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The limitation of traditional measurement technology in gas-liquid two-phase flow measurement is broken through.The liquid velocity field and vortex rope morphology from two vertical directions were measured synchronically by using particle image velocimetry and pulsed shadowgraphy technique. Experimental measurements show that the vortex rope in the draft tube presents three kinds of unstable circulation flow evolution, single spiral, double spiral, and overstocked rupture. When a single spiral vortex rope is partially split into two, it becomes a double spiral vortex rope. When the local spiral rise angle of a single spiral vortex rope decreases, it becomes an overstocked rupture. When there is a single spiral vortex rope in the draft tube, the vortex rope rotates precession around the central axis along with the liquid main flow, and the flow is divided into two parts, the outer main flow zone and the central stagnation zone, according to the axial velocity. The shear layer between the main flow zone and the stagnation zone rolls up to form several vortices. The position of liquid vortices determines the spatial morphology of the spiral vortex rope.

The mechanism of turbulence modulation by bubbles is crucial for understanding and predicting turbulent bubbly flow. In this study, we conducted an experimental investigation of turbulence modulation by bubbles of different sizes in homogeneous isotropic turbulence using two-phase stereo-particle image velocimetry measurement techniques. Two bubble generation methods, electrolysis and porous medium, were employed to generate bubbles in micrometer and millimeter sizes, respectively. The oscillating grid system was utilized to generate homogeneous isotropic turbulence, allowing precise control of turbulent boundary conditions. The ratio of the fluctuating velocities and the comparison between turbulent kinetic energy and average kinetic energy indicated that the generated turbulence was nearly homogeneous and isotropic. With increasing turbulence intensity, micron-sized bubbles transition from suppressing turbulence to enhancing it, while millimeter-sized bubbles exhibit the opposite behavior. Turbulence modulation by millimeter-sized bubbles appears to be nearly isotropic, whereas micrometer-sized bubbles do not exhibit isotropy. [ABSTRACT FROM AUTHOR]

The creation and control of homogeneous isotropic turbulence in laboratory settings is of practical significance. In this paper, we propose a novel design for a turbulence generation apparatus driven by two pairs of oscillating grids, with the object of striking a balance between the stochastics of momentum injection and the flexibility of control of turbulent kinematic energy (TKE). To validate the performance of this system, tomographic particle image velocimetry measurements are conducted. Based on Reynolds decomposition, we characterize the secondary flow pattern and examine the impact of the mean velocity on the TKE transport. Analysis of one- and two-point statistics indicates that the generated turbulence exhibits reasonable approximations of homogeneity and isotropy. The TKE and the dissipation rate are linearly proportional to the grid oscillation frequency, with the Taylor-scale Reynolds number R e λ ranging from 47 to 154, suggesting a convenient control of turbulence with low-to-moderate intensity. [ABSTRACT FROM AUTHOR]