• Corpus ID: 252519379

Multiphase lattice Boltzmann modeling of cyclic water retention behavior in unsaturated sand based on X-ray Computed Tomography

@inproceedings{Wang2022MultiphaseLB,
  title={Multiphase lattice Boltzmann modeling of cyclic water retention behavior in unsaturated sand based on X-ray Computed Tomography},
  author={Qiuyu Wang and Marius Milatz and Reihaneh Hosseini and Krishna Kumar},
  year={2022}
}

• Qiuyu Wang, M. Milatz, +1 author K. Kumar
• Published 23 September 2022
• Geology

The water retention curve (WRC) defines the relationship between matric suction and saturation and is a key function for determining the hydro-mechanical behavior of unsaturated soils. We investigate possible microscopic origins of the water retention behavior of granular soils using both Computed Tomography (CT) experiment and multiphase lattice Boltzmann Method (LBM). We conduct a CT experiment on Hamburg sand to obtain its WRC and then run LBM simulations based on the CT grain skeleton. The multiphase LBM simulations capture the hysteresis and pore-scale behaviors of WRC observed in the CT experiment. Using LBM, we observe that the spatial distribution and morphology of gas clusters varies between drainage and imbibition paths and is the underlying source of the hysteresis. During drainage, gas clusters congregate at the grain surface; the local suction increases when gas clusters enter through small pore openings and decreases when gas clusters enter through large pore openings. Whereas, during imbibition, gas clusters disperse in the liquid; the local suction decreases uniformly. Large pores empty first during drainage and small pores fill first during imbibition. The pore-based WRC shows that an increase in pore size causes a decrease in suction during drainage and imbibition, and an increase in hysteresis. measured using an in situ Computed Tomography (CT) experiment as described in (Milatz, 2020). The WRC shows a hysteretic behavior, showing higher suction during drainage than imbibition for the same degree of saturation. At the microscopic scale, air entrapment (Mualem, 1974; Poulovassilis, 1970), the ink-bottle effect (Ravikovitch and Neimark, 2002), contact angle hysteresis ( i. e. , the difference in solid-liquid phase distribution (liquid and of gas compare examining and of and We finally present the dependence of water retention behaviors on local pore size. State (EOS) is required to define the dependence of pressure on density. We employ the Carnahan-Starling (C-S) EOS similar to Hosseini et al (Hosseini et al., 2022) and obtain the same coexistence curves at different temperatures T . The threshold densities defines the boundary that separates liquid ( l ) densities above and gas ( g ) densities below. We use a ρ l = 0 . 2725 mu · lu − 3 and ρ g = 0 . 0484 mu · lu − 3 in the multiphase simulation to identify liquid nodes and gas nodes, where mu and lu are the mass and length units in lattice Boltzmann methods. The values of ρ l and ρ g are selected at which first derivatives of the EOS are zero. Fluid densities between ρ l and ρ g define a transition zone. The solid ( s ) density ρ s , also known as wall density, is set to ρ s = 0 . 35 mu · lu − 3 , corresponding to a constant contact angle of 5 ◦ , which means nearly perfect wetting. We reveal a pore size dependent hysteretic behavior of WRC. Larger pores are emptied sooner but filled later than the smaller pores. Larger the pores show a lower the suction as gas clusters inside large pores can sustain a larger radius of curvature. Furthermore, large pores exhibit greater hysteresis in local suction than small pores as their pore suction keeps decreasing after small pores are fully saturated during imbibition.