On the control of global modes in swirling jet experiments

Abstract

The aim of this work is to control the self-excited global mode that concomitants vortex breakdown in turbulent swirling jets. This mode is characterized by a co-rotating counter winding single helical instability wave that originates from the jet center. Experiments show that the amplitude of this global mode is effectively reduced by exciting a double-helical mode in the outer shear layer. This mode is shown to be convective unstable at growth rates that are well predicted by spatial linear stability analysis. The dampening of the global mode occurs through an energy transfer between the inner and the outer shear layer. In preparation of closed-loop experiments, a reduced order model of the flow dynamics is developed based on five leading POD modes. The model is calibrated to flow-transients recorded via time-resolved PIV. A state estimator is designed that predicts the flow state from two hotwire probes. The performance of the estimator is validated in open-loop experiments. First results support the design of the model and state estimator.