Internal flow and external jet characteristics of double serpentine nozzle with different aspect ratio

In order to increase the survivability of the fighter aircraft, the serpentine nozzle has been applied in series of stealth bombers and unmanned aerial vehicles due to its excellent potentiality of evidently suppressing the infrared radiation signatures and radar cross section emitted by engine exhausts. Among the geometric parameters of the serpentine nozzle, the aspect ratio (AR) at the nozzle exit is one of the most critical parameters for the nozzle design as the infrared suppression effect could be greatly enhanced with the increment of AR by strengthening the mixing between the exhaust plume and atmosphere; the aim of this paper is to study the influence of the AR on the flow characteristics of the double serpentine nozzle. The flow features of six double serpentine convergent nozzles, i.e. AR = 3, 5, 7, 9, 11, 15 respectively, were numerically simulated with the shear stress transport κ–ω turbulent model adopted, which had been validated by the experimental data. The characteristics of internal flow and external jet, and the aerodynamic performances of these six nozzles were compared. Results show that the Ma contours at the symmetric plane are different due to the distinct flow accelerations caused by the change of the curvature and the duct height for diverse AR, and the surface pressure and the shock wave features are different correspondingly. The lateral divergence and the lateral convergence characteristics of the nozzle configuration lead to opposite lateral flow under diverse AR, and the change of lateral width induced different lateral pressure gradient, then lead to various lateral vortex distributions. The length of potential core is the contribution of the comprehensive effects of geometry parameters, and it is decreased with the increase of AR due to the dominated effect of the increased mixing area; however, the declining rate is slowed down. The AR of 5 should be chosen for the best aerodynamic performance of the double serpentine nozzle under the qualifications to completely shield the high-temperature turbine.