Resonator-like behavior of a wall-bounded precessing vortex core in a diffuser with wall asymmetries

Abstract

This paper reports a detailed investigation of the interaction between a wall-bounded precessing vortex core (PVC) occurring in swirling flows after vortex breakdown and a wall asymmetry. Experiments are carried out in an axisymmetric diffuser downstream of an axial swirl generator inducing a swirling flow with a swirl number of S = 1.1. Wall pressure measurements and two-component particle image velocimetry (PIV) are conducted for Reynolds numbers (Re) ranging from 20 000 to 76 000 in the initial axisymmetric configuration and several asymmetric configurations, with an additional cylindrical protrusion placed on the diffuser wall at different streamwise and circumferential positions. It is first confirmed that synchronous pressure fluctuations at the PVC frequency are only produced in asymmetric configurations. Furthermore, the analysis of the pressure data in several asymmetric configurations revealed for the first time a resonator-like behavior of a wall-bounded PVC. While a change of the protrusion circumferential position in a given cross section of the diffuser only affects the phase of the synchronous pressure fluctuations, the amplitude of the latter features successive minima (pressure node) and maxima (pressure anti-node) as the protrusion is moved along the diffuser in the streamwise direction. In addition, as the protrusion is moved closer to a pressure node, the phase of the synchronous pressure fluctuations exhibits a sudden variation of ±π. Similar results are observed for all tested values of Reynolds number, whereas the PVC frequency linearly increases with Re. A reconstruction of the PVC helical structure based on PIV measurements showed that these consecutive pressure nodes are spaced by a distance equal to approximately one third of the PVC helical pitch. Finally, it also revealed that two different states are observed, depending on the position of the protrusion along the diffuser: the synchronous pressure component reaches its maximum value as the PVC center is approaching either its closest or farthest angular position with respect to the protrusion. The transition from one state to another one depends on the streamwise position of the protrusion with respect to the pressure nodes. These unprecedented experimental observations pave the way to novel theoretical developments for a better understanding and modeling of synchronous pressure fluctuations induced by wall-bounded PVC in asymmetric geometries.