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1.
A novel experimental technique for studying the stability of the breakup of a two-phase flow is presented. High-speed backlit
photography is used to capture realisations of the unstable mixing layer, and the edge velocity is derived in order to measure
the effects of parameters such as gas/liquid momentum ratio, Reynolds and Weber Number. This has traditionally been an edge
detection problem that introduces additional uncertainty. Here, a cross-correlation solution is presented, which overcomes
the limitations of threshold techniques. Practical application is demonstrated for an atomising annular liquid sheet under
several conditions. Sensitivity due to edge blurring and noise is quantified by artificial analysis. Sensitivity analysis
shows accuracy and precision to permit sub-pixel precise velocity and stability measurements up to 0.6 sheet thicknesses from
the nozzle exit at the conditions studied. 相似文献
2.
A correlation image velocimetry (CIV) technique has been developed to study the evolution of the leading edge, or tip, of
isothermal high-pressure fuel sprays. Adaptations of the analysis permit determination of both the average spray tip motion
and the spatial distribution of velocity along the spray edge. From these measurements, three distinct regions of the tip’s
evolution have been observed and scaling relations developed. Further investigation has revealed significant uniformity in
the radial evolution of the spray tip, despite the apparent similarity to turbulent jet flow. Examination of pdfs of the average
tip velocity reveals among the many repeatable injection events a significant amount of variability and that this variability
extends to regions near the nozzle, implying that among the sources of shot-to-shot viability is the atomisation process itself. 相似文献
3.
This paper details the use of magnified digital in-line holography (MDIH) and digital particle image velocimetry (DPIV) to
measure the evaporation rates of fuel micro-droplets undergoing heating. The technique can be used to measure instantaneous
evaporation along an individual droplet trajectory, or if applied to a series of droplets, the average evaporation over a
number of successive measurement locations. The advantage of this technique over traditional optical techniques is greater
spatial resolution and depth of field for the high magnification factors used. An application of the technique to the evaporation
measurement of diesel fuel droplets ranging from 10 to 90 μm is presented. Results reveal that similar to larger droplets,
temperature plays the dominant role in evaporation processes, with little sensitivity to initial droplet size found for a
peak reactor temperature of 660 K. 相似文献
4.
Multigrid cross-correlation digital particle image velocimetry (MCCDPIV) is used to investigate the stability and structure of low Reynolds number axisymmetric jets. The in-plane velocities, out-of-plane vorticity and some of the components of the Reynolds stress tensor are measured. Two Reynolds numbers based on the orifice outlet diameter are examined (680 and 1,030) at two different positions: one close to the orifice, ranging from 2D
0 to 5D
0 (D
0 is the orifice diameter); and the other further from the orifice, ranging from 10D
0 to 14.4D
0. The results show that the lower Reynolds number jet (Re=680) is marginally unstable in the near-orifice region and is best described as laminar. Further downstream some intermittent structures are observed in the jet, and the growth in integrated turbulent kinetic energy with axial position indicates that the jet is also unstable in this region. For the higher Reynolds number jet (Re=1,030) the increasing size and intensity of vortical structures in the jet in the near-orifice region observed from the MCCDPIV data and the growth in integrated turbulent kinetic energy indicate that the jet is unstable. Further downstream this jet is best described as transitional or turbulent. From flow visualisation images in the near-orifice region it seems that, for both Reynolds numbers, shear layer roll-up occurs when the jet exits the orifice and enters the quiescent fluid in the tank, resulting in vortical structures that appear to grow as the jet proceeds. This is indicative of instability in both cases and is consistent with previous flow visualisation studies of low Reynolds number round jets. Discrepancies observed between the flow visualisation results and the MCCDPIV data is addressed. On the basis of flow visualisation results it is generally assumed that round jets are unstable at very low Reynolds number, however the present work shows that this assertion may be incorrect. 相似文献
5.
