Experimental comparison of two laser-based velocimeters for flows with alluvial sand

 Nonintrusive measurements in a sediment-laden flow using two laser-based techniques, Discriminator Laser-Doppler Velocimetry (DLDV) and Particle Tracking Velocimetry (PTV), are compared. DLDV was previously developed at the Iowa Institute of Hydraulic Research, while PTV was specially configured for this application. Mean and fluctuating velocity components for both flow fractions were simultaneously measured in a laboratory-scale, submerged water jet loaded with alluvial sand. This information cannot be obtained using existing measurement techniques. The jet Reynolds number was 6120, and the sediment sieve diameter ranged from 0.5 to 0.6 mm. Small mass loadings of sand with inertial time constant τ _p of 0.6 ms were examined. The configuration, operation, and results obtained using the DLDV and PTV are presented. For each technique, means to precisely distinguish between the light scattered by suspended sand and that originating from seed-particles following the water were implemented. The agreement in measurement for the two methods validates one another since they are based on completely different principles of operation. The capabilities of DLDV and PTV to reliably measure sand and water velocities in sediment-laden flows are further indicated by the agreement of the present findings with those obtained previously in similar studies. The comparison suggests that PTV, due to its whole field nature, could become a powerful tool for flow and particle-related diagnostics, yielding fundamental information in an area with a long history of conflicting experimental evidence. Received: 27 August 1996 / Accepted: 25 June 1997     

Effect of particle number density in in-line digital holographic particle velocimetry

A digital in-line holographic particle tracking velocimetry (HPTV) system was developed to measure 3D (three-dimensional) velocity fields of turbulent flows. The digital HPTV (DHPTV) procedure consists of four steps: recording, numerical reconstruction, particle extraction and velocity extraction. In the recording step, a digital CCD camera was used as a recording device. Holograms contained many unwanted images or noise. To get clean holograms, digital image processing techniques were adopted. In the velocity extraction routine, we improved the HPTV algorithm to extract 3D displacement information of tracer particles. In general, the results obtained using HPTV were not fully acceptable due to technical limitations such as low spatial resolution, small volume size, and low numerical aperture (NA). The problems of spatial resolution and NA are closely related with a recording device. As one experimental parameter that can be optimized, we focused on the particle number density. Variation of the reconstruction efficiency and recovery ratio were compared quantitatively with varying particle number density to check performance of the developed in-line DHPTV system. The reconstruction efficiency represented the particle number distribution acquired through the numerical reconstruction procedure. In addition the recovery ratio showed the performance of 3D PTV algorithm employed for DHPTV measurements. The particle number density in the range of C _o = 13–17 particles/mm³ was found to be optimum for the DHPTV system tested in this study.     

Smoke-wire flow visualization of the near-wake region behind a circular disc at low Reynolds numbers

The flow structures in the near field of the unducted wake region behind a circular disc for annular flow at low Reynolds numbers were studied by smoke-wire flow visualization technique. A twisted-dual-wire was employed to perform the time evolving visualization. Three typical characteristic flow modes: Q-tip, open-top toroid, and closed toroid, were identified in the near disc region. For Reynolds number between 130 and 390, the Q-tip flow mode which subject to a periodic up-down oscillatory motion was observed. The open-top toroid mode which experiences the expelling vortex shedding was found for Reynolds number between 390 and 455. The free separation surface turns around and merges to the central axisymmetric axis to form the conventionally observed toroidal recirculation bubble for Reynolds number higher than 455. The closed toroid mode exhibits both expelling and shear-layer vortex sheddings. With the identified flow modes at low Reynolds numbers, the recirculation contours, recirculation length, and the shedding frequency in each mode were measured and discussed.List of symbols B.R. blockage ratio (=D ² /D _a ² ) - D _a outer diameter of annular jet, 30 mm - D diameter of circular disc, 20 mm - f frequency of vortex shedding, Hz - L _r axial length of recirculation zone - R radius of circular disc, 10 mm - u _a average exit velocity of annular jet - ₀ stream function with value of zero - mass density of annular flow - u average axial velocity - r radial coordinate, originated from center of circular disk - r ₀ radial coordinate of the boundary of the recirculation zone - Re _a Reynolds number of annular jet based on the disc diameter - Z axial coordinate, originated from center of circular disk - w _max maximum half-width of the recirculation zone - St Strouhal number (=fD/D _a )     

