Characteristics of oscillating-grid flow
Abstract
The turbulent flow generated by a vertically oscillating grid in a still water tank with a square cross-section has been studied. Theoretical models of oscillating-grid flows were reviewed within the context of grid-generated turbulence in general to examine their basic assumptions. The work was however mainly experimental in focus, based on measurements of turbulence characteristics using planar particle image velocimetry. For three strokes of 16, 26 and 41mm, variation of flow characteristics with distance from the oscillating grid, i.e., cross-sectionally averaged statistics obtained on a basis of the assumption of horizontal homogeneity, were explored to examine decay of turbulence intensities, appropriate scaling of turbulence characteristics, and the effects of stroke regarded as an initial condition. The decay exponent of the turbulence intensity measured in the present study was found to be larger than a widely accepted model and decreased with increasing stroke. The assumption of horizontal homogeneities of the oscillating-grid turbulence was investigated, and then inhomogeneities were characterized in terms of point-averaged statistics, i.e., time averaged at a point. The horizontal homogeneities of turbulence intensities for all three strokes were found to be much less than those of the fixed-grid turbulence so that the assumption of horizontal homogeneity might not be justified. For the cases of strokes of 16 and 26 mm, the flow characteristics were found to be similar to each other and also to share some features of simple turbulence shear flows of jets or wakes. For the case of the stroke of 41mm, the flow characteristics were observed to be different from those of other smaller strokes and the horizontal inhomogeneities were much higher than those of the other smaller strokes. The advection and shear production of turbulent kinetic energy due to inhomogeneities might change the decay behavior. The near field of the oscillating-grid flow was also investigated using conventional time- and phase-averaged statistics, the latter appropriate for studying unsteady flow features. The near-field was found to be mixture of jet-like (under a grid crossbar) and wake-like (under a single grid bar) flows. The flow characteristics under the grid-crossbar exhibited more inhomogeneous than under the single bar. Although several interesting flow features were identified, a direct link to inhomogeneities in the far-field was not evident.
Degree
Ph.D.
Advisors
Lyn, Purdue University.
Subject Area
Civil engineering|Fluid dynamics|Gases
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