|dc.description.abstract||This research is to develop a four-dimensional particle image velocimetry (PIV) technique, to measure quantitatively full spatio and temporal velocity fields in turbulence, and thus to investigate the canonical structures of fine scale turbulence. The four-dimensional PIV technique is consisted of a high-speed successive scanning laser sheet, a pair of synchronized high-speed stereo CCD cameras, a fast image processing system, and a home-built synchronizer for these components. Thus, four-dimensional velocity fields in a turbulent flow can be extracted from these full spatial images with temporal variations, each image field of view 1.4 cm×1.2 cm (480×420 pixels) in which the spatial resolution is about 29μm that may be sufficient to resolve the Kolmogorov scale of turbulence.
Applying this full spatiotemporal PIV technique to a zero-mean-shear turbulence that was generated by vertically-oscillated grids in a water tank, the corresponding velocity fields and thus fine scale structures of the turbulent flow can be obtained. Two cases are studied, one grid turbulence and two grids turbulence, the former having a decaying near-homogeneous turbulence while the latter creating a stationary near-isotropic turbulence within the core region between the two grids, as verified by previous LDV measurements (Shy et al. 1997). From these four-dimensional velocity field measurements, the associated vorticity fields, principle strain rate directions, and kinetic energy dissipation rate fields can be obtained. Following that fine scale structures of turbulence may be identified from these kinetic energy dissipation rate fields.
Both results of one-grid/two-grids turbulence reveal that fine scale structures are correlated with flow velocity gradients; the larger the velocity gradient, the higher kinetic energy dissipation rate that marks the fine scale structure of turbulence. It is found that the distribution of fine scale structures where highest values of the dissipation rate are concentrated has a very high degree of intermittency and only occurs about 2% in the whole measuring data volume at any given times. For one-grid/two-grids turbulent flows, three canonical fine structures coexist, including “line-like”, “sheet-like”, and “blob-like” structures. This finding is similar to the descriptions of turbulent vorticity distributions proposed by Burger (1948) and Townsend (1951) and is also consistent with measurements of scalar dissipation rates by Shy et al. (1999) using the same apparatus and three-dimensional laser-induced fluorescence technique, but it differs drastically that found by Buch & Dahm (1991) in a free-shear turbulent jet flow in which they concluded that the only universal canonical structure is the “sheet-like” structure. This discrepancy indicates that zero-mean-shear and free-shear turbulent flows are different.
For the present one-grid/two-grids turbulent flows, the average diameter of the “line-like” structure is about 1~2 Kolmogorov scale. We estimate all fine scale structures existing in both one-grid/two-grids turbulent flows, and it is found that the “line-like” structure (its length is at least three times greater than its average diameter) has 20%/25%, the “sheet-like” structure (its length and width are about the same but its thickness is very small) has 12%/8%, and the “blob-like” structure (its length, width and thickness are about the same) has 30%/30%, respectively. The remaining fine scale structures that are difficult to define and in between the “line-like” and “sheet-like” structures have about 38% for both one-grid/two-grids turbulent flows. These results are useful to validate existing turbulent models and the results of direct numerical simulations as well as for developing a new turbulent model.||en_US|