Unsteady flow in rotating drums using laser Doppler velocimetry

Non-destructive measurements by laser Doppler velocimetry is employed to study unsteady flow in a hollow drum filled with liquid. The drum is suddenly accelerated from rest or is suddenly decelerated from a steady rotation to rest. Pure water and glycerin-water mixtures are used as the test liquid in which polyethylenelatex particles are mixed as the light scattering tracer. The boundary layer formation, the time history of velocity, momentum and kinetic energy of the liquid, the wall-to-fluid force transfer, and the transient response time are determined. Also determined are the effects of side walls and fluid viscosity on the transient flow response. Of importance is the disclosure of Ekman layer instability near the inner radial wall of the test drum. It is actuated by the centripetal acceleration-induced buoyancy force.List of symbols A wetted surface area of test drum, cm² - a reciprocal of characteristic velocity, = t _sH, s/cm - B width of test drum, cm - b axial coordinate of test drum, cm - D diameter of test drum, cm; D ₁, inner diameter; D ₂, outer diameter - d diameter of laser beam, mm - d _p particle diameter, mm - E kinetic energy of liquid, kg · cm²/s²; E _s, steady value - F force transferred from drum walls to liquid, N - f focal length of lens, mm - G one-half of spacing between two parallel split beams, Fig. 1 - H characteristic length of test drum, cm; = V/A - M momentum of liquid, kg·cm/s; M _s, steady value - m mass of control volume, kg - r radial coordinate of test drum, cm - S fringe spacing, mm - t time, s - t _p time for particle to travel through fringe spacing, s - t _s transient time, s - u liquid velocity, cm/s - V liquid volume in test drum, cm³ - V _s effective volume of sample volume, mm³ - v velocity of tracer particle, cm/s; = S/t - W waist diameter of parabola in Fig. 2, mm - (x, y, z) coordinates for paraboloid in Fig. 2, mm - crossing angle of splitting beams, degrees - wavelength of laser length, cm - v kinematic viscosity, cm²/s - liquid density, kg/cm³ - Doppler frequency, l/s - s at steady state - 1 outer - 2 inner On leave from the Dept. of Mechanical Engineering, Musashi Institute of Technology, Tokyo, Japan     

Experimental study of sedimentation characteristics of spheroidal particles

The drag of non-spherical particles is a basic, important parameter for multi-phase flow. As the first step in research in this area, the terminal velocities, Ut, of hemispherical and spherical segment particles with maximal diameters of 6-21 mm were measured in static fluids by using a high-speed video camera. The drag coefficient, CD, measured for Reynolds number, Re of 10^1-10^5, has been obtained and compared with those for a sphere. The Re based on the terminal velocity has a logarithmic linear relationship with Ar number for both the facet facing upwards or downwards for the two experimental spheroidal particles, and their Co values are greater than those of spheres. A shape function that depends on the initial orientation of the particle facet is presented to correct for the shape effects.     

Simultaneous measurement of size and velocity of microbubbles moving in an opaque tube using an X-ray particle tracking velocimetry technique

An X-ray particle tracking velocimetry (PTV) technique was developed to simultaneously measure the sizes and velocities of microbubbles in a fluid without optical aberration. This technique is based on a combination of in-line X-ray holography and PTV. The X-ray PTV technique uses a configuration similar to that of conventional optical imaging techniques, and is easy to implement. In the present work, microbubbles generated from a fine wire by electrical heating were used as tracer particles. The X-ray PTV technique simultaneously recorded size and velocity data for microbubbles (_b=10–60 m) moving upward in an opaque tube (inner diameter =2.7 mm). Due to the different refractive indices of water and air, phase contrast X-ray images clearly show the exact size and shape of overlapped microbubbles. In all of the working fluids tested (deionised water and 0.01 M and 0.10 M NaCl solutions), the measured terminal velocity of the microbubbles rising through the solution was proportional to the square of the bubble diameter. The proposed technique can be used to extract useful information on the behaviour of various bio/microscale fluid flows that are not amenable to analysis using conventional methods.     

