Vortex pairing in a circular jet under controlled excitation. Part 2. Coherent structure dynamics

摘要: The coherent structure dynamics in the near field of a circular jet has been experimentally explored by inducing ‘stable’ vortex pairing through controlled excitation (see Zaman & Hussain 1980) and applying phase-averaging techniques. Hot-wire measurements were made 7·62 cm air with laminar exit boundary layer at Reynolds number ReD = 3·2 × 104, excited Strouhal StD 0·85. At particular phase during process, spatial distributions phase-average longitudinal lateral velocity perturbations (〈u)〉, 〈v〉), vorticity, streamlines, background stresses turbulence intensities have educed. These data obtained for four different locations occupied vortices same (preceding, during, following event), region 0 < x/D 5. Spatial these measures successive phases process are also educed an attempt to further understand vortex-pairing dynamics. flow physics is discussed on basis over physical extent vortical structures, phase-locked specific event thus do not involve use Taylor hypothesis.The computed pseudostream functions compared corresponding streamlines drawn method isoclines. Transition examined vorticity diffusion, superimposed random fluctuation stress circumferential correlation measurements. peak drops rapidly owing transition interaction but, farther downstream, decay can be attributed destruction stress, especially latter's ‘saddle points’. Controlled enhances initial coherence but ineffective delaying turbulent breakdown end potential core; appears occur evolution lobe structures. found much larger than [lsim ] 3, two comparable core. zone average cross-section merging pair that single either before or after completion pairing. During such correlations largest early while entrainment, as well rapid diffusion later phase. regions alternate positive negative associated structures their interactions help explain ‘negative production’.