Simultaneous quantitative flow measurement using PIV on both sides of the air–water interface for breaking waves

An experimental method for simultaneously measuring the velocity fields on the air and water side of unsteady breaking waves is presented. The method includes a novel technique for seeding the air flow such that the air velocity can be resolved in the absence of wind. Low density particles that have large Stokes drag and ability to respond to high-frequency flow fluctuations are used to seed the air flow. Multi-camera, multi-laser particle image velocimetry setups are applied to small-scale unsteady breaking waves, yielding fully time-resolved velocity fields. The surface tension of the fluid is altered and controlled to form spilling breaking waves. Results for the velocity and vorticity fields of representative spilling breakers, which show shedding of an air-side vortex and well-documented generation of water-side vorticity, are presented and discussed.     

A numerical investigation of flow past a bluff body using three-state anemometry

An application of a new flow measurement technique is described which allows for the non-intrusive simultaneous measurement of flow velocity, density, and viscosity. The viscosity information can be used to derive the flow field temperature. The combination of the three measured variables and the perfect-gas law then leads to an estimate of the flow field thermodynamic pressure. Thus, the instantaneous state of a flow field can be completely described. Three-state anemometry (3SA), a derivative of particle image velocimetry (PIV), which uses a combination of three monodisperse sizes of styrene seeding particles is proposed. A marker seeding is chosen to follow the flow as closely as possible, while intermediate and large seeding populations provide two supplementary velocity fields, which are also dependent on fluid density and viscosity. A simplified particle motion equation, aimed at turbomachinery applications, is then solved over the whole field to provide both density and viscosity data. The three velocity fields can be separated in a number of ways. The simplest and that proposed in this paper is to dye the different populations and view the region of interest through interferometric filters. The two critical aspects needed to enable the implementation of such a technique are a suitable selection of the diameters of the particle populations, and the separation of the velocity fields. There has been extensive work on the seeding particle behaviour which allows an estimate of the suitable particle diameters to be made. A technique is described in this paper to allow the separation of particles in a range of micrometer sized velocity fields through fluorescence (separation through intensity also being possible). Some preliminary results by direct numerical simulation (DNS) of a 3SA image are also presented. The particle sizes chosen were 1 μm and 5 μm, tested on the near-wake flow past a cylinder to investigate viscosity only, assuming uniform flow density. The accuracy of the technique, derived from simulations of swirling flows, is estimated as 0.5% RMS for velocity, 2% RMS for the density and viscosity, and 4% RMS for the temperature estimate. © 1998 John Wiley & Sons, Ltd.     

An economical droplet fog generator suitable for laser Doppler anemometry and particle imaging velocity seeding

An inexpensive device for generating fogs of micron-sized droplets is described. The liquid does not pass through small orifices. It is easily constructed and suitable for liquids containing particulates, seeding high-pressure environments and laser Doppler anemometry, and particle imaging velocimetry. Phase Doppler anemometry measurements show size distributions as a function of pressure for three working fluids. The device consistently produced a narrow size range with a mean diameter of a few microns.     

Deduction of aerosol size distribution from particle sampling by whisker collectors

A method of deducing airborne particle size distributions from the deposition on a collector is described. The method basically consists in collecting submicron-sized particles on whisker filters for subsequent electron-microscopic examination. The empirical size distributions on the collectors can be approximated by log-normal functions. Moreover, it has been found that the variation in particle distribution across a four-stage whisker filter can be interpreted on the basis of a simple model of the collection process. The effective absorption coefficient derived from this modeling is used to correct the empirical data for the effect of a selective collection characteristic.     

