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Kosiwczuk, W Cessou, A Trinite, M Lecordier, BThis paper describes a novel derivative of the PIV method for measuring the velocity fields of droplets and gas phases simultaneously using fluorescence images rather than Mie scattering images. Two-phase PIV allows the simultaneous and independent velocity field measurement of the liquid phase droplets and ambient gas in the case of two-phase flow mixing. For phase discrimination, each phase is labelled by a different fluorescent dye: the gas phase is seeded with small liquid droplets, tagged by an efficient fluorescent dye while the droplets of the liquid phases are tagged by a different fluorescent dye. For each phase, the wavelength shift of fluorescence is used to separate fluorescence from Mie scattering and to distinguish between the fluorescence of each phase. With the use of two cross-correlation PIV cameras and adequate optical filters, we obtain two double frame images, one for each phase. Thus standard PIV or PTV algorithms are used to obtain the simultaneous and independent velocity fields of the two phases. Because the two-phase PIV technique relies on the ability to produce two simultaneous and independent images of the two phases, the choice of the labelling dyes and of the associated optical filter sets is relevant for the image acquisition. Thus a spectroscopic study has been carried out to choose the optimal fluorescent dyes and the associated optical filters. The method has been evaluated in a simple two-phase flow: droplets of 30-40 mu m diameter, produced by an ultrasonic nozzle are injected into a gas coflow seeded with small particles. Some initial results have been obtained which demonstrate the potential of the method

An original method, based on the direct numerical simulation (DNS) of two phase flows, is proposed to evaluate the capability for the PIV algorithms to measure turbulence properties. 3D particle fields are produced from the DNS and are used to generate synthetic images with well-defined flow properties. The imposed velocities in the synthetic images are evaluated thanks to different PIV algorithms and these results can be compared with the initial conditions of simulation. The first part of the present paper describes the synthetic image generator and the DNS of two phase flows. The second part is focused on the analysis of synthetic images of homogeneous turbulence by using two different PIV algorithms: a conventional PIV algorithm and a `sub-pixel' iterative approach.

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Up to now, the use of CCD cameras, for crosscorrelation development in Particle Image Velocimetry (PIV), is reduced to relatively slow flows. An original storage method of two images on the two half frames of a video camera permits now to decrease the interval between exposures (lO ps). Therefore, it is possible to study high speed flows. Applications are shown of a jet and a turbulent flame propagation.

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