Algorithms for fully automated three-dimensional particle tracking velocimetry

A three-dimensional Particle Tracking Velocimetry (3-D PTV) technique has been developed to provide time-resolved, three-dimensional velocity field measurements throughout a finite volume. This technique offers many advantages for fundamental research in turbulence and applied research in areas such as mixing and combustion. The data acquired in 3-D PTV is a time sequence of stereo images of flow tracer particles suspended in the fluid. In this paper, the implementation of the technique is discussed in detail, as well as the results of an extensive statistical investigation of the performance of the algorithms. The technique has been optimized to allow fully automatic processing of long sequences of image pairs in a computationally efficient manner, hereby providing a viable, practical tool for the study of complex flows.List of symbols x, y, z Particle position - u, v, w Particle velocity This work was supported by a grant from Ford Motor Company, Powertrain Research Department. Their support is gratefully acknowledged.     

Turbulent shear stress profiles in a bubbly channel flow assessed by particle tracking velocimetry

Particle tracking velocimetry (PTV) is applied to a bubbly two-phase turbulent flow in a horizontal channel at Re = 2 × 10⁴ to investigate the turbulent shear stress profile which had been altered by the presence of bubbles. Streamwise and vertical velocity components of liquid phase are obtained using a shallow focus imaging method under backlight photography. The size of bubbles injected through a porous plate in the channel ranged from 0.3 to 1.5 mm diameter, and the bubbles show a significant backward slip velocity relative to liquid flow. After bubbles and tracer particles are identified by binarizing the image, velocity of each phase and void fraction are profiled in a downstream region. The turbulent shear stress, which consists of three components in the bubbly two-phase flow, is computed by analysis of PTV data. The result shows that the fluctuation correlation between local void fraction and vertical liquid velocity provides a negative shear stress component which promotes frictional drag reduction in the bubbly two-phase layer. The paper also deals with the source of the negative shear stress considering bubble’s relative motion to liquid.     

Bubble deformation and flow structure measured by double shadow images and PIV/LIF

An experiment on bubble motion in a simple shear layer was performed in order to obtain fundamental knowledge of the force on the bubble and its lateral motion induced by the surrounding flow field. We explored the flow structure in the vicinity of the bubble in one plane and its deformation in two planes by particle image velocimetry (PIV)–laser-induced fluorescence (LIF) and a projection technique for two perpendicular planes, respectively. For our experiment, we chose a single air bubble with an equivalent bubble diameter D _eq of 2~6 mm in a vertical shear flow. Velocity measurements were made using a digital high-speed CCD camera for PIV with fluorescent tracer particles. The second and third CCD cameras were used to detect the bubbles shape and motion via backlighting from an array of infrared LEDs. We quantitatively studied the three-dimensional wake structure from measurements of the two-dimensional vortex structure and approximated three-dimensional shape deformation arranged from two perpendicular bubble images.     

Simultaneous PIV and PTV measurements of wind and sand particle velocities

Wind-blown sand is a typical example of two-phase particle-laden flows. Owing to lack of simultaneous measured data of the wind and wind-blown sand, interactions between them have not yet been fully understood. In this study, natural sand of 100–125 μm taken from Taklimakan Desert was tested at the freestream wind speed of 8.3 m/s in an atmospheric boundary layer wind tunnel. The captured flow images containing both saltating sand and small wind tracer particles, were separated by using a digital phase mask technique. The 2-D PIV (particle imaging velocimetry) and PTV (particle tracking velocimetry) techniques were employed to extract simultaneously the wind velocity field and the velocity field of dispersed sand particles, respectively. Comparison of the mean streamwise wind velocity profile and the turbulence statistics with and without sand transportation reveal a significant influence of sand movement on the wind field, especially in the dense saltating sand layer (y/δ < 0.1). The ensemble-averaged streamwise velocity profile of sand particles was also evaluated to investigate the velocity lag between the sand and the wind. This study would be helpful in improving the understanding of interactions between the wind and the wind-blown sand.     

