Nature of unsteady and acoustic behavior in an elliptic sonic jet with aft-deck

Abstract

An experimental investigation is carried out on the under expanded sonic jet issuing from an elliptic nozzle with aft-deck at two different Mach numbers (Mj). Flow characteristics including shock-cell structure, jet separation, and pressure variation over the aft-deck wall are investigated with the help of schlieren images, oil flow visualization, and steady pressure measurements. A complex shock-structure is found to exist in the x-z and x-y planes due to the shape and curvature of the aft-deck for Mj = 1.35 and 1.56. The shock-induced jet separation is identified at x/L ∼ 0.75 for Mj = 1.56, whereas no separation is identified for Mj = 1.35. Synchronized acoustic and unsteady measurements reveal the coupling between the shock-induced jet separation on the aft-deck wall and the far-field noise emissions. Acoustic spectra elucidate the strong tonal noise component corresponding to the flapping Mode-B at St ∼ 0.68 for Mj = 1.56. Moreover, the noise components including screech tones, broad-band shock-associated noise, and turbulent mixing noise are dominant for the azimuthal angle ϕ = 0°. The dominant flow mode from proper orthogonal decomposition analysis shows the existence of strong jet flapping and upstream-traveling acoustic waves for the separated jet.