A comparison of 2D and 3D PIV measurements in an oblique jet

A stereo PIV (SPIV) acquisition and analysis system was developed to measure three velocity components in planar flow fields. The analysis software is based on a third order mapping function method. The system was calibrated by imaging a square grid in three measurement planes with two Kodak Megaplus cameras oriented at 30 to the bisector between them. The camera images were dewarped into real coordinates by employing a set of transform matrices computed for each calibration plane. Bias and rms errors were determined by comparing displacements measured directly with displacements estimated from the dewarping and recombination algorithm. The bias errors in the directions parallel with the measurement plane were negligible while the bias in thez direction was about 0.6 pixel. The rms errors, 0.2–0.3 pixels, were largest in thez direction. These errors were thought to result from limitations in the calibration method. The SPIV system was tested in a two-dimensional oblique jet with Reynolds number of 1800. The three dimensional results were taken in a vertical (x, y) plane parallel with the jet span. The SPIV results were compared with LDV data and two-dimensional PIV data obtained in a vertical (y, z) plane of the same jet. The SPIV measurements yielded accurate values for the in-plane mean and rms velocity components. The measured out-of-plane mean component was underestimated due to the bias error mentioned above. The rms component was accurate in part of the field but overestimated in another part due to local variations in rms error. It is expected that in the future, the out-of-plane errors can be minimized by improving the calibration and transformation procedures.     

Enhancement of measurement accuracy of X‐ray PIV in comparison with the micro‐PIV technique

The X‐ray PIV (particle image velocimetry) technique has been used as a non‐invasive measurement modality to investigate the haemodynamic features of blood flow. However, the extraction of two‐dimensional velocity field data from the three‐dimensional volumetric information contained in X‐ray images is technically unclear. In this study, a new two‐dimensional velocity field extraction technique is proposed to overcome technological limitations. To resolve the problem of finding a correction coefficient, the velocity field information obtained by X‐ray PIV and micro‐PIV techniques for disturbed flow in a concentric stenosis with 50% severity was quantitatively compared. Micro‐PIV experiments were conducted for single‐plane and summation images, which provide similar positional information of particles as X‐ray images. The correction coefficient was obtained by establishing the relationship between velocity data obtained from summation images (V_S) and centre‐plane images (V_C). The velocity differences between V_S and V_C along the vertical and horizontal directions were quantitatively analysed as a function of the geometric angle of the test model for applying the present two‐dimensional velocity field extraction technique to a conduit of arbitrary geometry. Finally, the two‐dimensional velocity field information at arbitrary positions could be successfully extracted from X‐ray images by using the correction coefficient and several velocity parameters derived from V_S.     

Visualization of low Reynolds boundary-driven cavity flows in thin liquid shells

Abstract  

Classic examples of low-Reynolds recirculating cavity flows are typically generated from lid-driven boundary motion at a solid–fluid interface, or alternatively may result from shear flow over cavity openings. Here, we are interested in an original family of boundary-driven cavity flows occurring, in contrast to classic setups, at fluid–fluid interfaces. Particle image velocimetry (PIV) is used to investigate the structure of internal convective flows observed in thin liquid shells. Under the specific configuration investigated, the soap bubble’s liquid shell is in fact in motion and exhibits sporadic local “bursts”. These bursts induce transient flow motion within the cavity of order Re ∼ O(1). The combination of PIV and proper orthogonal decomposition (POD) is used to extract dominant flow structures present within bubble cavities. Next, we show that thermally induced Marangoni flows in the liquid shell can lead to forced, (quasi) steady-state, internal recirculating flows. The present findings illustrate a novel example of low-Reynolds boundary-driven cavity flows.     

超声粒子图像测速技术及应用

心血管疾病的产生与动脉血流的流动状况密切相关。然而,目前普遍应用的超声多普勒成像技术不能精确测量复杂血流流场信息。本文提出了一种基于超声造影微泡的超声全流场粒子图像测速技术,能够获得多维流速速度信息,且不依赖于声束与速度向量之间的夹角。本文首先着重阐述了超声全流场粒子测速技术的基本原理以及系统组成,并对直管流和旋转流场流体动力学特性进行了实验测试研究,实验结果表明本技术能够测量全流场速度,并可作为表征复杂血流流场的有力手段。      

Bloch enhancement of electric field gradient in Fe and Cu host alloys

The Bloch enhancement factor α(k _f ) of the electric field gradient has been evaluated for the half-filledd-core Fe host metal and completely filledd-core Cu host metal in single orthogonalized plane wave (OPW) approximation. For this purpose the radially-dependent antishielding factors,γ(r) have been calculated in non-orthogonal Hartree-Fock perturbation theory (NHFPT). The results show that the contributions of antishielding to α(k _f ) from the plane wave-plane wave part and the core part of the OPW state are individually large but opposite in sign and thereby lead to partial cancellation. The net effect of antishielding on α(k _f ) is found to be − 5.6% in Fe and 14% in Cu.     

