Effect of relative jet momentum in supersonic dual impinging jets

Multi-jet impingement flows relevant to applications in Short Take-Off and Vertical Landing aircraft have been investigated from the standpoint of evaluating the induced lift forces and moments caused by fountain flows. However, for a pair of supersonic jets, the outcome of the difference in their momentum-flux on flow properties is relatively unknown. This study reports on the experimental characterization of the supersonic dual impinging jet flowfield by systematically varying their relative jet momentum-flux and impingement height. A nozzle pair involving a sonic converging and a Mach 1.5 Converging–Diverging (CD) is employed during the study. The CD nozzle is operated at a fixed nozzle pressure ratio of 3, and the expansion ratio (ER) of the converging nozzle is varied from 0.96, 1.19, to 1.59. Results indicate that the fountain flow exhibits a strong dependence on relative jet momentum and a relatively weak dependence on impingement height. Increase in jet momentum-flux of the converging nozzle led to an obvious increase in the nearfield noise levels over a majority of the impingement heights. At resonance-dominated impingement heights, however, a systematic reduction in both the nearfield noise and ground plane unsteady pressures is observed. The resonance in the jet from CD nozzle is found to be the chief source of unsteadiness in the flowfield, which grew weaker and exerted lesser influence with increase in jet momentum-flux of the converging nozzle. Proximity of the fountain flow to the resonating jet is identified to be the main cause for the change in resonance characteristics of dual impinging jets. The cause for this change is determined to be due to the alterations introduced to the highly sensitive processes that constitute the feedback mechanism, when the fountain flow directly interacts with the jet shear layer.