Simultaneous free-surface deformation and near-surface velocity measurements

 A newly developed non-intrusive approach has been devised for studying near-surface flows where it is important to be able to construct correlations between small-sloped free-surface deformations and near-surface velocities. This method combines digital particle image velocimetry (DPIV) and the reflective mode of the free-surface gradient detector (FSGD) technique into a single measurement system, providing us with an approach to be able to characterize correlations between elevation and kinematic properties, such as velocity and vorticity, which is essential in understanding near surface turbulence. Furthermore, as the free-surface elevation is directly proportional to the pressure for low Froude number flows, this method will allow for the measurement of pressure near the free surface. This will also be useful in calculating the pressure-velocity term in the turbulent kinetic energy equation for near-surface flows. The approach is explained and demonstrated by measuring these correlations for a vertical shear layer intersecting a free surface. Received: 2 August 1999/Accepted: 23 July 2000     

Optical density and velocity measurements in cryogenic gas flows

This paper presents the application of optical measurement techniques in dense-gas flows in a heavy-gas channel to determine planar two-component (2C) velocity profiles and two-dimensional (2D) temperature profiles. The experimental approach is rather new in this area, and represents progress compared with the traditional techniques based on thermocouple measurements. The dense-gas flows are generated by the evaporation of liquid nitrogen. The optical measurement of both the velocity and density profiles is accomplished by the implementation of particle image velocimetry (PIV) and background-oriented schlieren (BOS) systems. Supplemental thermocouple measurements are used as independent calibrations to derive temperatures from the density data measured with the BOS system. The results obtained with both systems are used to quantify the dilution behavior of the propagating cloud through a global entrainment parameter . Its value agrees well with the results obtained by earlier studies.     

Simultaneous velocity field measurements in two-phase flows for turbulent mixing of sprays by means of two-phase PIV

This paper describes a novel derivative of the PIV method for measuring the velocity fields of droplets and gas phases simultaneously using fluorescence images rather than Mie scattering images. Two-phase PIV allows the simultaneous and independent velocity field measurement of the liquid phase droplets and ambient gas in the case of two-phase flow mixing. For phase discrimination, each phase is labelled by a different fluorescent dye: the gas phase is seeded with small liquid droplets, tagged by an efficient fluorescent dye while the droplets of the liquid phases are tagged by a different fluorescent dye. For each phase, the wavelength shift of fluorescence is used to separate fluorescence from Mie scattering and to distinguish between the fluorescence of each phase. With the use of two cross-correlation PIV cameras and adequate optical filters, we obtain two double frame images, one for each phase. Thus standard PIV or PTV algorithms are used to obtain the simultaneous and independent velocity fields of the two phases. Because the two-phase PIV technique relies on the ability to produce two simultaneous and independent images of the two phases, the choice of the labelling dyes and of the associated optical filter sets is relevant for the image acquisition. Thus a spectroscopic study has been carried out to choose the optimal fluorescent dyes and the associated optical filters. The method has been evaluated in a simple two-phase flow: droplets of 30–40 μm diameter, produced by an ultrasonic nozzle are injected into a gas coflow seeded with small particles. Some initial results have been obtained which demonstrate the potential of the method.     

Simultaneous measurements of velocity and density in buoyancy-driven mixing

A variant of the particle image velocimetry (PIV) technique is described for measuring velocity and density simultaneously in a turbulent Rayleigh-Taylor mixing layer. The velocity field is computed by the usual PIV technique of cross-correlating two consecutive images, and deducing particle displacements from correlation peaks of intensity fields. Different concentrations of seed particles are used in the two streams of different temperature (density) fluids, and a local measure of the density is obtained by spatially averaging over an interrogation window. Good agreement is reported between the first- and second-order statistics for density obtained from this technique and from a thermocouple. Velocity-density correlations computed by cross-correlating individual time series are presented. The errors in the density measurements are quantified and analyzed, and the issue of spatial resolution is also discussed. Our purpose for this paper is to introduce the PIV-S method and validate its accuracy against corresponding thermocouple measurements.     

