Investigation of the Characteristics of an Axisymmetric Jet Subjected to Azimuthal Forcing

Abstract

Characteristics of an axisymmetric jet subjected to azimuthal forcing were investigated. A circular variable diameter iris was positioned near the nozzle exit to impart azimuthal excitation to the jet at desired Strouhal numbers. The experiments were conducted at the Reynolds number of 5,500. Two excitation frequencies corresponding to the Strouhal number of 0.12, and 0.09 were used. It was found that both excitation frequencies attenuated the large scale structures in the near-field of the jet, however, the harmonic forcing introduced low frequency instabilities within the first two diameters of the nozzle exit plane. The harmonic forcing also introduced more energy into the flow than the sub-harmonic forcing and was evident from the distribution of the turbulent kinetic energy, power spectral density, and visualization of the instabilities. The forcing also had an effect on the streamwise vortex filaments located between successive large-scale structures of the jet. The diameter of the streamwise vortex filaments and meandering increased for both forcing cases. The filaments played a pivotal role in the entrainment of ambient fluid and the distortion of the vortices due to additional strain.