Influence of inlet vorticity and aspect ratio on axis-switching and mixing characteristics of heated rectangular jets

To investigate interactive characteristics between the inlet condition and axis-switching phenomenon, heated rectangular jets are numerically investigated for seven aspect ratios (AR = 1.0 − 6.0) with three jet temperatures of 300, 500, and 1000 K. The corresponding Reynolds number is 11,400 – 21,000. To examine the relative importance of ω_θ and ω_x on axis-switching, seven inlets are obtained by an inflow generator. For the flows of nozzle part, various geometry-driven and turbulence-driven vortices are observed. It is shown that the rotational directions of turbulence-driven vortices have the rotational direction hindering the axis-switching flow. By using the crossover location of the jet half-widths and the mixing length, the axis-switching condition depending on AR and jet temperature is explained. The crossover location is changed to the ω_x-variation, which is approximately eight times as sensitive as the ω_θ-variation. Based on the results of AR and temperature changes, the strong influence on axis-switching flow and mixing is confirmed for the ω_x-change. Also, the heat transfer effect on mixing enhancement becomes strong for the weak axis-switching condition. As AR increases, the mixing performance decreases, but mixing enhancement of the axis-switching flow is confirmed for AR ≤ 4. Finally, to explain the ω_x-variation related to the axis-switching mechanism, the ω_x transport equations are analyzed by three terms of the Prandtl's first kind and the baroclinic torque. The roles of the vortex stretching and baroclinic torque terms are analyzed for the positive effect of the ω_x-variation on the axis-switching phenomenon.