Influence of Nozzle Convergence Angle on the Flow Characteristics of Synthetic Jets in a Turbulent Crossflow Boundary Layer

The effect of nozzle convergence angle of a circular synthetic jet actuator issuing into a turbulent crossflow is investigated using unsteady Reynolds-averaged Navier–Stokes simulations. The study examines three convergence angles, $30 deg, 45 deg,$ and $90 deg$ with constant neck and jet-exit diameters, actuation frequency, and amplitude. The crossflow Reynolds number is based on the momentum thickness, $Reθ=895$ and boundary layer thickness, $δ/d=7.75$⁠, where $d$ is the jet-exit diameter. The results showed that the jet-exit momentum was significantly enhanced as the convergence angle increased. The increased jet momentum resulted in enhanced mixing in the near field of the jet, rapid development of hairpin vortices attached to the wall, and deeper penetration of these vortices into the crossflow boundary layer. The hairpins and near-wall tertiary vortices promoted sweep and ejection motions, which enhanced wall shear stress in the actuated cases, demonstrating greater potential for flow separation control, particularly for the nozzle with a $90 deg$ convergence angle.