Study of oscillating flow structure and characterization of merging process in self-excited twin jets: Numerical and analytical approach

In this paper, the characteristics of oscillatory flow have been numerically investigated in the isothermal closed cavity of self-excited oscillating twin jets for several nozzle spacing-to-width ratios, $0\le S/e\le 12,$ and volumetric flowrates of $Q=0.04,0.08,\text{and}0.12m^3/s$. The results showed that in $0\le S/e\le 6.5$ two jets merge at a short distance after the nozzles and oscillate with a frequency equal to that of the single oscillating jet. At $6.5<S/e<9$, the oscillatory behavior of the twin jet enters a transient and unstable oscillatory mode. By increasing the nozzle spacing to $9\le S/e\le 12$, the oscillating behavior of the jet becomes stable, and jet flows oscillate with a much higher frequency than the self-excited oscillating single and twin jets with $0\le S/e\le 6.5$. In this research, the critical nozzle spacing (minimum nozzle spacing in which merging does not occur) was calculated for different flow rates. Also, an analytical method was presented based on the concept of half velocity edges to predict the critical nozzle spacing. The results showed that the analytical model is accurate compared to the numerical results. It was also revealed that the capability of self-excited twin jets to intensify the oscillating features in the flow field will be evident at nozzle spacings greater than the critical value.