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     

General density gradients in general domains: the “two-tank” method revisited

 An experimental technique for the creation of any statically stable density profile is presented. This technique is essentially a generalized, unsteady version of the “two-tank” method that is well known to the stratified flow community. It involves specifying the desired density profile and then solving an inverse problem in order to determine the necessary flow rates of light and dense fluid into the test-section tank. In addition to creating nonlinear density profiles, this technique is also useful for creating linear profiles in tanks whose planform areas vary with the vertical coordinate. The execution of this technique is carried out with computer-controlled peristaltic pumps. Several tests of the method are presented. The first consists of creating a hyperbolic tangent density profile in a rectangular tank. The second consists of creating, again in a rectangular tank, a density profile that is representative of those found in oceans and lakes. Finally, the third test involves creating a linear profile in a tank whose planform area is not constant. In all cases, the measured density profile shows excellent agreement with the requested profile. Received: 12 June 2001 / Accepted: 12 October 2001     

Particle imaging velocimetry of a wet aqueous foam with an underlying liquid film

 We investigate the utility of particle imaging velocimetry (PIV) for performing kinematic measurements in wet aqueous foam with a liquid film beneath it. The flow velocities are measured near the walls of a square cross-section horizontal duct. The flow velocities are useful for validating the rheological models. We show that there is a discrepancy between the velocity profiles in the wet foam and the Bingham plastic model of flow. The velocity measurements reveal a more complex flow pattern, which may be analysed following three different regimes: a plug flow, a shear flow in a vertical plane and a three-dimensional shear flow. The transition between the plug flow and the shear flows may be explained by a shear-induced migration of bubbles. Received: 25 April 2000 / Accepted: 26 February 2001     

Comparative evaluation of three heat transfer enhancement strategies in a grooved channel

 Results of a comparative evaluation of three heat transfer enhancement strategies for forced convection cooling of a parallel plate channel populated with heated blocks, representing electronic components mounted on printed circuit boards, are reported. Heat transfer in the reference geometry, the asymmetrically heated parallel plate channel, is compared with that for the basic grooved channel, and the same geometry enhanced by cylinders and vanes placed above the downstream edge of each heated block. In addition to conventional heat transfer and pressure drop measurements, holographic interferometry combined with high-speed cinematography was used to visualize the unsteady temperature fields in the self-sustained oscillatory flow. The locations of increased heat transfer within one channel periodicity depend on the enhancement technique applied, and were identified by analyzing the unsteady temperature distributions visualized by holographic interferometry. This approach allowed gaining insight into the mechanisms responsible for heat transfer enhancement. Experiments were conducted at moderate flow velocities in the laminar, transitional and turbulent flow regimes. Reynolds numbers were varied in the range Re = 200–6500, corresponding to flow velocities from 0.076 to 2.36 m/s. Flow oscillations were first observed between Re = 1050 and 1320 for the basic grooved channel, and around Re = 350 and 450 for the grooved channels equipped with cylinders and vanes, respectively. At Reynolds numbers above the onset of oscillations and in the transitional flow regime, heat transfer rates in the investigated grooved channels exceeded the performance of the reference geometry, the asymmetrically heated parallel plate channel. Heat transfer in the grooved channels enhanced with cylinders and vanes showed an increase by a factor of 1.2–1.8 and 1.5–3.5, respectively, when compared to data obtained for the basic grooved channel; however, the accompanying pressure drop penalties also increased significantly. Received on 5 April 2001     

Elongational effects in the flow of viscoelastic fluid through a wavy channel

The numerical simulation of the viscoelastic flow through a wavy channel was carried out using the modified Giesekus model. It was found that the excess pressure loss relates to the stretch-thickening properties of elongational viscosity and the geometry of the wavy channel through a large elongational component of the flow at the winding part of the channel. The profiles of the axial component of the velocity become significantly asymmetric when the excess pressure loss occurs. Furthermore, the velocity profiles of a 0.1 wt% solution of polyacrylamide were measured using laser Doppler velocimetry. The results of these measurements are compared to the numerical results. Received: 30 June 1998 Accepted: 20 May 1999     

The Giesekus fluid in ω–D form for steady two-dimensional flows

Numerical results of the simulation of the Giesekus model in ω–D form, which has previously been introduced in Part I of this study, are presented. The model has been applied to the flow of a concentrated polymer solution through a planar 3.97:1 contraction. To obtain an accurate fit of the rheological properties of the fluid a four-mode model is used. The predictions of the numerical simulations are directly compared with the experimental results published by Quinzani et al. in 1994. For the velocity fields a good quantitative agreement is reached, especially in the upstream channel. Regarding the shear stress and first normal stress difference, qualitative predictions of the experimental profiles are obtained.     