Periodic cavitation shedding in a cylindrical orifice

Cavitation structures in a large-scale (D = 8.25 mm), plain orifice style nozzle within a unique experimental rig are investigated using high-speed visualisation and digital image processing techniques. Refractive index matching with an acrylic nozzle is achieved using aqueous sodium iodide for the test fluid. Cavitation collapse length, unsteady shedding frequency and spray angles are measured for cavitation conditions from incipient to supercavitation for a range of Reynolds numbers, for a fixed L/D ratio of 4.85. Periodic cavitation shedding was shown to occur with frequencies between 500 and 2,000 Hz for conditions in which cavitation occupied less than 30% of the nozzle length. A discontinuity in collapse length was shown to occur once the cavitation exceeded this length, coinciding with a loss of periodic shedding. A mechanism for this behaviour is discussed. Peak spray angles of approximately θ ≈ 14° were recorded for supercavitation conditions indicating the positive influence of cavitation bubble collapse on the jet atomisation process.     

Pressure drop and thermal performance in rotating two-pass ducts with various cross rib arrangements

The present study investigates the pressure drop characteristics of rotating two-pass ducts. The duct has an aspect ratio (W/H) of 0.5 and a hydraulic diameter (D _h ) of 26.67 mm. Rib turbulators are attached in the four different cross arrangements on the leading and trailing surfaces of the test ducts. The ribs have a rectangular cross section of 2 mm (e) × 3 mm (w) and a rib angle-of-attack of 70°. The pitch-to-rib-height ratio (p/e) is 7.5 and the rib-height-to-hydraulic-diameter ratio (e/D _h ) is 0.075. The measured results for each region show that the highest pressure drop appears in the turning region in the stationary case, but appears in the upstream region of the second pass in the rotating case. The heat transfer and the pressure coefficients in the first pass are similar for the stationary and rotating cases in all the tested rib arrangements. After the turning region, however, the heat transfer and pressure drop are high in the cases with the cross NN- and PP-type ribs in the stationary ducts. In the rotating ducts, they are high in the cases with the cross NP- and PP-type ribs.     

The application of a 3D PTV algorithm to a mixed convection flow

A 3D particle-tracking velocimetry (PTV) algorithm is applied to the wake flow behind a heated cylinder. The method is tested in advance with respect to its accuracy and performance. In the accuracy tests, its capability to locate particles in 3D space is tested. It appears that the algorithm can determine the particle position with an accuracy of less than 0.5 camera pixels, equivalent to 0.3 mm in the present test situation. The performance tests show that for particles located in a 2D plane, the algorithm can track the particles with a vector yield reaching 100%, which means that a velocity vector can be determined for almost all particles detected. The calculated velocity vectors for this situation have a standard deviation of less than 1%. The performance is also tested on a mixed convection flow behind a heated cylinder in which the 2D flow transits into a 3D flow. As there is no exact solution of such a flow available, the 3D PTV results are compared with visualisation results. The results show that the 3D PTV method can capture the main features of the 3D transition of the 2D vortex street.     

Measurements of heat transfer coefficients between turbulent liquid jets and a radially finned heated disk placed at the bottom of an open vertical circular cavity

