Scentific Research

The research group of Zhou Gongdan, Researcher of the China-Pakistan Joint Research Center on Earth Sciences, has made important progress on the research of particle segregation and solid-liquid phase flow dynamics of debris flow. Relevant findings were recently published in the internationally renowned journals of geology and fluid dynamics: Journal of Geophysical Research: Solid earth, Physics of fluids, Physical Review Fluids.

Debris flow is a multi-phase fluid composed of a large number of solid particles, a large amount of water medium and a small amount of gas; driven by gravity, it can flow rapidly in a valley or on a slope, and its flow state is often complex and variable. Most debris flows have the characteristics of high speed and long runout. Different from general sediment-laden flow and mud flow, debris flow can carry solid particles with broad gradation. The segregation of solid particles composed of a large amount of sediments and rocks profoundly affects the fluidity of debris flow and the impact and damage of debris flow to structures. Therefore, the mechanism and effect of particle segregation is listed as the frontier research topic of debris flow dynamics.

Although we have made great progress in the segregation mechanism of dry granular flow, since the particle mediums in debris flow has a more complex gradation and the particle mediums often move in the presence of non-Newtonian fluids, the existing segregation theory of dry granular flow cannot explain the phenomenon of particle segregation in debris flow properly. There is also little research on the effect of particle segregation on the flow pattern and fluidity of debris flow. To deal with this problem, Zhou Gongdan's research group has carried out a large number of physical model experiments and coupled fluid-particle numerical simulations to systematically analyze the effects of different fluid densities and viscosities on particle segregation of solid-liquid phase flow (solid phase refers to dense double particle size granular flow). The research revealed the critical conditions for particle segregation in debris flow, and that particle segregation was weakened by reducing the contact stress gradients and formation of plug zones in the particle flow based on micromechanics. A generalized visco-inertial constitutive model for solid-liquid phase flow is also established, which can characterize the particle segregation effect in granular-fluid mixtures. The research group also proposed particle segregation rate and particle diffusion rate equations (with Stokes number and Inertial number as the dominant functions) considering the effect of fluid, and finally established a new solid-liquid phase continuum dynamic model which could take viscous effects of granular-fluid mixtures (particle segregation rate and diffusion rate) into consideration.

The research has provided an important dynamic model for defining the particle segregation mechanism of debris flow and the dynamic characteristics of solid-liquid phase flow, and accurately judging the movement path and influence range of debris flow.

Theses published (*corresponding authors):

Gordon G. D. Zhou, Kahlil Fredrick E. Cui*, L. Jing, D. Song, T. Zhao, Yu Huang (2020). Particle size segregation in granular mass flows with different ambient fluids. Journal of Geophysical Research: Solid earth. 125(10), e2020JB019536. https://doi.org/10.1029/2020JB019536

Kahlil Fredrick E. Cui, Gordon G. D. Zhou*, L. Jing (2021). Viscous effects on the particle size segregation in geophysical mass flows: insights from immersed granular shear flow simulations. Journal of Geophysical Research: Solid earth. 126(8), e2021JB022274.

Kahlil Fredrick E. Cui, Gordon G. D. Zhou*, L. Jing, X. Q. Chen, D. Song (2020). Generalized friction and dilatancy laws for immersed granular flows containing large and small particles. Physics of fluids, 32, 113312, doi: 10.1063/5.0024762

Kahlil Fredrick E. Cui, Gordon G. D. Zhou*, L. Jing (2022). Diffusion and segregation in immersed granular shear flows. Physical Review Fluids. 7(1): 004300. DOI: https://doi.org/10.1103/PhysRevFluids.7.014305