Simultaneous velocity and scalar measurements in premixed recirculating flames

 The use of a laser-Doppler velocimeter has been extended to the analysis of turbulent heat transfer in a strongly sheared disc-stabilised propane-air flame through its combination with either laser Rayleigh scattering or digitally-compensated fine-wire thermocouples. The laser velocimeter was based on a conventional forward scattering system from the green light of a 5W Argon-Ion laser, while the Rayleigh signals used the blue line of the same laser. The procedure for the numeric compensation of the thermocouple signals included analysis of the effect of velocity and temperature on the time constant of the thermocouple and was optimised to allow combined velocity–temperature samples acquired by a purpose-built digital interference with a frequency up to 2000 Hz, without deterioration of the thermocouple by particle accretion. The maximum effective data rate for the combined Rayleigh/LDV system is shown to be around 130 Hz, which corresponds to a data rate of valid Doppler signals around 400 Hz and statistics based on more than 15 000 measurements is made possible. The results obtained with the two systems agree qualitatively, although the use of thermocouples attenuates the measured velocity-temperature correlations. The results are used to assess the extent to which turbulent mixing in flames is altered by the accompanying heat release and quantify the processes of non-gradient diffusion in a strongly recirculating premixed flame. Received: 15 November 1996/Accepted: 2 September 1997     

Simultaneous velocity and temperature measurements in a premixed flame

Procedures which allow the correlation of velocity signals from a laser anemometer and temperature signals from a compensated, small-diameter thermocouple are described together with the error sources associated with the use of the technique in premixed flames. The digital compensation procedure includes the effect of velocity and temperature on the time constant of the thermocouple and the influence of its exposure to the solid particles required by the laser anemometer are quantified and shown to be able to cause large differences in the measured probability-density-distribution of the reaction progress variable. The technique has been used to measure the probability-density-distribution of temperatures, conditioned by the arrival of velocity signals and velocity conditioned by the temperature signal and sample results are presented to help quantify the accuracy of the measurements.     

A differential-pressure probe for velocity measurements in swirling air flows

For measuring air velocities in swirling flows a differential pressure probe of small axial length was developed. The determination of the velocity is based on measuring the difference between the stagnation pressure and the base pressure of a circular disk whose surface is perpendicular to the main flow direction.     

Laser-induced fluorescence modulation techniques for velocity measurements in gas flows

Broadband fluorescence of iodine, excited at 514.5 nm by a single-mode argon-ion laser tuned to the quasi-linear part of an absorption line, was used to detect the Doppler shift and hence the velocity of iodine molecules seeded in a nitrogen jet flow. The slope of the absorption line profile was measured directly using a frequency shift introduced by acoustooptic modulators (AOMs). A velocity of 36 m/s was measured in a jet of N₂ at 60 Torr in 2 ms with an accuracy of 11%. To reduce experimental noise, the laser beams were switched at 125 KHz and signal-tuned amplification was used.     

A pulsed-wire technique for velocity and temperature measurements in natural convection flows

A pulsed-wire probe based on the use of one or two parallel wires, capable of measuring the velocity and the temperature in natural convection flows is described. These measurements are based on the analysis of the relaxation response of a pulsing wire, submitted to a very short electrical pulse. The analysis of the temperature variation on an optional second receiver wire, gives information about the velocity direction. The implementation simplicity of this probe, its good spatial precision, the lack of thermal contamination of the flow, as well as the possibility of obtaining simultaneous velocity and temperature measurements, allow the integration of the device in a multi-point measurement network, capable to deliver thermal and dynamic cartographies of unsteady convection flows.     

Simultaneous concentration and velocity field measurements in a shock-accelerated mixing layer

A novel technique to obtain simultaneous velocity and concentration measurements is applied to the Richtmyer–Meshkov instability. After acceleration by a Mach 2.2 shock wave, the interface between the two gases develops into a turbulent mixing layer. A time-separated pair of acetone planar laser-induced fluorescence images are processed to yield concentration and, through application of the Advection-Corrected Correlation Image Velocimetry technique, velocity fields. This is the first application of this technique to shock-accelerated flows. We show that when applied to numerical simulations, this technique reproduces the velocity field to a similar quality as particle image velocimetry. When applied to the turbulent mixing layer of the experiments, information about the Reynolds number and anisotropy of the flow is obtained.     

