Particle motion in stratified turbulent flows
Chemical and Thermal Process Engineering
Final Report Abstract
In this project 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 swarms rising in water. The experiment takes advantage of a recently developed PSV technique for particle/bubble flows and provides the first comprehensive data set for computing multipoint measurements of flows laden with finite-sized bubbles. The results show that the level of anisotropy in the flow produced by the rising bubbles is strong in general, and not negligible at any scale in the flow. Moreover, the results show that (i) the differing behavior of the second-order longitudinal and transverse structure functions when measured for separations in different directions shows that both velocity components and separation directions of the 2 dimensional data need to be considered in order to fully characterize the anisotropy of the flow; (ii) the bubble size and void fraction are both important parameters determining the amount of anisotropy in the flow; (iii) higher-order structure functions reveal greater anisotropy across the scales of the flow, such that extreme fluctuations in the flow are the most anisotropic. Since the PSV data captures two-dimensional data, we were able to consider the energy transfer between scales for two separation directions in the flow. The results revealed a downscale energy transfer on average for horizontal separations, but an upscale energy transfer on average in the vertical direction. However, the horizontal energy transfer was much stronger than that in the vertical direction. We also investigated extreme events in the flow by considering the normalized probability density functions of the velocity increments in the flow. The results showed that the probability of extreme fluctuations increases with decreasing scale, just as in single-phase turbulence. However, the results also showed that for a given scale the probability of extreme events decreased with increasing Reynolds number, contrary to what occurs in single-phase turbulence. To explore the origin of these extreme fluctuations in the bubble-laden flows, we visualized regions of extreme small-scale velocity increments in the FOV and observed that they are typically located at the boundary of the wakes produced by the bubbles. For the cases with smaller bubbles and lower void fractions, vast regions outside of the bubbles wakes exhibit weak fluctuations, and so this combined with the extreme fluctuations at the bubble wake boundaries leads to strong intermittency. For larger bubbles which produce larger flow Reynolds numbers, and with larger void fractions, the wake regions become less rare in the flow and hence the flow is less intermittent than the former case, even though the Reynolds number is higher. Furthermore, the extreme values were also observed to reach larger values (compared to the standard deviation) for the smaller bubbles, which again causes the smaller bubble cases to exhibit greater intermittency than the larger bubble cases, in addition to the effect arising from the fraction of the flow modified by the bubble wakes.
Publications
- (2020). Explicit algebraic relation for calculating Reynolds normal stresses in flows dominated by bubble-induced turbulence. Phys. Rev. Fluids, 5(8), 084305
Ma, T., Lucas, D. & Bragg, D. A.
(See online at https://doi.org/10.1103/PhysRevFluids.5.084305) - (2021). Assessment of the validity of a loglaw for wall-bounded turbulent bubbly flows. Int. J. Heat Fluid Flow, 91, 108857
Bragg, A. D., Liao, Y., Fröhlich, J., & Ma, T.
(See online at https://doi.org/10.1016/j.ijheatfluidflow.2021.108857) - (2021). Scale-dependent anisotropy, energy transfer and intermittency in bubble-laden turbulent flows. J. Fluid Mech., 927, A16
Ma, T., Ott, B., Fröhlich, J., & Bragg, D. A.
(See online at https://doi.org/10.1017/jfm.2021.760) - (2022). An experimental study on the multiscale properties of turbulence in bubble-laden flows, J. Fluid Mech., 936, A42
Ma, T., Hessenkemper, H., Lucas, D., & Bragg, D. A.
(See online at https://doi.org/10.1017/jfm.2022.86) - (2022). Study on bubble-induced turbulence in pipes and containers with Reynolds-stress models. Experimental and Computational Multiphase Flow, 1-12
Liao, Y., & Ma, T.
(See online at https://doi.org/10.1007/s42757-021-0128-0)