Coherent structure identification from wavelet analysis of particle image velocimetry data

 A new technique based on wavelet transform is applied to bidimensional velocity fields obtained by particle image velocimetry (PIV) measurements, in order to extract and characterize swirling motion associated with coherent structures. The proposed technique is based on the selectivity property of the wavelet transform and permits the detection of regions of the flow field associated with coherent structures and their spatial localization. Furthermore, being the method based on the analysis of the local energy content at separated scales, it is possible to extract the typical wavenumber associated with structures and therefore the typical length-scale. The procedure is validated by the application to velocity vector fields obtained from PIV measurements in different flow conditions and turbulence levels. Results are compared with those obtained by other more standard procedures, and the advantages and limitations of the proposed method are then discussed. Received: 16 October 2000 / Accepted: 18 June 2001 Published online: 29 November 2001     

Quantitative rheological evaluation of phase inversion in two-phase polymer blends with cocontinuous morphology

 The present work is focused on the rheological properties of two-phase polymer blends in the phase inversion region. A large number of PS/PMMA- and PSAN/PMMA-blends has been investigated in order to establish a rheological criterion which allows the quantitative determination of the phase inversion concentration φ_PI by rheological means. Three rheological criteria based on the viscous and elastic blend properties such as maximum of dynamic viscosity η′, slope of G′, and maximum of G′ at a constant evaluation frequency have been tested. By correlating the rheological results to data from quantitative morphological analysis we could prove that the chosen rheological criteria are differently suitable for the determination of φ_PI. It has turned out that the G′ criterion is the most robust and most suitable one yielding an excellent correlation with morphological data. Based on these findings we propose a new simple equation for the prediction of φ_PI-values. Received: 14 March 2001 Accepted: 15 May 2001     

Flow mapping of the mercury flow

 An ultrasonic velocity profile (UVP) method has been successfully applied to the investigation of mercury flow contained in a stainless steel wall in the configuration of a liquid metal target of a spallation neutron source called SINQ at the Paul Scherrer Institute. One- and two-dimensional stationary flow has been fully investigated in the form of velocity profiles. Using velocity profiles obtained by the UVP method, a two-dimensional flow map was efficiently produced. A steady state vector map was successfully made and time dependent flow mapping is feasible. Received: 17 May 2000 / Accepted: 27 December 2000 Published online: 29 November 2001     

Gelation study of high processability and high reliability ternary systems based on benzoxazine, epoxy, and phenolic resins for an application as electronic packaging materials

 Fourier transform mechanical spectroscopy technique (FTMS) is utilized as a powerful tool to study the sol-gel transition of covalent bonded polymeric network. Winter and Chambon criteria resulting from the fractal-geometry characteristic of the gel networks allow the determination of the gel point with only single experiment using this technique. The gelation behaviors of low melt viscosity ternary systems of benzoxazine, epoxy, and phenolic resins are investigated and analyzed by the technique in order to study the effect of epoxy diluent on the rheological property development before and after the gel points. The gel time at 140 °C ranges from 5 min to 30 min and less than 5 min at 180 °C for all tested ternary system compositions. The gelation of the ternary mixture shows an Arrhenius-type behavior and the gel time can be well-predicted by the Arrhenius equation. Received: 23 November 1999 Accepted: 2 January 2001     

Flow measurement on an oscillating pipe flow near the entrance using the UVP method

