Abstract
This article proposes a technique to estimate the cross-sectional scalar interface (outer boundary) in an inhomogeneous turbulent flow from a conditioned particle image velocimetry (PIV) experiment, which is suitable for medium to high Reynolds numbers. The scalar interface is estimated directly by using conditioned PIV particle images which have distinguishably high particle seeding density in the area of interest, whereas conventionally in water based experiments, scalar interface is often determined from planar laser induced fluorescence (PLIF) or equivalent dye images. By comparing quantities in the vicinity of this scalar interface, it also shows that in terms of separate turbulent and non-turbulent regions, this technique could also replace the function of PLIF images in water experiments, with slightly lower spatial resolution. At the same time, if velocity information is also required simultaneously then the cost of a separate camera-laser system can be saved. The effect of particle field inhomogeneity on the PIV accuracy can be well reduced to an insignificant level by an image local normalisation treatment. This article shows that the interfacial layer could be detected fairly accurately by enhancing the particle images by wavelet based thresholding methods. The degree of detection accuracy is quantified by synthetic particle image analyses, where a scalar interface can be artificially pre-defined. The proposed technique is tested in two water based experiments but is expected to be particularly useful in gas-phase based experiments or some combustion applications, where liquid-phase dye cannot be applied.
Highlights
Scalar interface separating the scalar marked area from the rest is often interesting to know in inhomogeneous high Reynolds number turbulent flows, for both liquid and gas phased applications
The principle of estimating the scalar interface by a particle image velocimetry (PIV) image is that in any experiment in which if different regions can be distinguished by the difference of dye concentration, the dye may be replaced by PIV particles of a very high seeding concentration
Before moving on to discuss the solution, an important issue needs to be addressed: are the highly seeded particles in the structure region really able to replace the function of a traditional scalar dye, e.g. sodium fluorescein as commonly used in water based experiments, to mark out the passive scalar interface, to a degree which is sufficient in a turbulent flow problem?
Summary
Scalar interface separating the scalar marked area from the rest is often interesting to know in inhomogeneous high Reynolds number turbulent flows, for both liquid and gas phased applications. TNTI is usually defined by the transitional zone between the outside irrotational region and the inside highly concentrated, fully developed turbulent region (see 12; 28; 13, for example) In such applications, since turbulent velocity field and scalar interface are often both required, it usually needs simultaneous determination of the two. It is worth noting that there does exist a similar method (26) to achieve a similar objective, where interface is marked by simple spatial filtering of raw particle images, it is easy to implement
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