Simultaneous measurement of temperature,density and velocity in gas flows by modulated photoluminescence

A novel possibility to determine the temperature, density and velocity simultaneously in gas flows by measuring the average value, amplitude of modulation and phase shift of the photoluminescence excited by a temporally or spatially modulated light source is investigated. Time-dependent equations taking the flow, diffusion, excitation and decay into account are solved analytically. Different experimental arrangements are proposed. Measurements of velocity with two components, and temporal and spatial resolutions in the measurements are investigated. Numerical examples are given for N ₂with biacetyl as the seed gas. Practical considerations for the measurements and the relation between this method and some existing methods of lifetime measurement are discussed.     

Simultaneous mapping of the velocity and deformation field at a free surface

The oblique interaction of a turbulent vortex ring with a clean water surface is experimentally investigated during the transition stage using Digital Particle Image Velocimetry (DPIV) and the shadowgraph technique to map the surface velocity and deformation field simultaneously. The transitional vortex-ring/free-surface interaction leads to the formation of a trifurcation pattern at the free surface. Similar to the laminar flow case, the vortex ring initially bifurcates into two symmetric and separately connected vortex loops. The turbulent break-up of those vortex loops results in the formation of longitudinal wake vortices that symmetrically connect to the surface and eventually lead to a trifurcation pattern. In the absence of large-deformation surface waves, the simultaneous DPIV and shadowgraph measurements reveal good agreement between the surface vorticity and deformation field for small- and large-scale vortical structures. The simultaneous measurement technique is not restricted to the qualitative shadowgraph visualization, but can be easily extended to quantitative methods such as grating-imaging techniques, Color Schlieren, or Color Surface Mapping (CSM) techniques.This work has been supported by the Office of Naval Research, ONR-URI grant N00014-92-J-1610.     

Simultaneous size and velocity measurements of cavitating microbubbles using interferometric laser imaging

In this paper, interferometric laser imaging droplet sizing—ILIDS—is applied to incipient cavitation in the wake of a marine propeller model with the aim to evaluate simultaneously bubbles velocity and diameter. Until now, the feasibility of this technique has been demonstrated especially in sprays of water droplets in air where an optimal light scattering is obtained thanks to the spherical shape and to the given relative refractive index. In the present setup, to allow simultaneous size–velocity measurements, a single camera is used and the object distance over lens diameter ratio is kept as small as possible, thus increasing the size measurement resolution. These details, together with the algorithms used for image analysis at each single frame and in two consecutive frames, allow deriving cavitation bubble size and velocity distributions in the propeller wake.     

4D Magnetic resonance velocimetry for mean velocity measurements in complex turbulent flows

An adaptation of a medical magnetic resonance imaging system to the noninvasive measurement of three-component mean velocity fields in complex turbulent engineering flows is described. The aim of this paper is to evaluate the capabilities of the technique with respect to its accuracy, time efficiency and applicability as a design tool for complex turbulent internal geometries. The technique, called 4D magnetic resonance velocimetry (4D-MRV), is used to measure the mean flow in fully developed low-Reynolds number turbulent pipe flow, Re=6400 based on bulk mean velocity and diameter, and in a model of a gas turbine blade internal cooling geometry with four serpentine passages, Re=10,000 and 15,000 based on bulk mean velocity and hydraulic diameter. 4D-MRV is capable of completing full-field measurements in three-dimensional volumes with sizes on the order of the magnet bore diameter in less than one hour. Such measurements can include over 2 million independent mean velocity vectors. Velocities measured in round pipe flow agreed with previous experimental results to within 10%. In the turbulent cooling passage flow, the average flow rates calculated from the 4D-MRV velocity profiles agreed with ultrasonic flowmeter measurements to within 7%. The measurements lend excellent qualitative insight into flow structures even in the highly complex 180° bends. Accurate quantitative measurements were obtained throughout the Re=10,000 flow and in the Re=15,000 flow except in the most complex regions, areas just downstream of high-speed bends, where velocities and velocity fluctuations exceeded MRV capabilities for the chosen set of scan parameters. General guidelines for choosing scanning parameters and suggestions for future development are presented.     

