Vortex dynamics in an oscillating flow

In this seminar I will present recent research into the interaction between vortex shedding from consecutive inline bodies (cylinders upstream of a conical bluff body) in an oscillating flow field. The motivation was the discovery of a method to control thermoacoustic instabilities in premixed flames stabilised by a bluff body. By placing an array of cylinders upstream of the flame, it was found that lock-in between vortex shedding from cylinders and the bluff body due to an applied acoustic field could be used to damp or drive thermoacoustic instabilities via interference. To understand the underlying vortex dynamics, PIV and hot wire experiments were undertaken. It was found that the lock-in between the vortex shedding from the cylinders is amplified at the expected Strouhal number of 0.2. Results show that the forcing frequency, symmetric vortex shedding from the cylinders and the bluff body is the dominant mode. However, when the forcing frequency approaches twice the natural shedding frequency of the cylinders, spectra exhibited peaks at both the forcing frequency and a sub-harmonic, hallmarks of classical lock-in of the transverse (VK street) vortex shedding mode. This prompted us to revisit the phenomenon vortex lock-in for a single circular cylinder in an oscillating flow but with a new twist by placing the cylinder at the node and anti-nodal positions of a standing wave to assess the lock-in behaviour. Velocity fields reveal the existence of bimodal shedding during lock-in - both symmetric and transverse modes. POD was used to investigate the symmetric and transverse shedding modes revealing that symmetric shedding is inherent in the near wake region only. These results led to a proposed scaling based on Strouhal number/reduced velocity to predict the amplitudes of the velocity fluctuations required for lock-in.