Propulsive characteristics of twin-starting jets with simultaneous initiations

The effects arising from interactions between two identical starting jets on their propulsive characteristics have been investigated numerically for different dimensionless distances S/D (the distance between two jet axes normalised by nozzle diameter, from 1.1 to 4) and stroke ratios L/D (the length-to-diameter ratio of jet column, from 2 to 5). The two jets are arranged in parallel and initiated simultaneously with identical conditions. Their leading vortical structures evolve from an axisymmetric to a plane-symmetric configuration, with deceleration in regions where the two jet wakes approach each other. The generation of axial thrust is affected, primarily dominated by variations in the pressure thrust component. This results from the combination of the mutually induced pressure (MIP) and the coupling effects of vortex rings (CEVR for S/D >1.5 and CEVR-R for S/D <1.5). The MIP governs the fluctuations introduced into thrust development, while CEVR (CEVR-R) is responsible for the reduction in average thrust. These effects become more pronounced as S/D decreases, but remain almost unaffected by L/D. Adjusting the acceleration and deceleration rates of the velocity program shows limited effects on either the thrust fluctuations or the average thrust reduction. Furthermore, the interaction induces two lateral force components with equal magnitude but opposite directions on the outer walls of the two nozzles, with their magnitude exceeding 15 % of the axial thrust. The introduction of an additional vertical wall within the nozzle exit plane effectively eliminates the lateral force. However, it consequently enhances both the average thrust reduction and the thrust fluctuations induced by the interaction.