Decay of rotating turbulence: some particle tracking experiments

Abstract

Recent development of measurement techniques based on particle image velocimetry (PIV) are enabling more detailed measurements to be made over extended regions of a flow than have been previously possible. These techniques are of particular value for turbulent flows where the structures present within such flows are incompletely understood and are not readily accessible to traditional measurement techniques. Unfortunately the considerable processing time and specialised equipment required with most PIV techniques limits their applicability when ensemble statistics are required for an evolving turbulent flow. This paper reports on the development and application of an efficient, fully automated particle tracking system. The system was developed as part of a study of the decay of turbulence in a rotating environment. Ensemble descriptions of the temporally evolving flow were required over an extended measurement domain. For each set of parameters particles were tracked with a sampling frequency of 12.5Hz over 60 seconds for 25 realisations. Typically 350 particles were identified and tracked at each time step. Processing speeds in the region ten to fifteen sample images per minute were achieved using a PC/AT compatible computer. The results of the experiments were found to be in broad agreement with previous investigations. However it was found that the method of generating the initial turbulent flow had a profound affect on the subsequent evolution due to the forcing of a strong, large scale systematic flow.