In the present study we investigate three-scalar mixing in a turbulent coaxial jet. In this flow a centre jet and an annular flow, consisting of acetone-doped air and ethylene respectively, are mixed with the co-flow air. A unique aspect of this study compared to previous studies of three-scalar mixing is that two of the scalars (the centre jet and air) are separated by the third (annular flow); therefore, this flow better approximates the mixing process in a non-premixed turbulent reactive flow. Planar laser-induced fluorescence and Rayleigh scattering are employed to measure the mass fractions of the acetone-doped air and ethylene. The results show that the most unique aspects of the three-scalar mixing occur in the near field of the flow. The mixing process in this part of the flow are analysed in detail using the scalar means, variances, correlation coefficient, joint probability density function (JPDF), conditional diffusion, conditional dissipation rates and conditional cross-dissipation rate. The diffusion velocity streamlines in scalar space representing the conditional diffusion generally converge quickly to a manifold along which they continue at a lower rate. A widely used mixing model, interaction through exchange with mean, does not exhibit such a trend. The approach to the manifold is generally in the direction of the ethylene mass fraction. The difference in the magnitudes of the diffusion velocity components for the two scalars cannot be accounted for by the difference in their dissipation time scales. The mixing processes during the approach to the manifold, therefore, cannot be modelled by using different dissipation time scales alone. While the three scalars in this flow have similar distances in scalar space, mixing between two of the scalars can occur only through the third, forcing a detour of the manifold (mixing path) in scalar space. This mixing path presents a challenging test for mixing models since most mixing models use only scalar-space variables and do not take into account the spatial (physical-space) scalar structure. The scalar JPDF and the conditional dissipation rates obtained in the present study have similarities to those of mixture fraction and temperature in turbulent flames. The results in the present study provide a basis for understanding and modelling multiscalar mixing in reactive flows.

In a previous study we investigated three-scalar mixing in a turbulent coaxial jet (Cai et al.J. Fluid Mech., vol. 685, 2011, pp. 495–531). In this flow a centre jet and a co-flow are separated by an annular flow; therefore, the resulting mixing process approximates that in a turbulent non-premixed flame. In the present study, we investigate the effects of the velocity and length scale ratios of the annular flow to the centre jet, which determine the relative mean shear rates between the streams and the degree of separation between the centre jet and the co-flow, respectively. Simultaneous planar laser-induced fluorescence and Rayleigh scattering are employed to obtain the mass fractions of the centre jet scalar (acetone-doped air) and the annular flow scalar (ethylene). The results show that varying the velocity ratio and the annulus width modifies the scalar fields through mean-flow advection, turbulent transport and small-scale mixing. While the evolution of the mean scalar profiles is dominated by the mean-flow advection, the shape of the joint probability density function (JPDF) was found to be largely determined by the turbulent transport and molecular diffusion. Increasing the velocity ratio results in stronger turbulent transport, making the initial scalar evolution faster. However, further downstream the evolution is delayed due to slower small-scale mixing. The JPDF for the higher velocity ratio cases is bimodal at some locations while it is always unimodal for the lower velocity ratio cases. Increasing the annulus width delays the progression of mixing, and makes the effects of the velocity ratio more pronounced. For all cases the diffusion velocity streamlines in the scalar space representing the effects of molecular diffusion generally converge quickly to a curved manifold, whose curvature is reduced as mixing progresses. The curvature of the manifold increases significantly with the velocity and length scale ratios. Predicting the observed mixing path along the manifold as well as its dependence on the velocity and length scale ratios presents a challenge for mixing models. The results in the present study have implications for understanding and modelling multiscalar mixing in turbulent reactive flows.

We demonstrate for the first time, to the best of our knowledge, the ability of extreme ultraviolet (XUV) Hartmann wavefront sensors to characterize high charge vortex beams produced by high-order harmonic generation up to the order of 25. We also show that phase matched absorption limited high harmonic generation is able to maintain the high charge vortex structure of the XUV beam even in a rather long (1 cm) generation medium.

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We demonstrate that high harmonic generation when driven by vortex driver contaminated with various orbital angular momentum (OAM) modes, the upconverted EUV harmonic deviates from perturbative conversion law to exhibit a spectrum of OAM orders.