New tracking algorithm for particle image velocimetry

The cross correlation tracking technique is widely used to analyze image data, in Particle Image Velocimetry (PIV). The technique assumes that the fluid motion, within small regions of the flow field, is parallel over short time intervals. However, actual flow fields may have some distorted motion, such as rotation, shear and expansion. Therefore, if the distortion of the flow field is not negligible, the fluid motion can not be tracked well using the cross correlation technique. In this study, a new algorithm for particle tracking, called the Spring Model technique, has been proposed. The algorithm can be applied to flow fields which exhibit characteristics such as rotation, shear and expansion.The algorithm is based on pattern matching of particle clusters between the first and second image. A particle cluster is composed of particles which are assumed to be connected by invisible elastic springs. Depending on the deformation of the cluster pattern (i.e., the particle positions), the invisible springs have some forces. The smallest force pattern in the second image is the most probable pattern match to the correspondent original pattern in the first image. Therefore, by finding the best matches, particle movements can be tracked between the two images. Three-dimensional flow fields can also be reconstructed with this technique.The effectiveness of the Spring Model technique was verified with synthetic data from both a two-dimensional flow and three-dimensional flow. It showed a high degree of accuracy, even for the three-dimensional calculation. The experimental data from a vortex flow field in a cylinder wake was also measured by the Spring model technique.     

Theory and application of the laser shadow technique

In a previous publication a laser shadow technique for the quantitative investigation of turbulent flow has been introduced. Only simple two-dimensional flow fields have been considered for this purpose. In the present paper a method is provided that allows the application of this laser shadow technique to axisymmetric flow fields. The basic principle invoked is the Abel transform which has e.g. been applied extensively to determine the local emission coefficients in axisymmetric light sources. It is shown how this concept can be utilized for the quantitative optical investigation of axisymmetric turbulent flow.     

Phosphorescent tracer particles for Lagrangian flow measurement and particle tracking velocimetry

A new technique for manufacturing neutrally buoyant phosphorescent tracer particles for use in Lagrangian flow measurement and particle tracking velocimetry is presented. The particles can be manufactured with inexpensive equipment and materials, using three ingredients: paraffin wax, Keywax (a wax–rubber polymer) and LumiNova^® phosphorescent pigment. Particles can be made with a range of diameters (150–4,000 μm) and, when seeded throughout the flow, can be excited at the peak excitation wavelength of the pigment using a focused source of ultraviolet light. Under a range of lighting conditions, it is possible to excite a single particle or a chosen region of the flow to record and analyze their Lagrangian flow path. To demonstrate this technique, sample images are provided for flow in a laboratory channel with a side embayment.     

An indirect pressure-gradient technique for measuring instantaneous flow rates in unsteady duct flows

A new technique is presented for making measurements of the instantaneous flow rate in unsteady laminar pipe flows. It utilizes a relationship between flow rate and pressure-gradient history that is an exact solution to the Navier–Stokes equations for parallel, developed flow of constant-property Newtonian fluids undergoing arbitrary unsteadiness from an initially steady or stationary state. The method does not rely on any assumption about velocity profiles, and applies instantaneously in momentarily reversing flow. Experimental comparisons between direct measurements of the cumulative flow and the results of this technique indicate that it is capable of providing measurements of cumulative flow and flow rate which are accurate to within a few percent at any instant during a flow transient, provided the instantaneous pressure gradient can be measured with this accuracy.     

Measurement of surface shear stress vector distribution using shear-sensitive liquid crystal coatings

The global wall shear stress measurement technique using shear-sensitive liquid crystal (SSLC) is extended to wind tunnel measurements. Simple and common everyday equipment is used in the measurement; in particular a tungsten-halogen light bulb provides illumination and a saturation of SSLC coating color change with time is found. Spatial wall shear stress distributions of several typical flows are obtained using this technique, including wall-jet flow, vortex flow generated by a delta wing and junction flow behind a thin cylinder, although the magnitudes are not fully calibrated. The results demonstrate that SSLC technique can be extended to wind tunnel measurements with no complicated facilities used.     