Combined stereoscopic particle image velocimetry and line integral convolution methods

The flow fields for a sphere sedimenting through a Newtonian and two non-Newtonian liquids near a wall in a square tank are investigated using 3-D stereoscopic particle image velocimetry (PIV) and line integral convolution (LIC) methods. The PIV data were taken using an angular stereoscopic configuration with tilt and shift arrangements for the Scheimpflug condition and a pair of liquid correction prisms. Data were recorded from planes perpendicular and parallel to the wall for each fluid case over a range of distances from the wall. The PIV and LIC results highlight significant differences in the wake structure for all three cases. Out of plane flow was also found to persist up to two sphere diameters downstream in the wake for all cases.     

非标定波长调制吸收光谱气体测量研究

为消除可调谐激光调制吸收光谱气体测量技术对于标定过程的依赖,研究了二次谐波信号的非标定波长调制气体测量方法.通过对测量的二次谐波线型进行分析,给出相同工况下二次谐波模拟信号,并利用测量与模拟二次谐波信号进行线性拟合直接计算气体浓度.实验室内采用非标定波长调制气体测量方法,利用 6336.24 cm^-1处特征吸收谱线对10 cm长气体吸收池内的CO₂进行了测量.结果表明,非标定波长调制气体测量方法可适应各种不同条件,适合于现场气体在线测量.当调制系数在1.8—3. 关键词： 波长调制 二次谐波 吸收光谱 半导体激光器     

Triangle Distribution and Equation of State for Classical Rigid Disks

The triangle distribution function f ⁽³⁾ for three mutual near neighbors in the plane describes basic aspects of short-range order and statistical thermodynamics in two-dimensional many-particle systems. This paper examines prospects for constructing a self-consistent calculation for the rigid-disk-system f ⁽³⁾. We present several identities obeyed by f ⁽³⁾. A rudimentary closure suggested by scaled-particle theory is introduced. In conjunction with three of the basic identities, this closure leads to an unique f ⁽³⁾ over the entire density range. The pressure equation of state exhibits qualitatively correct behaviors in both the low-density and the close-packed limits, but no intervening phase transition appears. We discuss extensions to improved disk closures, and to the three-dimensional rigid-sphere system.     

Particles of different density in thermocapillary liquid bridges under the action of travelling and standing hydrothermal waves

We observed particles of different density ratio α = ρ _p /ρ _f in thermocapillary liquid bridges with steady and with time-dependent flow under normal- and under microgravity. Particle accumulation structures (PAS) visualize some features of the hydrothermal wave in the liquid bridge. Relatively fast formation of PAS from particles which are considerably less dense than the fluid (α = 0.42) in oscillatory thermocapillary flow of top-heated liquid bridges was observed and explained by an additional buoyancy-assisted mechanism which brings the particles into the surface flow. This PAS from particles with α = 0.42 will persist under normal gravity for infinite time. In contrast to these less dense particles the heavier particles with α > 1 settle down under normal gravity on the lower end face of the liquid bridge after some time and are no longer in suspension and PAS will fade out. On the other hand, particles with α = 0.42 will be less suited for experiments under microgravity than particles with α > 1 because most of them will be trapped in the vortex centre of the thermocapillary flow. The sedimented particles with α > 1 are a means to visualize some features of standing hydrothermal waves which are visualzed and discussed for the first time.     

Visualization and characterization of small-scale spanwise vortices in turbulent channel flow

High-resolution particle-image velocimetry (PIV) measurements are made in the streamwise-wall-normal plane of turbulent channel flow at Re_τ=566, 1184 and 1759, facilitating documentation of the population trends and core diameters of small-scale spanwise vortices. Swirling strength, an unambiguous vortex-identification criterion and hence a local marker of rotation, is used to extract small-scale spanwise vortex cores from the instantaneous velocity fields. Once the small-scale vortices are properly extracted from the PIV realizations, their characteristics are studied in detail. The present results indicate that the very-near-wall region (y < 0.1h) is densely populated by spanwise vortices with clockwise (negative) rotation. This behavior supports the notion that hairpin-like vortices are generated very close to the wall and grow into the outer layer as they advect downstream. In contrast, counterclockwise (positive) spanwise vortices are scarce in the very-near-wall region, but their presence steadily increases within the logarithmic layer presumably due to a localized generation mechanism. The average core diameter of negative spanwise vortices is found to be larger than the average diameter of positive vortices, with few positive vortices having core diameters exceeding 80y.     