Generation and control of tracer particles for optical flow investigations in air

The production of monodisperse tracer particles with suitable properties for optical flow investigations, such as small size, spherical shape, smooth surface, appropriate density and diffraction index, non-evaporating and agglomerating, electrically neutral, non-toxic and easily removable, is a challenging task due to the sensitivity of the particle size distribution to the boundary conditions. In order to obtain general design and operating rules for atomizers which are mostly applied for air flows, this dependence is investigated here. It is shown that high concentrations of narrow band particle size distributions, with a mean diameter below 1 µm, can easily be generated by means of multi-hole nozzles under over-critical pressure conditions, when the kinetic energy, transferred into the seeding liquid through the nozzle exits, is well balanced with the liquid volume inside the atomizer. In addition, flow visualization pictures are presented which permit a useful assessment of the functioning of the nozzles and reveal operating features of the atomizers which were not previously known. In particular, it is shown that existing explanations of the importance of certain design features of the Laskin nozzle are of minor importance for the generation of appropriate tracer particles.     

Effective particle size range in laser-Doppler anemometry

The present paper points out that all existing laser-Doppler anemometer systems do not only operate within a finite range of Doppler frequencies but also work within a relatively narrow range of signal amplitudes. It is shown that this corresponds to a finite, and usually to an extremely small, range of particle diameters which contributes to the final LDA measurements. Because of this, the particle size distribution has to be matched to the LDA-system used for measuring particle velocities. If this is not taken into account in particle seeding, low data rates will result in spite of very high particle passage rates through the measuring control volume. This is shown experimentally and is supported by theoretical considerations.The present investigation results in conclusions regarding optimum particle size distributions for laser-Doppler anemometry. If fluid velocity measurements are attempted rather than particle velocity measurements, the particles still have to satisfy well known size requirements that are flow, fluid and particle density dependent.The experimental study employs a combined optical system for simultaneous measurements of particle velocity, particle size and particle concentration. The system is used to measure those particles of a spectrum of oil droplets that contribute to the validated signal output of counter and transient recorder based LDA-electronic signal processing systems.     

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     

Simultaneous air/fuel-phase PIV measurements in a dense fuel spray

A new diagnostic has been developed that is capable of obtaining simultaneous two-phase velocity measurements in a gasoline direct-injection fuel spray. This technique utilizes a two-laser (double-pulse) two-camera (double-frame) setup to simultaneously image the injected fuel and entrained air to determine the 2D velocity vector fields of both phases using cross-correlation particle image velocimetry (PIV). The air phase is visualized through fluorescence from seeding particles introduced into the static measurement volume while Mie scattering signals are collected from the fuel droplets. The combination of different laser wavelengths and a spectral signal shift for the air phase allows spectral separation of the signals. Independent timing of the laser pulses permits optimized adaptation of the velocity dynamic range for the two phases to account for the large difference in velocities between air and fuel droplets.     

Effect of thermophoresis on fog droplet deposition on low pressure steam turbine guide blades

Deposition of droplets in the sub-micron size range, when discouraged by thermophoresis forces, has been studied by a simulation method employing uranine particles entrained in air and flowing through a cascade of full-sized low-pressure steam turbine blades. The particles, generated by a Collison atomiser, had a mass-median diameter range of 0.05–0.25 μm. The test blades were internally heated using hot air and had an output of 600 W/m² of swept surface. Circumferential tape around their surfaces provided a reception medium for the particles, the deposition density variation of which was found by fluorimetry. Prediction of the deposition on an unheated blade using the method of C.N. Davies showed acceptable agreement with the experimental results. The simulation method was validated by showing that the relevant non-dimensional numbers. Schmidt and Reynolds, can be acceptably matched for the air/particle and the steam/droplet cases. Tests on the heated blade showed that the deposition was reduced by 30–90% of the corresponding value for the unheated blade. The extent of the reduction decreased with particle size decrease.     