 The authors have carried out a study to investigate and clarify the characteristics of purely oscillating pipe flows over the developing region. The main objective of this study is to establish the method of time-dependent velocity profiles obtained by the ultrasonic velocity profile (UVP) measurement method. First, the relationship between the test fluids and the microparticles, as reflectors of ultrasonic pulses, was investigated. In addition, the relationship between the sound speeds of the test fluids and the wall materials was studied. Second, the UVP was used to obtain the instantaneous velocity profiles in oscillating pipe flows, and the developing characteristics of the flows were analyzed. Finally, the “entrance length” (by analogy with a unidirectional pipe flow) required for oscillating pipe flows was analyzed by examining the amplitude of the harmonic spectral components of the oscillating frequency. A fast Fourier transform (FFT) is proposed as the applicable method to estimate the “entrance length”. From the Fourier transform of the velocity on the centerline, nonlinear oscillation of fluid occurs in the “entrance length” of the oscillating flows, and the viscous dissipation of the higher-order velocity harmoncis determines the entrance region. The “entrance length” can be obtained from the dissipation length of the third-order harmonic. These results prove that the UVP method is highly applicable to carry out the flow measurement in the “entrance length” of oscillating pipe flow. Received: 20 March 2000 / Accepted: 10 August 2001     

Initiation of intense pressure fluctuations when a flow is turned in a duct with a blind-ended cavity

The initiation of narrow-band pressure fluctuations in the duct of a pipeline gas-compression station when the flow is turned near a blind-ended cavity was studied in a wind tunnel. The flow pattern under consideration was estimated from visualization of the flow in a water channel using the hydraulic analogy technique. It is believed that the high-intensity narrow-band pressure fluctuations observed in the flow result from the instability and regular rearrangement of large-scale vortex structures in the blind-ended cavity. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 104–111, March–April, 1998.     

Quantitative NMR velocity imaging of a main-chain liquid crystalline polymer flowing through an abrupt contraction

 The flow of isotropic and liquid crystalline (LC) hydroxypropylcellulose (HPC) aqueous solutions into an abrupt axisymmetric contraction has been quantitatively measured by pulsed-field-gradient NMR techniques. Steady-state axial velocity profiles, acquired upstream of the contraction, reveal a large contraction entry length for the LC solution. This entry flow field exists over an order of magnitude change in flow rate and is attributed to elasticity that is associated with polydomain liquid crystallinity. Pronounced, off-centerline velocity maxima (in an axisymmetric flow field) were present upstream of the contraction, in the entry flow region. Apparently, a more viscous and elastic core of fluid was present along the centerline; this fluid resisted elongational strain more than the fluid closer to the walls. Quantitative velocity profiles were extracted from displacement distributions and corrected for elongational dispersion. The isotropic solution velocity profiles matched those obtained from viscoelastic simulations using an approximate Doi-Edwards model, parameterized with independent rheological data. Received: 29 April 1999/Accepted: 30 August 1999     

Analytical solution of the poiseuille flow problem using the ellipsoidal-statistical model of the Boltzmann kinetic equation

An analytical solution (in the form of a Neumann series) of the problem of rarefied gas flow in a plane channel with infinite walls in the presence of a pressure gradient (Poiseuille flow) parallel to them is constructed within the framework of the kinetic approach in an isothermal approximation. The ellipsoidal-statistical model of the Boltzmann kinetic equation and the diffuse reflection model are used as the basic equation and the boundary condition, respectively. Using the resulting distribution function, the mass and heat flux densities in the direction of the pressure gradient per unit channel length in the y′ direction are calculated, and profiles of the gas mass velocity and heat flux in the channel are constructed. The results obtained for the continuum and free-molecular flow models are analyzed and compared with similar results obtained by numerical methods.     

Simultaneous two-phase PIV by two-parameter phase discrimination

 A flexible and robust phase discrimination algorithm for two-phase PIV employs second-order intensity gradients to identify objects. Then, the objects are sorted into solids and tracers according to parametric combinations of size and brightness. Solids velocities are computed by tracking, gas velocities by cross-correlation. Tests in a fully-developed turbulent channel flow of air showed that the two phases do not contaminate or bias each other's velocity statistics. Error magnitude and valid data yield were quantified with artificial images for three particle sizes (25, 33, and 63 μm), two interrogation area sizes (32 and 64 pixels), and volumetric solids loads from 0.0022% to 0.014%. At the channel centerline, the gas valid data yield was above 98% and the RMS error in gas velocity was less than 0.1 pixels for all variations of these parameters. The solid-to-tracer signal ratio was found to be the major parameter affecting the magnitude of the RMS error. Received: 20 September 2000/Accepted: 2 July 2001 Published online: 29 November 2001     

Hydrodynamics study in a plane ultrafiltration module using an electrochemical method and particle image velocimetry visualization