A study of some aspects of tracer particle responses to step changes in fluid velocity is presented. The effect of size distribution
within a seed material on measured relaxation time is examined, with polydisperse particles of the same median diameter shown
to possess a significantly higher relaxation time than their monodisperse counterparts when measured via a particle image
velocimetry algorithm. The influence of a shock wave–induced velocity gradient within a PIV interrogation window on the correlation
function is also examined using the noiseless cross-correlation function of Soria (Turbulence and coherent structures in fluids,
plasmas and nonlinear media. World Scientific, Singapore, 2006). The presence of a shock is shown to introduce an artificial fluctuation into the measurement of velocity. This fluctuation
is a function of the shock position, shock strength, spatial ratio and particle distribution. When the shock is located at
the middle of the window, the magnitude of the fluctuation increases monotonically with increasing spatial ratio, increases
asymptotically with shock strength, and decreases for increasing particle polydispersity. When the shock is located at the
left-hand edge of the window, the magnitude of the artificial fluctuation is highest for intermediate spatial ratios, going
to zero at infinitely high and low values. In this instance, particle polydispersity acts to increase the magnitude of fluctuations
in measured velocity. In both cases, particle polydispersity serves to broaden the PDF of measured velocity. For the cases
presented herein, with a shock located within the interrogation window, the root mean square of the artificial velocity fluctuations
reaches values in excess of 30% of the freestream velocity. 相似文献
6.
This paper presents an experimental investigation on swirling jets with well-defined initial conditions. The axial, radial,
and azimuthal velocity components, with their respective fluctuations were measured using high spatial–resolution particle
image velocimetry. These detailed measurements allow the initial conditions of the swirling jets to be established and the
jets to be characterized using various swirl number definitions. The significance of each term in the swirl number calculations
are quantified, and the effect of the common assumptions and simplifications are examined. The characteristics of the jets
in relation to the initial conditions are then investigated and compared with the previous studies using similar characterization
parameters. Jets with Reynolds number of approximately 5700 and swirl conditions ranging from a non-swirling reference case
to high swirl are studied. General properties of swirling jets such as higher spreading rate, higher centerline velocity decay,
and higher turbulence level are observed. When the degree of swirl is sufficiently high, vortex breakdown occurs. A swirl
number of 0.94 is recorded for a high swirl case prior to vortex breakdown, much higher than the critical swirl number reported
in the literature. This behavior is attributed to the effect of the initial conditions on the swirl number calculation. 相似文献
7.
In this experimental study, a thin annular moving water sheet is placed between two annular co-flowing air streams. The shear
at the interface gives rise to Kelvin–Helmholtz type instabilities and promotes development of a sinuous surface wave at the
gas–liquid interface. The amplitude of the surface wave is amplified as it travels downstream of the nozzle exit until it
ruptures forming spanwise and streamwise ligaments. The liquid sheet is illuminated with high-powered halogen lamps. High-speed
imaging is used in this study to qualitatively visualise the structure of the spray—of particular interest is the evolution
of the spray into a ligament structure during the primary break-up and the role the outer air stream plays in this process.
Sequences of images with high temporal resolution (∼2,000 fps) are recorded for image processing and analysis of the surface
waves and ligament formation. A preliminary analysis of the waveform of the outer gas–liquid interface of the annular liquid
sheet over a range of conditions shows the sheet Strouhal number to increase with increasing gas to liquid momentum ratio.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
8.
Dynamic mode decomposition (DMD) is a new diagnostic technique in fluid mechanics which is growing in popularity. A powerful
analysis tool, it has great potential for measuring the spatial and temporal dynamics of coherent structures in experimental
fluid flows. To aid interpretation of experimental data, error-bars on the measured growth rates are needed. In this article,
we undertake a massively parallel error analysis of the DMD algorithm using synthetic waveforms that are shown to be representative
of the canonical instabilities observed in shear flows. We show that the waveform of the instability has a marked impact on
the error of the measured growth rate. Sawtooth and square waves may have an order of magnitude larger error than sine waves
under the same conditions. We also show that the effects of data quantity and quality are of critical importance in determining
the error in the growth or decay rate, and that the effect of the key parametric variables are modulated by the growth rate
itself. We further demonstrate methods by which ensemble and orthogonal data may be introduced to improve the noise response.
With regard for the important variables, precise measurement of the growth rates of instabilities may be supplemented with
an accurately estimated uncertainty. This opens many new possibilities for the measurement of coherent structure in shear
flows. 相似文献
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