In this study, the characteristics of the scalar field in an axisymmetric turbulent water jet are investigated experimentally. In the experiments, the axial velocity, the concentration of the dye solution and the temperature of the fluid are measured by the hot-film probe, the fiber sensor and the cold-firm probe, respectively. In particular, the difference of statistics between scalars (concentration and temperature) with the different molecular diffusion coefficients is discussed. The Schmidt number of the diffusing matter is 3,800, and the Prandtl number of temperature is 7. As regarding the mean values, the r.m.s values and distribution of PDF, we cannot find any difference between the concentration field and temperature field. However, in the spectrum, it is found that the temperature spectrum shows the −5/3 law almost in the same range as the velocity spectrum, on the other hand, the concentration spectrum shows the −5/3 law in the wider range than the velocity spectrum. This means that the shape of spectrum depends on the diffusion coefficient. In order to make the higher resolution measurement of concentration, a new optical probe based on the LIF method is designed. This probe consists of the two optical fibers, the tip of which is processed like the shape of a lens. By the effect of lens, the laser beam can be focused on the narrower area in comparison with the past LIF measurements. In the present design, the width of focus of laser beam is set to 0.6 micrometers, and the focal length is set to 7.3mm. It is shown that this probe has the resolution less than the Batchelor scale at x / d ≥30 in the condition of present jet diffusion field (d = 4mm, Re = 20,000). Further, a new system to adjust the position of the optic fiber probe exquisitely is developed.

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