Effects of nozzle convergence angle on the flow characteristics of a synthetic circular jet in a crossflow

This paper investigates the effect of nozzle convergence angle of a synthetic jet actuator (SJA) issuing into a crossflow using unsteady Reynolds-Averaged Navier-Stokes (URANS). The SJA consists of a three-dimensional (3D) circular cavity with a diaphragm, a neck of diameter, D = 4 mm and height, h/D = 2.5 and a circular exit orifice of diameter, d = 2 mm. Three test cases were examined by varying the convergence angle (α) between the neck and the exit orifice: Case A with α = 90^o (square), Case B and Case C with α = 45^o and α = 30^o, respectively. The simulations were performed using k-ω SST turbulence model in StarCCM+ (v. 15.04) and the vibration of the diaphragm of the SJA was modelled using a dynamic mesh. The SJA without any modifications to the exit was validated and characterized in quiescent flow. The excitation frequency was 300 Hz and the phase-averaged centreline velocity during peak expulsion was U_cl = 7.93 m/s. For the crossflow turbulent boundary layer (TBL), the freestream velocity was U_∞ = 7.46 m/s, the Reynolds number based on the momentum thickness was Re_θ = 900 and the boundary layer thickness was δ₉₉/d = 7.75. The results showed that the maximum exit velocity during the expulsion increased with increasing convergence angle. Moreover, the depth of penetration of the reverse flow region behind the expelled jet was higher for Case A and C than Case B. During jet expulsion, vortex loops were formed due to the crossflow shear layer interactions, however, the number of loops significantly increased for Case A compared to Case B and C. Further downstream, the structure of the expelled jet attained similarity, as the hairpin vortex evolved into a vortex ring that was independent of the nozzle convergence angle.