|dc.description.abstract||This thesis focuses on the topic of laboratory isotropic turbulence and its applications to two-phase flows. The aim of this study is to provide a comprehensive and systematic analysis of what and how the spatio-temporal relationships between turbulence and two-phase flows. Experiments were conducted in an aqueous vibrating-grids-turbulence (VGT) and a gas phase near-isotropic turbulence to generate low to moderate Reynolds number turbulence with zero-mean velocity, where the Reynolds number, Rel, based on the Taylor microscale λ can be varied from 20 to 200 for the present study. In an effort to ascertain both the spatial and temporal properties of turbulence that are related to the large hierarchy of scales involved, a high-resolution, high-speed digital particle image velocimetry (DPIV) technique, together with the wavelet analyses, is developed so that the detailed three-dimensional spatio-temporal (x-y-t) measurements of the kinematic aspects of small-scale turbulent structures can be obtained simultaneously without any assumption and hypothesis.
The first objective of the present work is to identify the turbulent spatiotemporal intermittency in the dissipation and inertial ranges. We found that the characteristic spatial and temporal intermittent scales of intense vorticity structures in the dissipation range occur around 10 h and tk, where h and tk are the Kolmogorov length and time scales, respectively. These results are useful for further study of particle settling and/or premixed flames interacting with stationary near-isotropic turbulence. On the other hand, using the extended self-similarity (ESS) scaling, we found that in the inertial range the spatiotemporal scaling exponents for both high-order longitudinal and transverse velocity structure functions are identical. The second objective is to investigate the solid particle settling behaviours in a stationary homogeneous isotropic turbulence. The mean settling velocity, Vs, of solid particles was measured using both particle tracking velocimetry (PTV) in the VGT system and DPIV in the gaseous system. For the VGT system, we found that Vs > Vt and (Vs - Vt)/Vt reaches its maximum of about 7 % around St » 1, even when the particle Reynolds number Rep is as large as 25 at which Vt/vk » 10, where the Stokes number St = tp/tk, tp is the particle’s relaxation time, vk is the Kolmogorov velocity scale, and Vt is the particle’s terminal velocity. On the other hand, for the gaseous system the mean settling velocity reaches its maximum of 0.13 u¢ when St 1.0 and Vt/u¢ 0.5 for Rep ≤ 1 and Reλ = 120. In addition, non-uniform particle concentration fields (preferential accumulation) are observed and most significant when St 1.0. It is also found that the particle preferential accumulation is highly related to the scaling of the small intense vorticity structures; the particle cluster thickness is nearly the spacing between the small intense vorticity structures, and the time passage of the clustered particle is tk. By comparing the average wavelet spectra between unladen (neutral particle) and laden (heavy particle) turbulent flows at a fixed Rel, turbulence augmentation of energy spectra in the gravitational direction is found over the entire frequency domain, whereas in the transverse direction augmentation occurs only at higher frequencies beyond the Taylor microscale. Finally, a simple physical model based on the phenomenology of the energy balance concept for turbulence generation and dissipation by the descending heavy particles is proposed to predict the value of ul’/u’, where ul’ and u’ are the r.m.s. fluctuating velocity of laden and unladen turbulent flows, respectively. The predicted values agree reasonably well with the experimental results. This suggests that the assumptions based on the scaling of the small intense vorticity structures and the slip velocity at which the maximum probability distribution occurs are adequate, especially when St 1.0 for the present study.||en_US|