 Experiments have been performed to assess the impact of an extended surface on the heat transfer enhancement for axisymmetric, turbulent liquid jet impingement on a heated round disk. The disk, with an array of integral radial fins mounted on its surface, is placed at the bottom of an open vertical circular cavity. Hydrodynamic and heat transfer data were obtained for a dielectric fluorocarbon liquid FC-77. For a fixed circular heater of diameter D=22.23 mm, several geometric parameters were tested: the nozzle diameter (4.42≤d≤9.27 mm), the confining wall diameter of the vertical cavity (22.23≤D _c≤30.16 mm), and the nozzle-to-heater spacing (0.5≤S/d≤5.0). The FC-77 flow rates varied from =0.2 to 11.0 l/min producing Reynolds numbers in the wide interval 700≤Re _d ≤44,000. For d=4.42 mm, the heat transfer response to the separation distance S/d was small but increased gradually with increasing nozzle diameter up to d=9.27 mm. The thermal resistance R _th increased with the confining wall diameter D _c and also with the nozzle diameter d. A minimum value of the thermal resistance of R _th,min=0.4 cm² K/W was attained for a combination of d=4.42 mm, D _c=22.23 mm, S/d=1, and =7.5 l/min. Based on a simplified heat transfer model, reasonable agreement was obtained between measured values of the thermal resistance and the R _th-predictions. The total fin effectiveness ɛ_f was shown to increase with increasing nozzle diameter, but was invariant with the flow rate (or the jet exit velocity). More than a three-fold heat transfer enhancement was realized through the addition of the array of integral radial fins on the heated round disk. Received on 30 August 2000 / Published online: 29 November 2001     

The analysis of silica suspensions atomization

The paper contains the results of experimental investigation of air–water and air–silica suspension atomization process in effervescent nozzles with internal mixing obtained by the use of the digital microphotography method. In experiments the different aqueous solutions of silica Aerosil 300 of different concentration have been used. The suspensions containing up to 0.04 (kg solid particles/kg solution) have Newtonian rheological properties. The observations were carried out at liquid flow rates changed from 0.0014 to 0.011 (kg/s) and gas flow rates from 0.00015 to 0.0065 (kg/s). It corresponded to gas to liquid mass ratios (GLR) values from 0.014 to 0.46. The analysis of photos shows that the droplets which have been formed during the liquid atomization have very different sizes. The differences between characteristics of effervescent atomization for water and suspensions used have not been observed. The present study confirmed the previous reports which suggested that the small particles added to solution do not change spray characteristics. The experimental results show that C_D and SMD are non-linear functions of GLR. Their values are decreasing rapidly as GLR is increased from zero to around 0.07 and thereafter decreasing at a slower rate with further increase in GLR. In the same point (GLR = 0.07) the value of α is maximal. The first regime is characteristic for bubbly flow. The second is typical of annular flow regime. Boundary between bubbly and annular flow regime is observed at GLR = 0.07 for investigated systems. The correlations for C_D and Sauter mean diameter were proposed. The results may be used for example to verify numerical models or comparisons with respect to similar atomization processes.     

Asymptotic Axially Symmetric Deformations for Perfectly Elastic Neo-Hookean and Mooney Materials

For axially symmetric deformations of the perfectly elastic neo-Hookean and Mooney materials, formal series solutions are determined in terms of expansions in appropriate powers of 1/R, where R is the cylindrical polar coordinate for the material coordinates. Remarkably, for both the neo-Hookean and Mooney materials, the first three terms of such expansions can be completely determined analytically in terms of elementary integrals. From the incompressibility condition and the equilibrium equations, the six unknown deformation functions, appearing in the first three terms can be reduced to five formal integrations involving in total seven arbitrary constants A, B, C, D, E, H and k ², and a further five integration constants, making a total of 12 integration constants for the deformation field. The solutions obtained for the neo-Hookean material are applied to the problem of the axial compression of a cylindrical rubber tube which has bonded metal end-plates. The solution so determined is approximate in two senses; namely as an approximate solution of the governing equations and for which the stress free and displacement boundary conditions are satisfied in an average manner only. The resulting load-deflection relation is shown graphically. The solution so determined, although approximate, attempts to solve a problem not previously tackled in the literature.      

