COHERENT STRUCTURES IN THE SIMILARITY REGION OF A TWO-DIMENSIONAL TURBULENT JET: A VORTEX STREET
Abstract
The characteristics of the coherent structures of a plane jet were evaluated experimentally and with a kinematic numerical representation. Distributions of intermittency and interface crossing frequency were measured for 10 (LESSTHEQ) x/D (LESSTHEQ) 60. These properties, including the maximum interface crossing frequency, were found to scale as self-preserving flow variables. Spatially coherent patterns within the turbulent velocity field were evaluated utilizing two point velocity correlation measurements. From these, it was found that the instantaneous velocity fluctuation distributions are antisymmetric with respect to the centerline and coherent over the entire width of the flow field. In addition, the patterns are periodic in the streamwise direction. The two-dimensionality of the large structures was evaluated through the comparison of longitudinal and vertical integral length scales derived from correlations in both the fully turbulent zone and the intermittent region. In the intermittent region, the integral scales were approximately equal while in the fully turbulent zone, the longitudinal scale was approximately double the vertical one. Utilizing a combination of flow visualization, structural passage frequency measurement and similarity considerations, the convective velocity of the vortex-like structures was estimated to be one half the mean centerline velocity. From the experimental results, a hypothetical vortex street representation of the plane jet was formulated and evaluated numerically. This kinematic representation was found to be capable of generating realistic distributions of mean longitudinal and lateral velocity as well as the proper spreading and velocity decay rates. Although the correct magnitude of the Reynolds stress was calculated, its sign was in error. The small scales of turbulence were not included in the model and consequently, it was not possible to predict reasonable distributions of the fluctuation intensities, the energy spectra or interface properties.
Degree
Ph.D.
Subject Area
Fluid dynamics|Gases
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