Nihon Kikai Gakkai ronbunshu (Jul 2021)
Effect of distance between jets and pulsation phase difference on flow in multiple pulsating jets
Abstract
Numerical simulation using DNS (direct numerical simulation) were performed for multiple pulsating jets consisting of two jets with different phases of pulsation. The calculations were performed for multiple cases where the jets with Re = 1500 were arranged at short or long distances from each other, and had pulsations with St = 0.3 and 0 (in-phase), π / 2, π (anti-phase) phase differences respectively. In addition to evaluating the instantaneous vortex structure of the jets and the average contour in the cross section along the mainstream direction through the centerline of the two nozzles, we introduced entrainment and mixing parameters to evaluate quantitatively the average characteristics of the jets over the entire calculation area. For the jets arranged at short distance, the jets with in-phase pulsations caused flow switching, and entrainment and mixing were suppressed compared to the case without pulsations, on the other hand, the jets with anti-phase pulsations were expanded in the jets arrangement direction and the entrainment and mixing were improved. The jets with π / 2 phase difference caused flow vectoring to the side where the phase was delayed by π / 2. On the other hand, when the jets were arranged at long distance, switching or expanding flow did not occur for the jets with in-phase and anti-phase. However, for the jets with π / 2 phase difference, the jet flow expanded in the jets arrangement direction, and improving entrainment and mixing. These findings suggest that adding pulsations other than in-phase or anti-phase to the multiple pulsation jet can further improve the flow entrainment and mixing characteristics. These phenomena are due to the interference between the vortex rings formed by the two jets, and it’s state seems to be determined by the strength of the vortex rings and the positional relationship between them. Therefore, by adding a stronger pulsation, it can be expected that even if two jets are arranged at long distance, a flow similar to that when the jets are arranged at short distance will occur.
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