Liquids laden with positively-buoyant bubbles are commonly found in nature (e.g. natural gas seeps and multiphase plumes due to subsea oil spills) and in industrial processes (mixing tanks and pneumatic breakwaters). In many cases, the bubbles have diameters of a few millimeters and therefore possess turbulent wakes behind them as they rise in water. The dynamic interaction between the bubble wakes and between the wakes and any preexisting liquid turbulence create a complex turbulent field that is not fully understood and is at odd with the classic, forward-energy-cascade picture of turbulence. It is important to clarify this fundamental issue as many turbulence models are based on the classic picture.
To tackle the problem, I started a collaboration with numerical modelers – Bruno Fraga, Michael Dodd and Ronald Chan – in the 2018 Biannual Summer Research Program hosted by the Center of Turbulence Research at Stanford University. During the program, our team has developed two direct numerical simulation (DNS) codes to study homogeneous bubbly flows in a vertical channel using immersed boundary (IMB) method and volume-of-fluid (VOF) method. We seek to answer the following three fundamental questions: (1) do bubbly flows have an inertial subrange? (2) what is the origin of the peculiar -3-spectral slope in the velocity spectra reported in physical experiments? and (3) what is the map of interscale energy transfer in bubbly flows?
This is an on-going project with the final goal to develop the next generation turbulence models for the prediction of bubbly flows using under-resolved Reynolds-averaged Navier Stokes (RANS) and large eddy simulation (LES) approaches.