Assessment of tomographic PIV in wall-bounded turbulence using direct numerical simulation data

Simulations of tomographic particle image velocimetry (Tomo-PIV) are performed using direct numerical simulation data of a channel flow at Reynolds number of Re _τ = 934, to investigate the influence of experimental parameters such as camera position, seeding density, interrogation volume size and spatial resolution. The simulations employ camera modelling, a Mie scattering illumination model, lens distortion effects and calibration to realistically model a tomographic experiment. Results are presented for camera position and orientation in three-dimensional space, to obtain an optimal reconstruction quality. Furthermore, a quantitative analysis is performed on the accuracy of first and second order flow statistics, at various voxel sizes normalised using the viscous inner length scale. This enables the result to be used as a general reference for wall-bounded turbulent experiments. In addition, a ratio relating seeding density and the interrogation volume size is proposed to obtain an optimal reference value that remains constant. This can be used to determine the required seeding density concentration for a certain interrogation volume size.     

Local field correction PIV: on the increase of accuracy of digital PIV systems

Cross-correlation Particle Image Velocimetry (PIV) has become a well known and widely used experimental technique. It has been already documented that difficulties arise resolving velocity structures smaller than the interrogation window. This is caused by signal averaging over this window. A new cross-correlation PIV method that eliminates this restriction is presented. The new method brings some other enhancements, such as the ability to deal with large velocity gradients, seeding density inhomogeneities, and high dispersion in the brightness of the particles. The final result is a method with a remarkable capability for accurately resolving small scale structures in the flow, down to a few times the mean distance between particles. When compared to particle tracking velocimetry, the new method is capable of obtaining measurements at high seeding density concentrations. Therefore, better overall performance is obtained, especially with the limited resolutions of video CCDs. In this paper, the new method is described and its performance is evaluated and compared to traditional PIV systems using synthetic images. Application to real PIV data are included and the results discussed. Received: 9 March 1998 / Accepted: 25 August 1998     

Effect of particle polydispersity on particle concentration measurement by using laser Doppler anemometry

Measurement of particle concentration by laser Doppler anemometry (LDA) is studied on a vertical air jet seeded by a powder disperser with controlled particle and air flow rates. Particle arrival rate is utilized to retrieve particle number densities from conventional LDA operation. The effect of polydisperse nature of the particles is assessed. Comparisons between measured and estimated particle number densities suggest that only a certain portion of the particle population with a particle size to fringe spacing ratio around unity can be detected. Results indicate that reliable measurement of absolute particle concentration is possible for a particle population of narrow size distribution with an average diameter equivalent to fringe spacing. Present number density measurement technique which is useful for practical purposes with conventional LDA systems is found to yield physically reasonable profiles in both laminar and turbulent regimes.     

On the motion of particles in a fluid under the influence of a large velocity gradient

The motion of seeding particles as used in laser Doppler anemometry is investigated in the presence of a large velocity gradient across aerodynamic shocks under different flow conditions. Experimentally obtained results are presented and compared with theoretical predictions based upon the size distribution of the seeding particles used. It is found that the agreement of experimental and theoretical results depends on the flow conditions as well as on the particle material.     

Mechanism of the Impact of Particle Size Distribution to Bed-Inventory Overturn for Pant-Leg Circulating Fluidized Bed

An improved two particle sizes numerical model based on a uniform size model was established to investigate the influence of the average particle size on bed-inventory overturn inside a pant-leg circulating fluidized bed (CFB). The new model successfully simulated the dynamic performance of a pant-leg CFB that as average particle size shrank, the pant-leg CFB tended to overturn, while the uniform size model showed a contradicted trend. The success was attributed to the difference of the flow pattern with different particle size. The smaller particles tend to stay the upper furnace after fluidized by the primary air flow while the larger particles tend to fall back to the bottom soon after being carried to upper furnace by the primary air flow and smaller particles. As pointed out in our previous work, the lateral mass transfer resulted in a lateral pressure difference at the upper finance and inhibited further lateral mass transfer, which was regarded as a self-balancing process. The quick fall down of the large particles somehow weaken the lateral pressure different built-up at the upper furnace. Therefore, as the average particle size shrink, the weaken effect of the large particles on self-balancing ability of the pant-leg CFB increase, resulting a more tendency for bed-inventory overturn. It was such a characteristic behavior of the large particles, which was neglected in the uniform particle size model, caused the difference between the results of the two models described above.     

