The Preferred-Mode Coherent Structure in the Near Field of an Axisymmetric Jet With and Without Excitation

Abstract

The ‘preferred-mode’ coherent structure in the near field of an axisymmetric jet is educed for different Reynolds numbers and for the two limiting asymptotic states of the exit boundary layer: laminar and fully- developed turbulent. Distributions of phase-average longitudinal and lateral velocities, coherent vorticity, coherent and incoherent turbulence Reynolds stresses, and coherent strain rate and production are obtained via phase-locked hot-wire measurements over the spatial extent of the coherent structure, for controlled excitation at the jet Strouhal number of 0.3. These properties are found to be independent of the initial condition but show weak dependence on the Reynolds number up to about 10⁵. The naturally-occurring coherent structure in the unexcited jet, educed via an optimized conditional sampling scheme, shows no clear dependence on the jet Reynolds number up to 8 × 10⁵.