3D3C-coherent structure measurements in a free turbulent jet

Abstract

The near-field of a free turbulent air jet at Reynolds number 65000 was investigated by means of tomographic particle image velocimetry (TPIV). The averaged three-dimensional and three-component (3D3C) velocity fields were characterized according to standard jet quantities, such as centerline decay rate and jet-broadening rate amongst others. These values were in agreement with the few investigations [GRI06, GAN02] that have been performed in the non–fully developed turbulence regime of the flow. The main focus was laid on the investigation of coherent structures (CSs) that were analyzed by means of statistics. The CSs were separated and extracted by constrained ensemble correlation (EC) that leads the 3D3C similarity field around an event that was chosen as a marker of a distinct CS. Although it does not yield a velocity field around that event, it is possible to make conclusions about the velocity field. Therefore it is similar to a technique called linear stochastic estimation [ADR94]. Especially flow structures around streaks were the main issue of the research. Their average local structure and the frequency of appearance could be derived with this analysis technique, which yielded similar results as shown in Matsuda et al. [MAT05] and Hori et al. [HOR04] but without the use of Taylor’s frozen turbulence [TAY38]. Compared to reconstructed volumetric velocity fields [HOR04] the present paper shows better results in terms of obedience to incompressibility of the velocity field by at least one order of magnitude at a much higher Reynolds number. Regarding the investigation of spatial structures, a discrepancy between the alignment of streaks and their surrounding vortical structures was measured and appears to be a self-similar property of turbulent jets.