Turbulence kinetic energy budget in bubbly flows in a vertical duct

Understanding turbulence kinetic energy (TKE) budget in gas–liquid two-phase bubbly flows is indispensable to develop and improve turbulence models for the bubbly flows. In this study, a molecular tagging velocimetry based on photobleaching reaction was applied to turbulent bubbly flows with sub-millimeter bubbles in a vertical square duct to examine the applicability of the k–ε models to the bubbly flows. Effects of bubbles on TKE budget are discussed and a priori tests of the standard and low Reynolds number k–ε models are carried out to examine the applicability of these models to the bubbly flows. The conclusions obtained are as follows: (1) The photobleaching molecular tagging velocimetry is of use for validating turbulence models. (2) The bubbles increase the liquid velocity gradient in the near wall region, and therefore, enhance the production and dissipation rates of TKE. (3) The k–ε models can reasonably evaluate the production rate of TKE in the bubbly flows. (4) The modulations of diffusion due to the bubbles have different characteristics from the diffusion enhancement due to shear-induced turbulence. Hence, the k–ε models fail in evaluating the diffusion rate in the near wall region in the bubbly flows. (5) The k–ε models represent the trends of the production, dissipation, and diffusion rates of ε in the bubbly flow, although more accurate experimental data are required for quantitative validation of the ε equation.     

Template matching for improved accuracy in molecular tagging velocimetry

In 2D molecular tagging velocimetry (MTV), tags are written into a fluid flow with a laser grid and imaged at discrete times. These images are analyzed to calculate Lagrangian displacement vectors, often by direct cross correlation. The cross correlation method is inherited from particle imaging velocimetry, where the correlated images contain a random pattern of particles. A template matching method is presented here which takes advantage of the known geometry of laser written tag grids in MTV to achieve better accuracy. Grid intersections are explicitly located in each image by correlation with a template with several linear and rotational degrees of freedom. The template is a continuous mathematical function, so the correlation may be optimized at arbitrary sub-pixel resolution. The template is smooth at the spatial scale of the image noise, so random error is substantially suppressed. Under typical experimental conditions at low imaging resolution, displacement uncertainty is reduced by a factor of 5 compared to the direct cross correlation method. Due to the rotational degrees of freedom, displacement uncertainty is insensitive to highly deformed grids, thus permitting longer delay times and increasing the relative accuracy and dynamic range of the measurement. In addition, measured rotational displacements yield velocity gradients which improve the fidelity of interpolated velocity maps.     

Gaseous flow measurements in an internal combustion engine assembly using molecular tagging velocimetry

A molecular tagging velocimetry (MTV) system was applied for mapping in-cylinder flows in an internal combustion engine. The images were captured inside an optical engine assembly that reproduces operation of a 2.2 L four stroke gasoline engine. A recently developed algorithm to process MTV images is based on a fast-normalized spatial correlation approach implemented using MATLAB software. The code allows accurate detection of the MTV grid nodes displacements. It processes simultaneously velocity vector and circulation fields for individual cycles, and ensemble averages of those over a few hundred sequential cycles to obtain mean and standard deviation values. Then probability density functions are reconstructed to quantify cycle-to-cycle variability of the in-cylinder flow.     

Application of photobleaching molecular tagging velocimetry to turbulent bubbly flow in a square duct

To evaluate turbulence energy budget in bubbly flows, an image processing method in a photobleaching molecular tagging velocimetry is improved for accurate evaluation of velocity gradients. Turbulence properties in single-phase and two-phase dilute-bubbly flows in a square duct are measured using the improved method. As a result, the following conclusions are obtained: (1) The axial velocity and axial turbulent intensity measured by the present method agree well with those measured by laser Doppler velocimetry not only for the single-phase flow but also for the dilute-bubbly flow. (2) The present method can measure velocity components and velocity gradients in the vicinity of the wall, and therefore the present method is of great use in understanding the mechanism of turbulence generation and dissipation near the wall. (3) The present method can provide detailed information on turbulence structure such as turbulence kinetic energy budget. (4) Bubbles tend to increase not only the turbulence production but also the turbulence dissipation.     

