| 摘要(英) |
This study aims to measure and analyze important spatiotemporal characteristics of featureless turbulence of the well-known Taylor-Couette (TC) flow using high-speed, high-resolution digital particle image velocimetry (DPIV) and wavelet analyses. Featureless turbulence is generated by two concentric counter-rotating, at some specific rotating speeds, inner and outer long cylinders. Andereck et al. (1986) via liquid flow visualization had first noticed that no dominant large structures being larger than the annulus gap width can be observed, so they named it featureless turbulence. Ronney et al. (1995) showed that this liquid flow is nearly homogeneous across the annulus with the negligible mean velocity by laser Doppler velocimetry (LDV). DPIV measurement of gaseous featureless turbulence was first conducted here, and the radial, axial and azimuthal velocity data of this flow were acquired. By decomposing the obtained spatiotemporal velocity data by wavelet transform, the subsequent scale, energy spectrum and intermittency analyses were obtained for the analyses. Focuses are placed on the following three goals. (1) By what means can featureless turbulence be recognized? (2) What are the characteristic spatiotemporal scales relevant to energy production and dissipation of featureless turbulence? (3) Does the preferential sweeping exist for the particle settling? We found that every velocity component is of a near-Gaussian distribution with the negligible mean velocity comparative to its turbulence intensity everywhere in the flow, so that this flow is near homogeneous, consistent with the previous result (Ronney et al. 1995). Energy spectra showed a -5/3 decaying slope in the inertial subrange, proving the essential factor of fully-developed turbulence. Intermittency level was found to increase as the eddy scale decreases, validating existence of the violent small scale motion. The highest intermittency showed up at the same order of magnitude of the estimated smallest scale. In two-phase study, the settling phenomenon of heavy glass particles was observed. The glass particles that were subject to a linear drag force (Rep = VtDp/? < 1, where Vt, Dp and ? are the terminal velocity and the diameter of the heavy particle, and the kinematic viscosity of the fluid), St ≈ 6 or 13 (St = ?p/?K, where ?p and ?K are respectively the particle relaxation time and the Kolmogorov time scale), were found to response well to vortex structures in featureless turbulence, and showed a preferential sweeping along down flow side of the periphery of the strong vortex structure. An apparent change in motion of the particle due to the strong vortex pairing was observed, and in some cases the particle appeared in horizontal movement, but others the particle settling rate could be twice as the original value, about half of the turbulence intensity in raise. |
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