Abstract
The properties of bubble-laden turbulent flows at different scales are investigated experimentally, focusing on the flow kinetic energy, energy transfer and extreme events. The experiments employed particle shadow velocimetry measurements to measure the flow in a column generated by a homogeneous bubble swarm rising in water, for two different bubble diameters ($2.7$mm and$3.9$mm) and moderate gas volume fractions ($0.26\,\%\sim 1.31\,\%$). The two velocity components were measured at high resolution, and used to construct structure functions up to twelfth order for separations spanning the small to large scales in the flow. Concerning the flow anisotropy, the velocity structure functions are found to differ for separations in the vertical and horizontal directions of the flow, and the cases with smaller bubbles are the most anisotropic, with a dependence on void fraction. The degree of anisotropy is shown to increase as the order of the structure functions is increased, showing that extreme events in the flow are the most anisotropic. Our results show that the average energy transfer with the horizontal velocity component is downscale, just as for the three-dimensional single-phase turbulence. However, the energy transfer associated with the vertical component of the fluid velocity is upscale. The probability density functions of the velocity increments reveal that extreme values become more probable with decreasing Reynolds number, the opposite of the behaviour in single-phase turbulence. We visualize those extreme events and find that regions of intense small-scale velocity increments occur near the turbulent/non-turbulent interface at the boundary of the bubble wake.
Highlights
While great attention has been given to investigating flows containing suspensions of small inertial, heavy particles (Maxey 1987; Bec et al 2006, 2007; Fox 2014; Gustavsson & Mehlig 2016; Ireland, Bragg & Collins 2016a,b; Hogendoorn & Poelma 2018; Dou et al 2018a,b; Petersen, Baker & Coletti 2019; Tom & Bragg 2019; Berk & Coletti 2021), there has been less of a focus on bubbly turbulent flows, partly due to the increased complexity associated with performing experiments or simulations for such flows (Lohse 2018)
Surprisingly, while non-Gaussianity of the probability density functions (PDFs) of the velocity increments becomes stronger as Reynolds number is increased in single-phase turbulence (Frisch 1995), the opposite occurs for the bubble-laden turbulent flow considered here
In this paper we have presented an analysis of the multiscale properties of a bubble-laden turbulent flow, based on experimental data of a flow in a vertical column with bubble
Summary
Particle-laden flows are a central topic in fluid mechanics and omnipresent in nature and technology (Prosperetti & Tryggvason 2009; Balachandar & Eaton 2010). An experimental study on the multiscale properties number, and that this increase was more pronounced at the small scales than the large scales They considered the PDFs of the velocity increments and used extended self-similarity (Benzi et al 1993) to show that the flow intermittency is enhanced by the bubbles. An issue with our previous study (Ma et al 2021), is that the conclusion was drawn based on a one-dimensional dataset, which only allowed us to construct the velocity increments for separations in the spanwise direction of the bubble-laden turbulent channel flow Another important point to be quantified is how the bubbles influence the anisotropy of the turbulent flow across the scales.
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