序列标准图像进行仿真计算和误差分析,与原算法相比其速度矢量图的误差降低了2个百分点,流场速度值的平均误差小于±1%。该结果表明文中建立的方法是有效的,并可推广用于其它的流场分析。     

聚合物真实黏度流变仪收缩流道轴向速度场的PIV实验研究及分析EI北大核心CSCD

为研究测量聚合物真实黏度流变仪收缩流道轴向速度分布规律,建立了收缩流道实验模型及实验方案。利用粒子图像测速(PIV)系统拍摄牛顿流体聚丁二烯在收缩流道流场中粒子的图像,利用Tecplot、Origin软件对图像和数据进行分析处理,得到聚合物真实黏度流变仪收缩流道的轴向速度曲线。同时将PIV实验、Polyflow仿真、理论计算结果相比较,发现三者规律基本吻合。证明了从PIV实验中得到牛顿流体聚丁二烯收缩流道速度场的方法是可行的。     

Laser-induced fluorescence measurements of translational temperature and relative cycle number by use of optically pumped trace-sodium vapor

Sodium fluorescence induced by a narrow-bandwidth tunable laser has been used to measure temperature, pressure, axial velocity, and species concentrations in wind tunnels, rocket engine exhausts, and the upper atmosphere. Optical pumping of the ground states of the sodium, however, can radically alter the shape of the laser-induced fluorescence excitation spectrum, complicating such measurements. Here a straightforward extension of rate equations originally proposed to account for the features of the pumped spectrum is used to make temperature measurements from spectra taken in pumped vapor. Also determined from the spectrum is the relative fluorescence cycle number, which has application to measurement of diffusion rate and transverse flow velocity. The accuracy of both the temperature and the cycle-number measurements is comparable with that of temperature measurements made in the absence of pumping.     

Application of particle image velocimetry for measuring He II thermal counterflow jets

The particle image velocimetry (PIV) technique was successfully applied for measuring the velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen-deuterium solid particles were used as tracer particles for PIV measurement. In the application, the normal component velocity was measured. The jet velocity profile and spatial decay of the jet velocity were compared with those of turbulent round jets of ordinary viscous fluids. The velocity measured near the jet nozzle exit was compared with the theoretical prediction for the normal component flow velocity.     

Eliminating background noise effect in micro-resolution particle image velocimetry

A novel method is developed to improve the accuracy of micro-resolution particle image velocimetry (PIV) in microfluidics measurements. This method utilizes the Laplacian of Gaussian method and image-processing techniques to eliminate the background scattering noise. A high signal-to-noise ratio image has been obtained. This technique is especially suitable for improving micro-resolution PIV in micro, two-, or multiphase flow conditions, such as for submicron bubbly flow measurements in a microchannel. The method can easily be implemented with minimal modification of the conventional PlV system. The results of simulation and experiments demonstrated the feasibility of this, to our knowledge, new method.     

PIV experimental investigation of entrance configuration on flow maldistribution in plate-fin heat exchanger

Flow characteristics of flow field in the entrance of plate-fin heat exchanger have been investigated by means of particle image velocimetry (PIV). The velocity fields were measured using the two-frame cross-correlation technique. A series of velocity vector and streamline graphs of different cross-sections are achieved in the experiment. The experimental results indicate that performance of fluid maldistribution in conventional entrance configuration is very serious, while the improved entrance configuration with punched baffle can effectively improve the performance of fluid flow distribution in the entrance. Based on the analysis of the fluid flow maldistribution, a baffle with small holes is recommended to install in the entrance configuration in order to improve the performance of flow distribution. When the punched baffle is proper in length, the small holes is distributed in staggered arrangement, and the punched ratio gradually increases from central axis to the boundary along with the baffle length, the performance of flow distribution in plate-fin heat exchanger is effectively improved by the optimum design of the entrance configuration. The flow maldistribution parameter S in plate-fin heat exchanger has been reduced from 1.21 to 0.209 and the ratio of the maximum velocity to the minimum θ is reduced from 23.2 to 1.76 by installing the punched baffle. The results validate that PIV is well suitable to investigate complex flow pattern and the conclusion of this paper is of great significance in the optimum design of plate-fin heat exchanger.     

A stereo PIV system for measuring the velocity vector in complex gas flows

A stereo PIV system for investigating complex highly turbulent flows, typical for gas flows in aircraft engines and gas turbines, is developed, and the values obtained for the velocity are compared with the results measured using a laser Doppler anemometer. The features of the use of the PIV system for measurements in three-dimensional highly turbulent flows with high velocity gradients are investigated.