Low shear viscometry in relation to brushing

Summary The flow of decorative paints should be capable of prediction from a rheological study of the paint. By taking into consideration the loss of solvent from a film during the early drying stages, a good estimation of its flow properties in relatively high film thicknesses can be obtained.Compensation for loss of solvent during drying can be made for arbitrary drying conditions which eliminate much of the complications of testing films under practical conditions. This technique is generally applicable to brushing systems where only loss of solvent is expected to occur during the period of time in which flow takes place, and cannot be applied to systems in which other changes might occur.Factors affecting the rate of change of flow with film thickness have not been studied, although the technique described may be capable of extension in this field.Presented at the Joint Meeting of the British Society of Rheology and Research Association of British Paint, Colour and Varnish Manufacturers at Teddington, April 29, 1964.     

Point-to-plane corona discharge for high-speed reacting flow visualization

We present the results of a novel technique for the high-speed visualization of a flame reaction zone using a streamer-initiated point-to-plane unipolar pulsed corona discharge. Our results show images of the flame front under conditions of natural hydrodynamic flame instability, as well as external air flow modulation induced flame instability. This technique can potentially be used as a high-speed 2-D flow visualization diagnostic tool to monitor flow instabilities in reacting and non-reacting fluids that have a density gradient. We also show that this technique does not modify the flame characteristics in any measurable way, if the high electric field region of the streamer/corona discharge is located in the downstream region.     

Probing the velocity fields of gas and liquid phase simultaneously in a two-phase flow

The feasibility of simultaneous measurements of the instantaneous velocity fields of gaseous and liquid phase is demonstrated in a laminar, unsteady two-phase flow. Thus, the instantaneous relative velocity field can be measured in such media. This is achieved by combining Particle Image Velocimetry (PIV) and a gas-phase velocimetry technique, which is based on laser-induced fluorescence (LIF) from a gaseous tracer. The wavelength shift of LIF is exploited to separate it from Mie scattering from the liquid phase. The new technique and the PIV measurement system work independently in this approach. Thus, the measurement accuracy and precision of the new technique can be validated by comparing it to the PIV results in regions of the flow field where the relative velocity vanishes. Received: 18 October 1998/Accepted: 16 October 1999     

A numerical investigation of flow past a bluff body using three-state anemometry

An application of a new flow measurement technique is described which allows for the non-intrusive simultaneous measurement of flow velocity, density, and viscosity. The viscosity information can be used to derive the flow field temperature. The combination of the three measured variables and the perfect-gas law then leads to an estimate of the flow field thermodynamic pressure. Thus, the instantaneous state of a flow field can be completely described. Three-state anemometry (3SA), a derivative of particle image velocimetry (PIV), which uses a combination of three monodisperse sizes of styrene seeding particles is proposed. A marker seeding is chosen to follow the flow as closely as possible, while intermediate and large seeding populations provide two supplementary velocity fields, which are also dependent on fluid density and viscosity. A simplified particle motion equation, aimed at turbomachinery applications, is then solved over the whole field to provide both density and viscosity data. The three velocity fields can be separated in a number of ways. The simplest and that proposed in this paper is to dye the different populations and view the region of interest through interferometric filters. The two critical aspects needed to enable the implementation of such a technique are a suitable selection of the diameters of the particle populations, and the separation of the velocity fields. There has been extensive work on the seeding particle behaviour which allows an estimate of the suitable particle diameters to be made. A technique is described in this paper to allow the separation of particles in a range of micrometer sized velocity fields through fluorescence (separation through intensity also being possible). Some preliminary results by direct numerical simulation (DNS) of a 3SA image are also presented. The particle sizes chosen were 1 μm and 5 μm, tested on the near-wake flow past a cylinder to investigate viscosity only, assuming uniform flow density. The accuracy of the technique, derived from simulations of swirling flows, is estimated as 0.5% RMS for velocity, 2% RMS for the density and viscosity, and 4% RMS for the temperature estimate. © 1998 John Wiley & Sons, Ltd.     

旋转翼面尾迹的流态显示技术

本文介绍了一种可用于显示旋转弹翼面在非旋转和旋转过程中拖出的涡及其轨迹的流态显示技术。本方法在烟线技术的基础上,采用新的发烟装置,既提高了发烟的浓度,延长了发烟的时间,并不增加任何附加力矩而使发烟部件随旋转弹体一起转动,从而实现了在旋转过程中进行动态的流态显示。文中对于应用此方法得到的流态的真实性进行了分析讨论,分析表明用此方法显示的流态具有一定的真实性,可以提供定性的旋转翼面尾涡流动特性。     

Temperature imaging of turbulent dilute spray flames using two-line atomic fluorescence

To study the behavior of thixotropic yield stress fluids, information at the local scale is required in order to determine precisely the yield point value, and the shear rate and stresses can be obtained all over the flow. This study focuses on the flow in a large shear cell of a Laponite suspension. In order to be able to construct a local rheogram for this suspension, two different methods issued from fluid mechanics and solid mechanics are used. Local velocities are determined with a PIV technique, and local stresses are determined with the photoelasticimetry technique.     

