Recurrent properties of coherent structures in a thermal plasma jet

Abstract

Summary form only given. Coherent structures play an important role in the generation, dissipation and transfer of energy in turbulent flows. Coherent structures may be characterized as recognizable flow patterns which resist dissipation for a relatively long time periods, reappear frequently and have a typical life-cycle. Coherent structures may be detected in thermal plasma jets e.g. by analyzing differences between successive images of the plasma jet taken by a CCD camera as moving regions characterized by continually increasing or decreasing optical emission. It is also possible to detect these structures by a proper optical system when they pass through planes perpendicular to the plasma jet axis. This way of detection offers a possibility of long-term observation and thus a chance to investigate recurrent characteristics of the structures. We have studied recurrent properties of coherent structures in experiments with a dc argon plasma torch working at various gas flow rates. The plasma jet radiation was projected through 4 objective lens on face areas of linear arrays of optical fibers and photodiodes arranged at 4 directions and 2 levels above the nozzle. The experimental set-up provided records of plasma jet optical radiation with the sample rate 468 kHz/channel and record lengths 1 Msample/channel. The distributions of plasma radiation in measurement planes were reconstructed using the inverse Radon transform. The results show that the principal frequencies controlling the occurrence of similar coherent structures (indicated by similar differences between two successive radiation distributions) are the frequency 300 Hz and its first harmonic 600 Hz. The frequency 300 Hz is present in the plasma radiation records due to the ripple modulation of the dc power source. It is responsible for the corresponding modulation of the arc current and plasma temperature. The analysis shows that the recurrence of coherent structures with the frequencies 300/600 Hz does not imply just repetition of the plasma jet radiation intensity values but it incorporates also the renewals of plasma jet axis positions. The similarity of repeating structures decreases with the gas flow rate and with the distance from the plasma torch nozzle.