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

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

Digital particle image accelerometry

A high-resolution video-based technique for obtaining two-dimensional fluid acceleration field data has been developed. The algorithm uses a combination of cross-correlations and autocorrelations on doubly exposed images of particle-seeded flows. Autocorrelations of individual video frames in an image pair yield two instantaneous velocity fields from which accelerations can be computed. Cross-correlations between successive images in the pair are used to resolve directional ambiguity associated with the autocorrelation. Time intervals are made arbitrarily small through the use of a laser sheet generator circuit which is synchronized with the framing rate of the camera. The technique is validated using a fluid-filled Petri dish subject to a known periodic motion. Ongoing development, uncertainties, and limitations of the technique are discussed. Received: 22 October 1998/Accepted: 27 September 2000     

On the applicability of background oriented optical tomography for large scale aerodynamic investigations

 Density fields of the blade tip vortices from a helicopter in hover flight were visualized by a technique which does not require any installation on the helicopter or close to it. The results illustrate an encouraging prospect for the applicability of the technique. It offers the capability of at least qualitative investigations of unsteady density fields even in full-scale flight tests. The underlying principle is briefly described in this article and an extension to a three-dimensional quantitative technique by using multiple cameras is outlined. Received: 8 September 1999 / Accepted: 7 October 1999     

A four-sensor hot-wire probe system for three-component velocity measurement

 A velocity measurement system based on a miniature four-sensor hot-wire probe capable of simultaneous three-component measurements throughout a wide range of flow angles has been developed. The calibration technique allows measurements to be made throughout the acceptance cone of the probe without being restricted by the errors associated with analytic angle response equations. This technique is based upon look-up tables with values which tend to vary slowly, allowing a simple interpolation scheme to be used. Measurements made in a turbulent pipe flow verify the accuracy of the technique. Received: 27 September 1996/Accepted: 20 October 1997     

Determination of the growth rate of instability of low velocity free falling jets

An imaging technique with asymmetric magnification is used to visualise liquid cylindrical jet interface displacements. The wavelet transform technique is applied to the jet interface to localise the perturbations. Scale parameters of the perturbation waves are obtained by a recognition technique. Wave numbers and growth rates are determined for free falling jets. The most representative growth rate of perturbation is measured by a statistical analysis as well as the wavenumber and the initial amplitude of perturbation η₀. The concordance between the appearance of a critical velocity U _c in the stability curve and a minimum in the growth rate is shown. Important variations of η₀ has also been measured in a flow regime where it is usually supposed to be constant. Received: 7 October 1998/Accepted: 13 October 1999     

A digital mask technique for reducing the bias error of the correlation-based PIV interrogation algorithm

 In this paper the bias phenomenon in the evaluation of PIV recordings by using the correlation-based interrogation algorithm is discussed, and a digital mask technique, that can effectively reduce the bias error, is introduced. The correlation-based interrogation algorithm, when masked with a Gaussian window function, can achieve a higher evaluation accuracy not only for PIV recordings of flows with small velocity gradients, but also for that of flows with large gradients. Received: 14 October 1998/Accepted: 20 July 1999     

Compressible flow hot-wire calibration

 A resistance thermal anemometer operated in compressible flow is sensitive to density, velocity, and total temperature fluctuations. To address these effects, a relatively inexpensive compressible flow calibration facility has been developed and tested. Characteristics of a hot-film anemometer in compressible flow are determined and a correction technique to account for differences in mean density between the calibration facility and the flow under study is proposed. Received: 19 October 1995 / Accepted: 18 October 1996     

Efficient estimation of power spectral density from laser Doppler anemometer data

 A non-biased estimator of power spectral density (PSD) is introduced for data obtained from a zeroth order interpolated laser Doppler anemometer (LDA) data set. The systematic error, sometimes referred to as the “particle-rate filter” effect, is removed using an FIR filter parameterized using the mean particle rate. Independent from this, a procedure for estimating the measurement system noise is introduced and applied to the estimated spectra. The spectral estimation is performed in the domain of the autocorrelation function and assumes no further process parameters. The new technique is illustrated using simulated and measured data, in the latter case with direct comparison to simultaneously acquired hot-wire data. Received: 9 June 1997/Accepted: 14 October 1997     

Passive acoustic bubble sizing in sparged systems

Passive acoustic bubble sizing was investigated in both controlled tests and in a stirred, sparged tank typical of the biotechnology or minerals processing industries. Acoustic techniques have promise for industrial systems where other bubble analysis methods are impractical. Acoustic signals were studied for bubbles precisely formed at higher airflow rates. Acoustic pulses varied with bubble production rate as well as with bubble size. A technique of windowing pulses is proposed. Two alternative versions of this windowing technique were applied to a stirred, sparged tank, giving good agreement. It was shown that, in some cases, it may also be possible to acoustically estimate the spatial distribution of void fraction. Received: 13 June 2000/Accepted: 19 October 2000     

Quantitative flow visualization of fluidized-bed under normal- and down-flow-mode operations by neutron radiography

Heat transfer characteristics of tube-banks immersed in a fluidized-bed is dominated by the time-averaged as well as statistical characteristics of bed-material movement, especially, in the neighboring region of heat transfer tube. The neutron radiography and image processing technique have been successfully applied to the visualization of flow field and quantitative measurement of void fraction in the bed. This quantitative visualization technique is verified as a useful means in understanding the flow behavior and thus the heat transfer mechanisms. Received: 4 October 1998 / Accepted: 7 June 1999     