Analysis of entry flow to determine elongation flow properties revisited

The minimisation technique proposed by Binding (J. Non-Newtonian Fluid Mech., 27 (1988) 173) was used in our Generalised Engineering Bernoulli Equation framework to relate the entry pressure and stress power. We arrived at a final result similar to Binding's using assumed kinematics. Through subsequent assumptions to the kinematics we finally arrive at a result exactly equivalent to Cogswell's technique (Trans. Soc. Rheol., 16 (1972) 383). Thus, these two techniques are related in this general framework. The techniques were used to predict elongation flow properties of a polymer melt and polymer solution. The results for the polymer melt clearly show Cogswell's technique is adequate at high elongation rates. All these techniques require minimisation of the stress power with respect to the flow volume and discussion is given as to the validity of this minimisation technique. In addition, the approximate variational technique we propose gives clears limits as to when a technique, such as Cogswell's, can be applied.     

A quantitative laser sheet image processing method for the study of the coherent structure of a circular jet flow

A quantitative two-dimensional digital image processing technique is successfully developed to enhance qualitative flow visualization and to obtain quantitative results. The technique is applied to study some less known properties of the coherent structural interaction and evolution mechanism of a low Reynolds number circular jet flow under high level acoustic excitation. Before processing the quantitative data, many inherent errors and uncertainties of the instruments and the system are first discussed and corrected. In this research, the uniformity and the traceability of the flow marker are carefully tested, and the distortion of the imaging system and the fan-shape of the laser sheet are calibrated. Through the image processing technique, the spreading of the jet, the trajectory and the convective velocity of the vortex can be analyzed rapidly and simultaneously. By analyzing the constant jet fluid concentration contour, the mechanism of vortex roll-up and entrainment, which has been ambiguous by traditional pointwise measurements, are more solidly confirmed. Also, the detailed tearing process of the vortex and the evolution mechanism of partial pairing, which can not be clearly detected in the conventional flow visualization pictures, are made clearly visible and carefully delineated.A version of this paper was presented at the 11th Symp. on Turbulence, University of Missouri-Rolla, 17–19 Oct. 1988     

奇点法在叶轮机械任意回转面亚音速可压缩流动计算中的应用

本文在[3]的基础上,进一步将传统奇点法的应用范围从计算不可压缩流动推广到叶轮机械任意回转面亚音速可压缩流动的计算,并在数值处理方面做了有益的改进.算例和设计实例表明本方法计算可靠并具有实用性,为叶轮机械准三元流动计算中S_1和S_2两类流面间的反复迭代提供了一个有力的计算工具.     

非规则区域中拉普拉斯方程的有限分析5点格式

建立了非规则区域的有限分析5点格式,增加了有限分析法对不规则边界的适应性。应用所提出的方法对水利工程中常见的有压和无压流动进行了计算,与实验和前人的计算结果相比较,本文的方法都能得到较为满意的结果。本文的计算格式也可以应用到其他非规则区域的计算中。     

Holographic velocimetry for flow diagnostics

Currently, there are a number of flow diagnostic tools available for the evaluation of fluid dynamic systems. In spite of its great potential, holographic velocimetry is one technique which has not been widely used. It does, however, have great potential in this area due to its inherent three-dimensionality. As demonstrated in this study of fully developed turbulent flow in a pipe, full three-dimensional mapping can be achieved at any instant in a flow cycle. Comparisons of holographic results with analytical predictions and laser-Doppler-anemometry (LDA) measurements demonstrate the accuracy of the technique as well as some of its advantages and disadvantages relative to LDA. Although relatively poor spatial resolution is obtained, the fact that holographic velocimetry is both an instantaneous and full volume measuring tool makes it useful for a range of complex and high-speed flow-measurement applications.     