Generation of vortices by a streamwise oscillating cylinder

The wake of a streamwise oscillating cylinder is presently investigated. The Reynolds number investigated is 300, based on the cylinder diameterd. The cylinder oscillates at an amplitude of 0.5d and a frequencyf _e/f_s=1.8, wheref _e is the cylinder oscillating frequency andf _s is the natural vortex shedding frequency of a stationary cylinder. Under these conditions the flow is essentially two dimensional. A two-dimensional direct numerical simulation (DNS) scheme has been developed to calculate the flow. The DNS results display a street of binary vortices, each containing two counter-rotating vortical structures, symmetrical about the centerline, which is in excellent agreement with measurements. The drag and lift on the cylinder have been examined. The time averaged drag and lift are 1.4 and 0, respectively, which are the same as those on a stationary cylinder at the sameRe. However, the fluctuating drag was high, about 2.68. It has been found that, being symmetrically formed about the centerline, the binary vortices induce an essentially zero fluctuating lift, which may have a profound implication in flow control and engineering.     

Echo particle image velocimetry technique and its applications

Developments of many cardiovascular problems have been shown to have a close relationship with arterial flow conditions.However,current ultrasound/Doppler imaging techniques cannot resolve the complex nature of arterial blood flow.We have recently developed a novel contrast-based echo particle imaging technique(Echo PIV) without angel dependence for non-invasively measuring multi-component flow vectors.This study introduces the Echo PIV principles,system characterization and utility examination to characterize hemodynamics in pipe laminar flow and rotating flow.Echo PIV measurement results show its capability to resolve the complex hemodynamics including proximal flow velocity vectors,and velocity mapping. The Echo PIV method provides an easy,direct and accurate means of quantitatively yet non-invasively characterizing the complex vascular hemodynamics.     

High speed analogue autocorrelation for PIV transparency analysis using a ferroelectric liquid crystal spatial light modulator

This paper describes a novel automated high-speed optical autocorrelation system for particle image velocimetry (PIV) analysis. In this system, a bistable ferroelectric optically addressed spatial light modulator (BOASLM) is used for real time optical autocorrelation. Input data from a PIV transparency is scanned into the system by utilising two scanning mirrors and the output autocorrelation signals are detected with a two-dimensional scanning tracking system implemented by two acousto-optic modulators. This data extraction system is equivalent to a random access camera, which is used to track the autocorrelation signals from region to region in order to retrieve velocity data at the full frame rate of the BOASLM. The factors that influence the processing capacity of the optical system are discussed in detail and a critical comparison is made with digital analysis systems. A working prototype of an optical PIV analysis system is described and the measurement of 1296 velocity vectors from a PIV transparency in approximately five seconds is demonstrated.     

Heated granular fluids: The random restitution coefficient approach

We introduce the model of inelastic hard spheres with random restitution coefficient α, in order to account for the fact that, in a vertically shaken granular system interacting elastically with the vibrating boundary, the energy injected vertically is transferred to the horizontal degrees of freedom through collisions only, which leads to heating through collisions, i.e. to inelastic horizontal collisions with an effective restitution coefficient that can be larger than 1. This allows the system to reach a non-equilibrium steady state, where we focus, in particular, on the single-particle velocity distribution f (v) in the horizontal plane, and on its deviation from a Maxwellian. Molecular Dynamics simulations and Direct Simulation Monte Carlo (DSMC) show that, depending on the distribution of α, different shapes of f (v) can be obtained, with very different high-energy tails. Moreover, the fourth cumulant of the velocity distribution quantifying the deviations from Gaussian statistics is obtained analytically from the Boltzmann equation and successfully tested against the simulations. Received 24 November 2000 and Received in final form 8 February 2001     

Boiling up of jets of superheated ethanol-water solutions

The stability and evolution of the shapes of jets of strongly superheated aqueous solutions of ethanol flowing out of a high-pressure chamber are experimentally investigated at various initial parameters. It has been found that a low-boiling liquid causes shifting to lower temperatures of the boundaries between various kinds of boiling-up jets, such as cylindrical, conical, and open. Large-scale pulsations of the jet shapes are observed in the temperature range from 454 to 484 K, where the jet completely breaks down. The conditions for the formation of large-scale low-frequency pulsations with a power spectrum diverging inversely proportional to the frequency f (1/f spectra) have been revealed. Such fluctuations prove that the flow is unstable and large-scale low-frequency ejections are possible.     

