Development of Two-Focus LDV System for Analysis of Spatial Structure of Turbulence

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Turbulent flame structure has large influences on the turbulent burning velocity. Not only turbulence intensity but also turbulence length scales are very important factors to turbulent flame structure. In the many past studies, turbulence length scales were estimated by the Taylor's hypothesis using mean velocity and turbulence time scale obtained from auto-correlation function of turbulence. Turbulence length scales are by definition obtained from cross-correlation of turbulence. In this study, Two-Focus LDV System was developed in order to obtain turbulence length scales by definition. This LDV (Laser Doppler Velocimetry) system has two measuring volumes. Velocities at two points are measured at the same time. The separation distance between two measuring points is able to be changed perpendicularly to the measuring velocity vector. Cross-correlation function of turbulence is obtained by this LDV system changing the separation distance. This system was applied for turbulent flow fields in the wind tunnel and cnstant volume bomb. And the following conclusions were obtained. (1) Velocity measurement is possible even at the separation distance 0.5mm. (2) Integral length scale obtained by this system is rather different from the estimated one from Taylor's hypothesis, though this comparison was carried out in the wind tunnel where the mean velocity is 8 times of turbulence intensity. (3) Turbulence length scale can be obtained by this system even in the flow fields where no mean flow exists and Taylor's hypothesis is not applicable.