Eddy viscosity in decaying swirl flow in a pipe

Prediction of heat transfer coefficient for swirling flows can be made provided the values of the eddy viscosity are available. In the present work the axial and tangential velocity fields are surveyed in a pipe for the determination of eddy viscosity. The data thus obtained were utilised to determine the influence of the axial Reynolds number and swirl number on the eddy viscosity. An empirical relationship is suggested to determine the eddy viscosity as a function of Reynolds number and swirl number.Nomenclature A _T angular momentum, equation (10) - a coefficient, equation (1) - b coefficient, equation (1) - D pipe diameter - f friction factor - F(y) initial condition function, equation (8) - J ₀ Bessel's function of the first kind of order zero - J ₁ Bessel's function of the first kind of order one - R pipe radius - Re Reynolds number, u _av D/ - r radial coordinate - S _n swirl number, equation (6) - (S _n )_in swirl number at the inlet of the test pipe - u axial velocity - u _av mean axial velocity in pipe - W non-dimensional local tangential velocity, w/u _av - w tangential velocity - X non-dimensional axial coordinate, x/D - x axial coordinate - y non-dimensional radial coordinate, r/R - z non-dimensional parameter, 4(1+/)/Re(x/D) - kinematic eddy viscosity - _n eigenvalues, equation (7) - kinematic viscosity - density     

Viscoelastic properties of polymer solutions

Summary Measurements were taken of the thrust of liquid jets ejecting from a long capillary into air for aqueous solutions of polyacrylamide (ET 597) of various concentrations. The measurements were then used to determine axial normal stresses for two capillary diameters: 1.52 mm (L/D = 201.5) and 2.44 mm (L/D = 207.2). The results show that the calculated values of the axial normal stress are higher for the larger capillary diameter than for the smaller, as recently reported byPowell andMiddleman. It has been further found that this diameter effect becomes more pronounced as the concentration of solute is increased. For the same materials, we also measured the primary normal stress difference by means of aWeissenberg rheogoniometer. Comparison between the two kinds of measurements shows that the magnitude of axial normal stresses is much smaller than that of normal stress differences over the range of shear rates studied (200 20,000 sec^–1) for the materials investigated. This result seems to point out the necessity of measuring the wall normal stresses, which are believed to depend not only on the capillary diameter, but also on the concentration. The authors therefore contend that, in general, the measurement of axial normal stresses alone is not sufficient to completely determine the elastic properties of viscoelastic solutions.     

Mathematical models for mass transport through arcs and flames

Summary The transport of particles, caused by axial and radial diffusion and axial flow of convection, will be considered in a D.C. arc and in a laminar flame. The following mathematical model will be discussed. Assuming a steady state in both cases the mass transport may be described in cylindrical coordinates (r, z) by the following partial differential equation where C means the particle concentration, D the coefficient of diffusion, and W the axial velocity. D and W are taken to be constant; various boundary conditions, corresponding to different approximations of the physical situation, are considered. Solutions of (0.1) are obtained in an explicit form.     

Selective control of flow coherence in triangular jets

The triangular jet was investigated for use as a passive device to enhance fine-scale mixing and to reduce the coherence of large-scale structures in the flow. The suppression of the structures is vital to the enhancement of molecular mixing, which is important for efficient chemical reactions including combustion. The sharp corners in the jet injector introduced high instability modes into the flow via the non-symmetric mean velocity and pressure distribution around the nozzle. Both aerodynamic and hydrodynamic flows showed the difference between the flow at the corner (vertex) and at the flat side. While highly coherent structures could be generated at the flat side, the corner flow was dominated by highly turbulent small-scale eddies. The flow characteristics were tested using hotwire anemometry for mean flow and turbulence analysis, and flow visualization in air and water.List of symbols D inlet duct diameter - D _e equivalent diameter - D _i inside diameter - E _v velocity fluctuation energy - f _F forcing frequency - f _j preferred mode frequency - L length - Re Reynolds number - R _e equivalent radius (same area) - r _0.5 jet half-width - R _1.2 cross-correlation factor - r radial coordinate (circular duct) - St _e most energetic Strouhal number - St _j preferred mode Strouhal number - U _m centerline (maximum) velocity in radial u-profile - U ₀ jet exit velocity - u local axial mean velocity - x axial coordinate - X ₁ axial position of first of two hot-wires for axial cross-correlation - + y _F lateral coordinate at flat side of triangular duct - - y _V lateral coordinate at vertex side of triangular duct - (E _V)_j preferred mode energy - X axial distance between hot-wires - r radial distance between two hot-wires (circular jet) - y lateral distance between two hot-wires (triangular jet) - P/P pressure amplitude - momentum thickness - time