 The wall shear stress is determined at the surface of a plane sheet of Plexiglas, taking the place of a membrane, using an electrochemical method. Several microelectrodes are mounted flush to this plane plate, and maps of shear stress are determined for two inlet and outlet configurations and three channel heights. The heterogeneity of the wall shear stress is observed for both configurations. Furthermore, the study of the turbulence features of the flow shows a decreasing fluctuating rate of velocity gradient when the channel height is decreased. The wall velocity gradients and turbulent intensity rates analysis are confirmed by a flow visualization using the particle image velocimetry method. Received: 25 September 2000 / Accepted: 23 April 2001     

The ultrasonic velocity profile measurement of flow structure in the near field of a square free jet

 Coherent structures in the near field of a three-dimensional jet have been investigated. Experiments were carried out for a free jet issuing from a square nozzle using a water channel. Instantaneous velocity profiles were obtained in the axial and radial directions by using an ultrasonic velocity profile (UVP) monitor. Axial variations of dominant time-scales of vortex structures were examined from one-dimensional wavelet spectra. Wavenumber-frequency spectra were calculated by two-dimensional Fourier transform along the axial direction in a mixing layer, and it was found that a convective velocity of flow structures was nearly constant independently of their scales in space and time. Coherent structures in the axial direction were investigated in terms of proper orthogonal decomposition (POD). Eigenfunctions are similar to a sinusoidal wave, and reconstructed velocity fields by the lower-order and higher-order POD modes demonstrate large-scale and smaller-scale coherent structures, respectively. Received: 8 May 2000/Accepted: 23 January 2001 Published online: 29 November 2001     

Pulsatile flow with heat transfer of dusty magnetohydrodynamic Ree-Eyring fluid through a channel

The flow due to the pulsatile pressure gradient of dusty non-Newtonian fluid with heat transfer in a channel is considered. The system is stressed by an external magnetic field. The non-Newtonian fluid under consideration is obeying the rheological equation of state due to Ree-Eyring’s stress–strain relation. The equations of momentum and energy have been solved by using Lightill method. The velocity and temperature distributions of the two phase of the dusty fluid are obtained. The effects of various physical parameters of distributions the problem on these distributions are discussed and illustrated graphically through a set of figure.     

A comprehensive study of measurement uncertainty in tomographic reconstruction of void-profiles in a mercury-nitrogen flow

 The liquid–metal magneto-hydrodynamic (LMMHD) facility at the Bhabha Atomic Research Centre (Mumbai) has a two-phase mercury–nitrogen flow system which is currently used for various studies related to void-fraction, flow, pressure, slip-ratio, of the flow system. Non-invasive measurements of steady-state void-fraction profiles has been carried out by a 60 mCi Cesium-137 gamma-ray source and a NaI(Tl) detector mounted on a trolley. The reconstructed cross-sectional void-fraction profiles were in the riser section of the LMMHD loop, and in this process radial tomographic methods were used, e.g. least-squares and chord-segment-inversion (CSI). The present work investigates the possible uncertainties in the profiles thus measured. A simple statistical model has been developed for the CSI algorithm due to the inherent “square nature” of the data matrix. The inherent Poisson error has been also investigated in this exercise. Received: 1 December 1997/Accepted: 30 May 1998     

An experimental contribution to near-wall measurements by means of a special laser Doppler anemometry technique

 A new experimental technique for the investigation of near-wall turbulence using laser Doppler anemometry is presented, which allows an accurate measurement of the flow field very close to the wall, with good resolution and a high data rate. Such a technique is tested in a fully developed turbulent flow (with Reynolds numbers between 4,300 and 67,000) by carrying out a careful statistical analysis of the streamwise and wall-normal velocity components within the near-wall region, at distances from the wall ranging from approximately y ⁺ = 1 to y ⁺ = 100. The velocity profiles, Reynolds stresses and higher-order moments of the two-dimensional boundary layer are presented. The results, which are in agreement with the most recent data in the literature, testify the validity of the proposed experimental solution. Moreover, the accuracy of the results allows the friction velocity to be calculated as the intercept at the wall of the best linear fit of the total stress profile; in this way, an unambiguous examination of the normalized statistics is possible. Received: 17 April 2001 / Accepted: 15 August 2001