Stress and heat flux in non-inertial reference frames

We compute the stress tensor and the heat flux in a non-inertial reference frame, using the BGK model to any order in the product between the microscopic collision time and the angular velocity of the reference frame, a product that is assumed to be of order one. We find that both the stress and the heat flux change both in magnitude and in orientation with respect to its inertial counterparts, and that the non-inertiality always lowers their absolute value. Received: October 7, 1996 / Accepted: October 7, 1996     

Simultaneous temperature and 2D velocity measurements in a turbulent heated jet using combined laser-induced fluorescence and LDA

 This paper presents an efficient technique for the characterization of thermal transport properties in turbulent flows. The method is based on the temperature dependence of fluorescence, induced by laser radiation, of an organic dye. The laser-induced fluorescence technique is combined with 2D laser Doppler anemometry, in order to measure in the same sample volume simultaneously and instantaneously the temperature and velocity. The technique is demonstrated on a turbulent heated round jet: the mean and fluctuating dynamic and thermal fields are investigated, and the temperature-velocity cross-correlations are determined in order to characterize the turbulent diffusivity and the turbulent Prandtl number. Received: 30 October 1997/Accepted: 14 July 1998     

Turbulence structure in the immediate vicinity of the shear-free air–water interface induced by a deeply submerged jet

 A PIV based technique is developed to perform flow measurements in the vicinity of the air–water interface of a submerged confined jet. Both the interface movement and the velocity field immediately beneath it are measured simultaneously. A detailed turbulence structure in the surface influence region is thus obtained. Flow parameters evaluated without and w.r.t. the interface are quantified and compared against previous works obtained using the conventional Eulerian-based instrumentation which do not account for the interface fluctuation, and checked against analytical model characterising the turbulence close to a assumed flat air–water interface. Received: 15 March 1998/Accepted: 19 October 1998     

A microfluidic-based nanoscope

 A novel technique for noninvasively measuring the shapes of walls with resolution approaching tens of nanometers is presented. The nanoscope measures local wall position by measuring the velocity of a fluid with micron-scale spatial resolution as it flows over a surface. The location of the wall is estimated by assuming the no-slip velocity condition at the wall and extrapolating the velocity profile to zero. Nanoscope measurements were obtained in a 30 × 300-μm channel. The wall shape of the glass microchannel was determined to be flat to within a root mean square uncertainty of 62 nm. Numerical simulations show that noise in the velocity measurements contributes significantly to uncertainty in wall position. The technique can be used to measure surfaces that are immersed in liquids and in geometries that do not provide exposed surfaces, where traditional nanoscope techniques such as scanning probe microscopes (SPM) are not applicable. Received: 2 March 2001 / Accepted: 19 October 2001     

In situ force-balance tensiometry

 Although a fundamental physical parameter, surface tension is difficult to measure. Common tensiometry inaccuracy comes from failure to control air–liquid–solid contact conditions, or account for liquid meniscus geometry and buoyancy corrections. This paper describes an in situ tensiometry technique, based on withdrawal of a thin-walled tube from the liquid interface, that enforces a known air–liquid–solid contact condition. This technique can be pursued at any level of experimental hygiene. Experimental results for filtered tap water, an alcohol–water solution, and a surfactant–water solution show that results repeatable to three significant digits are obtained with modest effort for a variety of geometrical parameters. Received: 7 October 1997/Accepted: 23 April 1998     

Rheological characterisation of a high-density polyethylene with a multi-mode differential viscoelastic model and numerical simulation of transient elongational recovery experiments

The rheological characterisation of a high-density polyethylene is performed by means of measurements of the storage and loss moduli, the shear viscosity and the transient uniaxial elongational viscosity, the latter being obtained with the Meissner extensional rheometer. The rheological behaviour of the polymeric material is described by means of a multi-mode Phan Thien-Tanner fluid model, the parameters of which are successively fitted on the basis of the linear and non-linear properties. By using a semi-analytical technique and the finite element method, numerical investigations are performed for the shape recovery of the sample, and the predictions are compared with their experimental counterparts. Surface tension effects are also explored. We discuss the agreement between the experiments and the simulation results. Received: 15 October 1998 Accepted: 22 December 1998     

Gas-phase velocity field measurements in sprays without particle seeding

A laser-based technique is presented that can be used to measure the instantaneous velocity field of the continuous phase in sprays and aerosols. In contrast to most well established laser-based velocity measurement techniques, this method is independent of particle seeding and Mie scattering. Instead of that it is based on gaseous flow tracers and laser-induced fluorescence (LIF). Inhomogeneous tracer gas distributions, which are created by an incomplete, turbulent mixing process, are exploited for flow tracing. The velocity field can be measured close to the droplets, because frequency-shifted LIF is separated from Mie scattering by optical filters. Validation tests and results from a water spray in air are given. Accuracy and spatial resolution are discussed in detail. Received: 26 April 1999/Accepted: 16 October 1999     

The eduction of structures from flow imagery using wavelets Part I. The mixing layer

 Using two-dimensional Mexican hat wavelets, digitized imagery from the flow visualization pictures of a mixing layer by Brown and Roshko have been analysed. The objective of the present study is to “calibrate” the proposed wavelet technique for educing structures in a flow whose large-scale organization is well established. It is found that, with appropriate thresholds on wavelet-transform coefficients, considerable insight into the structure of the flow at different scales can be obtained. In particular, the analysis reveals that the small-scale structure within the large-scale vortices is approximately homogeneous, and has characteristic scales that do not vary significantly with downstream distance. Received: 5 March 1998 / Accepted: 28 October 1998