An optical flow MTV based technique for measuring microfluidic flow in the presence of diffusion and Taylor dispersion

A novel technique is presented for accurately measuring flow fields in microfluidic flows from molecular tagging velocimetry (MTV). Limited optical access is frequently encountered in microfluidic systems. Therefore, in this contribution we analyze the special case of tagging a line across the thin dimension of a microchannel and subsequent imaging along this line. This represents a set-up that is applicable to a wide range of microfluidic applications. A volume illumination has to be used, resulting in an integration of the visualized dye across the flow profile. This leads to the well-known effect of Taylor dispersion. Our novel technique consists of measuring motion from digital image sequences in a gradient-based approach. A motion model is developed which explicitly deals with brightness changes due to Taylor dispersion and additional molecular diffusion of dyes. The presented approach is specific to the case of a parabolic velocity profile. In the presence of these effects, an accurate computation of motion is only possible by applying this novel motion model. Our technique is tested on simulated sequences corrupted with varying levels of noise and on actual measurements. Measurements were conducted in a microfluidic mixer of precisely known flow properties. In the same mixer, a comparative study of our MTV technique to μPIV was performed. Also, the results were compared to bulk measurements of the fluid flow velocity. The novel algorithm compared favorably and also, measurements were conducted on inhomogeneous flow configurations.     

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.     

Molecular tagging techniques and their applications to the study of complex thermal flow phenomena1,211

This review article reports the recent progress in the development of a new group of molecule-based flow diagnostic techniques, which include molecular taggingvelocimetry(MTV)andmoleculartaggingthermometry(MTT), for both qualitative flow visualization of thermally induced flow structures and quantitative whole-fiel measurementsofflowvelocityandtemperaturedistributions.The MTV and MTT techniques can also be easily combined to result in a so-called molecular tagging velocimetry and thermometry(MTVT) technique, which is capble of achieving simultaneousmeasurementsofflowvelocityandtemperature distribution in flui flows. Instead of using tiny particles, the molecular tagging techniques(MTV, MTT, and MTVT)use phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, as the tracers for the flow velocity and temperature measurements. The unique attraction and implementation of the molecular tagging techniques are demonstrated by three application examples, which include:(1) to quantify the unsteady heat transfer process from a heatedcylindertothesurroundingflui flowinordertoexamine the thermal effects on the wake instabilities behind the heated cylinder operating in mixed and forced heat convection regimes,(2) to reveal the time evolution of unsteady heat transfer and phase changing process inside micro-sized, icing water droplets in order to elucidate the underlying physics pertinent to aircraft icing phenomena, and(3) to achievesimultaneousdropletsize,velocityandtemperaturemeasurements of "in-flight droplets to characterize the dynamic and thermodynamic behaviors of flyin droplets in spray flows.     

A spatial correlation technique for estimating velocity fields using molecular tagging velocimetry (MTV)

A direct spatial image correlation technique is presented for estimating the Lagrangian displacement vector from image pairs based on molecular tagging diagnostics. The procedure provides significant improvement in measurement accuracy compared to existing approaches for molecular tagging velocimetry (MTV) analysis. Furthermore, this technique is of more general utility in that it is able to accommodate other laser tagging patterns besides the usual grid arrangement. Simulations are performed to determine the effects of many experimental and processing issues on the sub-pixel accuracy of displacement estimates. The results provide guidelines for optimizing the implementation of MTV. Experimental data in support of this processing technique are provided.     