Efficient computation of unsteady vortical flow using flow-adaptive time-dependent grids

The objective of this study is to efficiently simulate vortex-dominated highly unsteady flows. In such flows, the locations as well as the extent of the regions requiring fine-mesh resolution vary with time. A technique has been developed to simulate these flows on a temporally adapting grid in which the adaption is based on the evolving flow solution. The flow in an axisymmetric constriction has been selected as an illustrative problem. The multiple and disparate length scales inherent in this complex flow make this problem ideally suited for evaluating the adaptive-grid technique. Adaption is based on the equidistribution of a weight function, through the use of forcing functions. The significance of this is that the method can be implemented into existing flow-analysis systems with minimal changes. The grid-generation equations developed are viewed as grid-transport equations. The time-dependent control functions perform the role of the convective speed in this transport mechanism. The equations provide the efficiency and flow tracking capability of parabolic equations, while maintaining the smoothness of computationally expensive elliptic equations. The efficiency and flow tracking capability of the approach is demonstrated for both steady and unsteady flows.     

Micro-flow analysis by molecular tagging velocimetry and planar Raman-scattering

The two dimensional molecular tagging velocimetry (2D-MTV) has been used to measure velocity fields of the flow in a micro mixer. Instead of commonly used micro particles an optical tagging of the flow has been performed by using a caged dye. The pattern generation is done by imaging a mask for the first time. This allows to generate nearly any imaginable pattern. The flow induces a deformation of the optically written pattern that can be tracked by laser induced fluorescence. The series of raw images acquired in this way were analyzed quantitatively with a novel optical flow based technique. The reference measurements have been carried out allowing to draw conclusions about the accuracy of this procedure. A comparison to the standard technique of μPIV has also been conducted. Apart from measuring flow velocities in microfluidic mixing processes, the spatial distribution of concentration fields for different species has also been measured. To this end, a new technique has been developed that allows spatial measurements from Planar Spontaneous Raman Scattering (PSRS). The Raman stray light of the relevant species has been spectrally selected by a narrow bandpass filter and thus detected unaffectedly by the Raman stray light of other species. The successful operation of this measurement procedure in micro flows will be demonstrated exemplary for a mixing process of water and ethanol.     

An engineering approach to blade designs for low to medium pressure rise rotor-only axial fans

Low to medium pressure rise axial fan equipment of the arbitrary vortex flow rotor-only type is widely used in industrial and commercial applications, with many of the installations and rotor designs being far from optimum. Complex computational methods exist for analyzing flows in, for example, high-speed axial flow compressors with multistage blade rows; however, the designers and manufacturers of low-speed, general-purpose axial flow fan equipment have been reluctant to embrace this technology. A simpler yet reliable design technique is presented that allows this category of ducted axial fan rotors, in the presence of swirl-free inlet flow, to be designed to achieve a specified duty with sufficient accuracy for engineering purposes. Practical blade design recommendations and limits, similar to those that exist for free vortex flow axial rotors, have been established for the arbitrary vortex flow rotor-only case.

The technique employs a straightforward engineering approach to arbitrary vortex flow axial fan rotor design, and the equation set can be solved by using relatively simple numerical methods. Estimates of pressure rise and shaft power characteristics for a proposed fan/rotor design can be computed and the design loop iterated until an acceptable set of blade parameters is identified. It is also possible to analyze the performance of an existing axial fan installation as a prelude to the design of a more efficient and effective replacement rotor.

Experimental data used in validating the design and analysis techniques are also presented. These data include comprehensive Cobra pressure probe surveys of local flow parameters downstream of three different low boss ratio, low solidity, arbitrary vortex flow rotors (all with circular arc camber line type blades) as well as fan performance characteristics for one of the experimental rotors configured as a direct-exhaust fan unit. Installation-dependent factors such as direct-exhaust losses and tip clearance effects are also examined. The analytical technique is shown to provide acceptable estimates of fan/rotor pressure rise performance and shaft power characteristics over a moderately wide range of blade angles and operating conditions.     

Streakline flow visualization with neutron radiography

Although flow visualization is an extremely useful tool in fluid mechanics research, many practical fluid flow problems have limited optical access for visualization. A technique has been developed which permits fluid flow to be visualized even in cases where the flow is completely shrouded by metal. The technique employs real-time thermal neutron radiography, which is similar to x-ray radiography except that a collimated beam of neutrons is used. Neutrons can easily penetrate metal casings, but are attenuated by elements such as hydrogen, boron, cadmium, and gadolinium. Various combinations of neutron-opaque tracer particles moving in neutron-transparent ambient fluids were tested for image contrast, resolution, and ability to accurately track the flow. Experiments in a simple pipe flow have demonstrated the feasibility of the technique. Namely, it was possible to visualize the motion of streaklines within a flow field shrouded by metal, which would not have been possible with any other technique.