Filtration characteristics of a binary multi-layer granular bed filter based on CFD-DEM coupling simulation

The coupled CFD-DEM method with the JKR (Johnson-Kendall-Roberts) model for describing the contact adhesion of dust to filter particles (FPs) is used to simulate the distribution pattern of dust particle deposition in the granular bed filter (GBF) with multi-layer media. The minimum inlet flow velocity must meet the requirement that the contact probability between dust and FPs is in the high contact probability region. The air flow forms vortices on the leeward side of the FPs and changes abruptly at the intersection of different particle size FPs layers. Dust particles form large deposits at the intersection of the first and second layers and the different particle size filter layers. Dual element multilayer GBF can further optimize the bed structure by interlacing filter layers with different particle sizes. Compared with single particle size multi-layer GBF, the bed pressure drop is reduced by 40.24%–50.65% and the dust removal efficiency is increased by 21.93%–55.09%.     

Verification of LDA and seed generator performance

An essential step in the application of laser-Doppler anemometry (LDA) to turbulent flow research is to verify the capability of the measurement system to respond adequately to the anticipated velocity fluctuations. Problems of seed particle velocity fidelity can arise due to inadequate detectability of micron-size seed particles by the LDA system or improper seeding techniques. As a result of the present investigation, we caution that reported LDA measurements in piston engines could be in error. A convenient method to check for the occurrence of these problems is described.This paper was presented at the Ninth symposium on turbulence, University of Missouri-Rolla, October 1–3, 1984     

Conditional sampling of velocity and scalars in turbulent flames using simultaneous LDV-Raman scattering

The laser Doppler velocimeter (LDV) measures the velocity distribution of particles which is often an acceptable representation of the distribution of gas velocities. However, in turbulent two stream mixing flows, the particle velocity distribution will differ from the gas velocity distribution when the particle densities in the two streams are unequal. This bias is explored in a reacting and nonreacting turbulent jet which is surrounded by coflowing air. By adding seed particles to only the coflow air and then to only the jet fluid, the limits of this bias are established. Additional measurements with an LDV triggered laser Raman scattering system demonstrate that the bias in the LDV sampling is propagated to the Raman measurements. An analytical equation is presented which will generate unbiased velocity and scalar distributions from measurements obtained from seeding only one stream at a time.     

Integrating cross-correlation and relaxation algorithms for particle tracking velocimetry

An integrated cross-correlation/relaxation algorithm for particle tracking velocimetry is presented. The aim of this integration is to provide a flexible methodology able to analyze images with different seeding and flow conditions. The method is based on the improvement of the individual performance of both matching methods by combining their characteristics in a two-stage process. Analogous to the hybrid particle image velocimetry method, the combined algorithm starts with a solution obtained by the cross-correlation algorithm, which is further refined by the application of the relaxation algorithm in the zones where the cross-correlation method shows low reliability. The performance of the three algorithms, cross-correlation, relaxation method and the integrated cross-correlation/relaxation algorithm, is compared and analyzed using synthetic and large-scale experimental images. The results show that in case of high velocity gradients and heterogeneous seeding, the integrated algorithm improves the overall performance of the individual algorithms on which it is based, in terms of number of valid recovered vectors, with a lower sensitivity to the individual control parameters.     

Particle size influence on effective permittivity of particle–gas mixture with particle clusters

The influence of particle size on the effective permittivity of a particle–gas mixture in the presence of particle clusters was studied using numerical analysis involving the three-dimensional finite element method. The effective permittivity of the mixture was obtained by calculating the electrostatic energy generated in the computation domain. Numerical results show that for fixed volume fraction of particles in the mixture, the effective permittivity of the mixture increases with decreasing particle size. Static experiments were carried out by using a differential capacitance sensor with parallel plates. The variation of the effective permittivity with particle size is shown by experimental data to agree with the numerical results. The methodology described and the results obtained in this paper may be used to help modify the measurement of particles volume fraction in the presence of particle clusters when a capacitance sensor is used.