Large eddy simulation of flow separation-induced disturbance evolution inside a sudden contracting nozzle

The contracting section of a nozzle defines outcomes like downstream velocity and its fluctuations. We studied the flow dynamics inside a three-dimensional sudden contracting nozzle using large eddy simulation. Flow obstruction by the nozzle contracting step induces flow detachment from the walls, creating separation bubbles and vortex shedding. The span-wise movement of fluid particles upstream of the nozzle step, along with the vortex shedding phenomena and unstable flow separation layer, develops a three-dimensional flow motion inside the nozzle throat. Turbulent kinetic energy inside the nozzle throat propagates in a definite pattern. A two-dimensional disturbance first develops on the flow separation layer, followed by a three-dimensional nature at the separation bubble reattachment zone near the step-wise walls, which then move toward the span-wise walls. Finally, flow motions from all the walls disturb the nozzle centerline region. We studied velocity fluctuations through different cross-sectional locations like the centerline and areas near the step and span-wise walls. Flow disturbance through the nozzle throat is categorized as two-dimensional, near-wall three-dimensional, bulk three-dimensional transition, and fully randomized zones based on the velocity fluctuations. Finally, we studied the cross-sectional stream-wise velocity distribution. After passing through the nozzle step section, the flow does not form a conventional channel velocity profile (maximum at the centerline). Instead, the peak average stream-wise velocity offsets symmetrically about the centerline.