Particle image velocimetry in aerodynamics: Technology and applications in wind tunnels

Particle image velocimetry (PIV) is increasingly used for aerodynamic research and development. The PIV technique allows the recording of a complete flow velocity field in a plane of the flow within a few microseconds. Thus, it provides information about unsteady flow fields, which is difficult to obtain with other experimental techniques. The short acquisition time and fast availability of data reduce the operational time, and hence cost, in large scale test facilities. Technical progress made in the last years allowed DLR to develop a reliable, modular PIV system for use in industrial wind tunnels. The features of this system are summarized and results of recent PIV applications are presented.     

On Poisson's ratio of glass and liquid vitrification characteristics

The apparent activation energy for viscous flow of a glass-forming liquid, E_a , and its ratio to temperature T_f , m_f (=?E_a /2.3T_f ), are seen as a measure of the viscosity's departure from the Arrhenius equation. As the viscosity, η, increases on cooling, E_a and m_f increase. On slow cooling, a liquid vitrifies at a lower T, higher η and higher m_f , and on rapid cooling it vitrifies at a higher T, lower η and lower m_f . It is argued that Poisson's ratio, υ _Poisson , and the instantaneous bulk modulus to shear modulus ratio, K _∞/G _∞, of a liquid (and glass) would decrease as it departs from Arrhenius behaviour or m_f increases. Available data on structural relaxation of metal alloy glasses confirm it. Therefore, as a glass spontaneously becomes denser with time, m_f increases and its state becomes dilationally stiffer. This finding is opposite to the correlation that, amongst glass-forming liquids of different chemical compositions, υ _Poisson and K _∞/G _∞ increase when departing from Arrhenius behaviour at T_g or m increases (V.N. Novikov and A.P. Sokolov, Nature 431 961 (2004)). Further analysis shows that a liquid's structure has a predominant effect on its elastic constants.     

A high-speed all-digital technique for cycle-resolved 2-D flow measurement and flow visualisation within SI engine cylinders

Particle image velocimetry (PIV) is now a well-established planar flow measurement technique for the study of in-cylinder flow fields in internal combustion engines. Here the authors describe a turnkey, high-speed digital imaging system that provides combined real-time flow visualisation and rapidly processed PIV data in an industrial optical research engine facility. The system is based on commercially available, high-speed imaging and laser technology and conventional digital cross-correlation processing to provide cycle-resolved PIV data and flow visualisation within timescales appropriate for engine development. A simple variation on the synchronisation scheme also allows the acquisition of tens to thousands of flow visualisation sequences and PIV maps at the same crank angle, thus giving the potential for the study of cycle-to-cycle flow variability and its effect on combustion stability in a suitably instrumented optical engine. The technique may also find applications in other unsteady or oscillatory flows of importance in aerodynamics, acoustics, mixing, and heat transfer.     

Heat transfer by laminar flow of an elastico-viscous liquid along a plane wall with periodic suction

The problem of heat transfer by the laminar flow of an elastico-viscous liquid along a plane wall with periodic suction has been considered. A perturbation technique has been used to obtain an approximate solution of the differential equations. The flow phenomenon has been characterized by the non-dimensional parameters like the elastic number (S), the Reynolds number (R), the Prandtl number (P) and the Eckert number (E). The effects of these parameters on the temperature distributions and the rate of heat transfer at the wall have been studied.     

Investigations of three-dimensional flow characteristics in a liquid ramjet combustor using the PIV method

Three-dimensional flow characteristics in a liquid fuel ramjet combustor were investigated using the PIV method. The combustor had two rectangular inlets that form a 90-degree angle with each other, with intake angles of 30 degrees. Three guide vanes were installed in each rectangular inlet to improve flow stability. The experiments were performed in a water tunnel test with the same Reynolds number as Mach 0.3 at the inlet. PIV software was developed to measure the characteristics of the flow field in the combustor. Accuracy of the developed PIV program was verified with a rotating disk experiment and standard data. The experimental results showed that the two main streams from the rectangular intakes collided near the plane of symmetry and generated two large longitudinal vortices, which was in agreement with three dimensional computational results. A large and complex threedimensional recirculating flow was measured behind the intakes.