Simultaneous velocity and concentration field measurements of passive-scalar mixing in a confined rectangular jet

Simultaneous velocity and concentration fields in a confined liquid-phase rectangular jet with a Reynolds number based on the hydraulic diameter of 50,000 (or 10,000 based on the velocity difference between streams and the jet exit dimension) and a Schmidt number of 1,250 were obtained by means of a combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) system. Data were collected at the jet exit and six further downstream locations. The velocity and concentration field data were analyzed for flow statistics such as turbulent fluxes, turbulent viscosity and diffusivity, and turbulent Schmidt number (Sc _T ). The streamwise turbulent flux was found to be larger than the transverse turbulent flux, and the mean concentration gradient was not aligned with the turbulent flux vector. The average Sc _T was found to vary both in streamwise and in cross stream directions and had a mean value around 0.8, a value consistent with the literature. Spatial correlation fields of turbulent fluxes and concentration were then determined. The R _u′ϕ′ correlation was elliptical in shape with a major axis tilted downward with respect to the streamwise axis, whereas the R _v′ϕ′ correlation was an ellipse with a major axis aligned with the cross-stream direction. Negative regions of R _u′ϕ′ were observed in the outer streams, and these negatively correlated regions decayed with downstream distance and finally disappeared altogether. The R _ϕ′ϕ′ correlation field was found to be an ellipse with the major axis inclined at about 45° with respect to the streamwise direction. Linear stochastic estimation was used to interpret spatial correlation data and to determine conditional flow structures. It is believed that a vortex street formed near the splitter plate is responsible for the negatively correlated region observed in the R _u′ϕ′ spatial correlations of turbulent fluxes. A positive concentration fluctuation event was observed to correspond to a finger of nearly uniform concentration fluid reaching out into the outer stream, whereas a negative event corresponds to a pocket of nearly uniform fluid being entrained from the outer stream into the center jet region. Large-scale vortical structures were observed in the conditional velocity fields with an elliptical shape and a streamwise major axis. The growth of the structure size increased linearly initially but then grew more slowly as the flow transitioned toward channel flow. Support of this work was provided by the National Science Foundation through grants CTS-9985678 and CTS-0336435 and by the Dow Chemical Company. The author greatly acknowledge Charles Lipp at Dow Chemical and Ken Junk at Emerson Fisher for their valuable assistance in the design and construction of the flow system.     

Concentration and velocity field measurements in turbulent flows using laser-induced fluorescence tomography (LIFT)

Laser-Induced Fluorescence is used to tomographically produce volume information of concentration distribution in a turbulent shear flow. Based on Adaptive Least Squares Correlation (ALSC) of grey level distributions in small patches cut out of consecutive tomographically constructed observation volumes, 3-D fields of velocity, vorticity and rate-of-strain are determined with high spatial resolution, satisfactory temporal resolution and high accuracy. This novel technique opens new perspectives for the study of mixing processes.     

Concentration and velocity measurements in the flow of droplet suspensions through a tube

Two optical methods, light absorption and LDA, are applied to measure the concentration and velocity profiles of droplet suspensions flowing through a tube. The droplet concentration is non-uniform and has two maxima, one near the tube wall and one on the tube axis. The measured velocity profiles are blunted, but a central plug-flow region is not observed. The concentration of droplets on the tube axis and the degree of velocity profile blunting depend on relative viscosity. These results can be qualitatively compared with the theory of Chan and Leal.List of symbols a particle radius,m - a/R, non-dimensional particle radius - c volume concentration of droplets in suspension, m³/m³ - c ₅ stream-average volume concentration of droplets in suspension, - D 2 R, tube diameter, m - L optical path length, m - L _ij path length of laser beam through thej-th concentric layer when the beam crosses the tube diameter at the point on the inner circumference of thei-th layer, m - N exponent in Eqs. (3) and (4) - Q volumetric flowrate of suspension, - R tube radius, m - Re _{S S} D/µ, flow Reynolds number - r radial position (r = 0 on a tube axis), m - r r/R, non-dimensional radial position - v velocity of suspension, m/s - v v/v _S , non-dimensional velocity - v ₀ centre-line velocity of suspension (r = 0), m/s - v _S Q/ R ², stream-average velocity of suspension, m/s - x streamwise position (x = 0 at tube inlet), m - x x/D, non-dimensional streamwise position - _c density of continuous phase, kg/m³ - _d density of dispersed phase, kg/m³ - _s stream-average density of suspension, kg/m³, equals density when homogenized - _d - _c, phase density difference, kg/m³ - µ_c viscosity of continuous phase, Pa · s - µ_d viscosity of dispersed (droplet) phase, Pa · s - µ_d/_c, viscosity ratio - interfacial tension, N/m This work was financially supported by the National Science Foundation (USA) through an agreement no. J-F7F019P, M. Sklodowska-Curie fund