On Turbulent Swirling Jets: Vortex Breakdown, Coherent Structures, and their Control

This thesis provides an experimental and theoretical investigation of turbulent swirling jets with the emphasis on the formation of large-scale coherent flow structures. The involved mechanisms are highly three-dimensional due to the coexistence of an axial and an azimuthal shear layer. Moreover, swirling jets are prone to a unique flow phenomenon that causes an abrupt change of the entire flow. The so-called vortex breakdown is manifested in the appearance of an internal recirculation bubble, which creates an inner and an outer shear layer. The complexity of this flow configuration poses a great challenge to fundamental research and deals as a benchmark for recent theoretical concepts. Furthermore, the present work is of great importance for the gas turbine industry, where swirling flows are frequently applied to improve combustion processes. The work consists of four major investigations. First, the dominant flow dynamics are characterized at different swirl intensities via time-resolved stereo particle image velocitmetry. This experimental work provides an overview of the main flow features with particular focus on the formation of vortex breakdown and the onset of global instability. A self-excited single-helical mode is found to arise from the axisymmetric breakdown state when the recirculation bubble reaches a sufficient streamwise extent. The preceding onset of breakdown is explained by the criticality of the rotating base flow. The second investigation consists of a detailed examination of the coherent flow structures that amplify in the swirled shear layer. The impact of swirl on the driving instabilities is addressed theoretically by means of spatial stability analysis based on the mean turbulent flow. The most dominant instability is of Kelvin–Helmholtz type, similar to the non-swirling jet. However, swirl effects the phase velocity of the helical instability waves and renders the shear layer as strongly dispersive. Moreover, the azimuthal shear destabilizes steady modes that gain significant amplitudes, which may lead to a breaking of the mean flow symmetry. The theoretical predictions are confirmed by hot-wire measurements of the pulsed and the single-mode actuated swirling jet. The third investigation focuses on the coherent structures associated with the swirling jet’s global mode that arises from the axisymmetric vortex breakdown state. A method based on the proper orthogonal decomposition is developed that allows to reconstruct the phase-averaged velocity field of the dominant coherent structures from uncorrelated flow snapshots. The obtained coherent structures are compared to the global eigenmode derived theoretically from a local spatial stability analysis. The global mode is characterized by a precessing vortex core located upstream of the breakdown bubble. It is interpreted as the global wavemaker that imposes its frequency onto the highly receptive outer shear layer, causing large-scale helical flow structures to evolve in the periphery of the jet. The last investigation focuses on the control of the swirling jet undergoing vortex breakdown. By combining the findings of the preceding studies, it is possible to derive a control scheme that enables to dampen the natural global mode at low amplitude forcing. The natural flow is shown to be globally unstable to a single-helical mode resulting in the precession of the vortex core. Convectively unstable modes forced at the nozzle lip lead to an enhanced growth of the outer shear layer. The resulting change of the mean flow leads to a global stabilization of the single-helical mode and to the suppression of the internal feedback that causes the precession of the vortex core.