In-cylinder engine flow measurement using stereoscopic molecular tagging velocimetry (SMTV)

The stereoscopic molecular tagging velocimetry (SMTV) technique is used to obtain the multiple point measurement of an instantaneous three-component velocity field inside the cylinder of an internal combustion (IC) engine assembly. A novel image processing technique is implemented to obtain the velocity data. The technique has the advantage that it eliminates the geometric details required to obtain the three components of the velocity field. The procedure involves two major steps: (i) calibration process and (ii) data acquisition and reduction. Cycle-to-cycle variations of the three-component velocity field and out-of-plane vorticity are presented inside an engine cylinder. Preliminary results show that cycle-to-cycle variations are more prominent in the velocity component perpendicular to the tumble plane, as opposed to the in-plane components. Such new insights will help better understand the details of these flows and further improve CFD models for IC engines.     

Dynamic measurement of temperature, velocity, and density in hot jets using Rayleigh scattering

A molecular Rayleigh scattering technique is utilized to measure gas temperature, velocity, and density in unseeded gas flows at sampling rates up to 10 kHz, providing fluctuation information up to 5 kHz based on the Nyquist theorem. A high-power continuous-wave laser beam is focused at a point in an air flow field and Rayleigh scattered light is collected and fiber-optically transmitted to a Fabry–Perot interferometer for spectral analysis. Photomultiplier tubes operated in the photon counting mode allow high-frequency sampling of the total signal level and the circular interference pattern to provide dynamic density, temperature, and velocity measurements. Mean and root mean square velocity, temperature, and density, as well as power spectral density calculations, are presented for measurements in a hydrogen-combustor heated jet facility with a 50.8-mm diameter nozzle at NASA John H. Glenn Research Center at Lewis Field. The Rayleigh measurements are compared with particle image velocimetry data and computational fluid dynamics predictions. This technique is aimed at aeronautics research related to identifying noise sources in free jets, as well as applications in supersonic and hypersonic flows where measurement of flow properties, including mass flux, is required in the presence of shocks and ionization occurrence.     

Density tagging velocimetry

Density tagging velocimetry, a novel optical technique for point-wise measurement of flow velocity is proposed here. This new method is based on the detection and subsequent tracking of a local density variation deliberately inserted in the flow. The experimental implementation comprising tagging, detection, and velocity evaluation reverts to and combines principles of well-known optical measurement techniques. Density tagging velocimetry has the potential for in-flight application and is particularly suited for measuring flow velocities in regions where the use of tracer particles is difficult or undesired. The applicability of this new technique is illustrated by a jet flow measurement.     

Flow field characteristics in the near field region of particle-laden coaxial jets

The effects of solid particles on the flow structure in the near field region of a coaxial water jet are investigated non-intrusively using molecular tagging velocimetry. Glass beads of 240 μm and specific gravity SG of 2.46 are used at three volume loadings of γ_v=0.03, 0.06, and 0.09% in the central water jet with a Reynolds number of 4.1×10⁴. Measurements are acquired for four annular to central jet velocity ratios in the range 0.11≤ U _o/U _i≤1.15 at downstream distances up to six inner jet diameters and the results are analyzed for the effects of solid particles on the characteristics of flow. It is found that the addition of particles does not affect the mean fluid velocity profile in this region. The results also indicate a small and moderate enhancement of axial turbulent velocity and radial gradients of velocity fluctuations, respectively, due to the presence of particles.     

Molecular Tagging Velocimetry (MTV) measurements in gas phase flows

 Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity fields of the intake flow into a “steady flow rig” model of an internal combustion engine. Received: 29 August 1997/Accepted: 16 September 1998     

全场测速技术进展

速度场测量技术的研究在流体力学和空气动力学中具有极高的学术意义和实用价值．文中归纳介绍了近十多年来人们积极探索的各种全场测速新技术,特别是激光诱导荧光高速流场测试技术、二维粒子跟踪和粒子图象测试技术以及三维测速的最新进展      

In vivo whole-field blood velocity measurement techniques

In this article a number of whole-field blood velocity measurement techniques are concisely reviewed. We primarily focus on optical measurement techniques for in vivo applications, such as laser Doppler velocimetry (including time varying speckle), laser speckle contrast imaging and particle image velocimetry (including particle tracking). We also briefly describe nuclear magnetic resonance and ultrasound particle image velocimetry, two techniques that do not rely on optical access, but that are of importance to in vivo whole-field blood velocity measurement. Typical applications for whole-field methods are perfusion monitoring, the investigation of instantaneous blood flow patterns, the derivation of endothelial shear stress distributions from velocity fields, and the measurement of blood volume flow rates. These applications require individual treatment in terms of spatial and temporal resolution and number of measured velocity components. The requirements further differ for the investigation of macro-, meso-, and microscale blood flows. In this review we describe and classify those requirements and present techniques that satisfy them.     

Dual-plane stereo particle image velocimetry (DSPIV) for measuring velocity gradient fields at intermediate and small scales of turbulent flows

A two-frequency dual-plane stereo particle image velocimetry (DSPIV) technique is described for highly resolved measurements of the complete nine-component velocity gradient tensor field u_i/x_j on the quasi-universal intermediate and small scales of turbulent flows. The method is based on two simultaneous, independent stereo particle image velocimetry (PIV) measurements in two differentially spaced light sheet planes, with light sheet characterization measurements demonstrating the required sheet thicknesses, separation, and two-axis parallelism that determine the measurement resolution and accuracy. The present approach uses an asymmetric forward–forward scatter configuration with two different laser frequencies in conjunction with filters to separate the scattered light onto the individual stereo camera pairs, allowing solid metal oxide particles to be used as seed particles to permit measurements in nonreacting as well as exothermic reacting turbulent flows.     

Velocity profile characterization in sub-millimeter diameter tubes using molecular tagging velocimetry

Fluid flow through microtubes is of interest to many industries and there exists a need for detailed measurements of the velocity field. Velocity profile data are critical for momentum, mass, and heat transport analysis, and thus the design of devices utilizing microgeometries. This paper outlines a measurement technique that has led to time-resolved measurements of velocity profiles in microtubes (less than 1,000 μm). The research program was experimental in nature and consisted of an extension of molecular tagging velocimetry to the microscale. Average velocity and rms profile data in the fully developed region, in addition to mass flow rate and pressure drop data, are presented for numerous Reynolds numbers ranging from 600 to 5,000 in a tube of diameter 705 μm. Received: 20 December 1999 / Accepted: 20 March 2001     

Ionic strontium fluorescence as a method for flow tagging velocimetry

A flow tagging technique based upon ionic fluorescence in strontium is investigated for applications to velocity measurements in gas flows. The method is based upon a combination of two laser based spectroscopic techniques, i.e. resonantly-enhanced ionisation and laser-induced ionic fluorescence. Strontium is first ionised and then planar laser-induced fluorescence is utilised to give 2D `bright images' of the ionised region of the flow at a given time delay. The results show that this method can be used for velocity measurements. The velocities were measured in two types of air–acetylene flames – a slot burner and a circular burner yielding velocities of 5.1 ± 0.1 m/s and 9.3 ± 0.2 m/s, respectively. The feasibility of the method for the determination of velocities in faster flows than those investigated here is discussed. Received: 5 November 1998/Accepted: 19 January 2000     

3-Component acceleration field measurement by dual-time stereoscopic particle image velocimetry

In this article, a multiplane stereo-particle image velocimetry (PIV) system was implemented and validated to measure the three-component acceleration field in a plane of turbulent flows. The employed technique relies on the use of two stereoscopic particle image velocimetry (SPIV) systems to measure pairs of velocity fields superimposed in space but shifted in time. The time delay between the two velocity fields enables the implementation of a finite difference scheme to compute temporal derivatives. The use of two synchronized SPIV systems allows us to overcome the limited acquisition rate of PIV systems when dealing with highly turbulent flows. Moreover, a methodology based on the analysis of the spectral error distribution is described here to determine the optimal time delay to compute time derivatives. The present dual-time SPIV arrangement and the proposed analysis method are applied to measure three-component acceleration fields in a cross section of a subsonic plane turbulent mixing layer.