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### Browsing by Author "Guo, Junwei"

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- ItemOpen AccessAge of Soils: A Measure of Creep History(2017) Guo, Junwei; Wong, Ron Chik-Kwong; Wan, Richard; Priest, Jeffrey; Sudak, Les
Show more Time-dependent behaviors of soils are critical for engineering design. Results of one-dimensional (1-D) constant rate of strain (CRS) tests on clays show that there is an existence of unique relationship between current stress and strain state for a given constant strain rate, irrespective to previous stress-strain-time history. In the present study, this relationship is employed to estimate the creep rate during the CRS test. It is found that the creep rate is consistently related to distance from current stress-strain state to the instant compression line, which is the creep void ratio or creep history, termed age of soils. Based on the creep rate as a function of creep strain or age of soils, the stress relaxation rate function is derived through the correspondence principle. Age contours are iso-creep rate lines defining the creep rate field in stress-strain space. Creep Balanced State equation states that CRS path will converge to a iso-creep rate line. This equation is used to determine the CRS path and quantify the rate effect on preconsolidation pressure. Age- and pressure-dependent secondary compression coefficients are incorporated in the above framework.Show more - ItemOpen AccessDirect simulation of stably stratified wall-bounded turbulence using the lattice Boltzmann method(2023-04-27) Guo, Junwei; Zhou, Qi; Wong, Ron Chik-Kwong
Show more The lattice Boltzmann method (LBM) is employed to simulate stratified plane Couette (SPC) flows in their statistically stationary turbulent state. The aim is to assess the suitability of the LBM for direct simulation of wall-bounded, sheared turbulence under the influence of stable stratification. The SPC flow is generated by two parallel plates moving in opposite directions with velocities ± U w, and the buoyancy is fixed at ± b w at the upper and lower plates, respectively. The Reynolds number Re = U w h / ν, where h is the half-gap height, and ν is the kinematic viscosity, varies from 1000 to 3000. The Richardson number Ri = b w h / U w 2 is set to 0 or 0.01. The LBM results are compared to direct numerical simulations using the conventional pseudo-spectral method, and good agreement is found in various turbulence statistics, such as mean and fluctuation velocity and buoyancy, Reynolds stress, turbulent heat flux, dissipation rate, wall fluxes of momentum and heat, and longitudinal and transverse turbulence spectra. The results from grid-sensitivity tests indicate that the uniform isotropic grid spacing Δ x in LBM needs to be no greater than approximately the near-wall viscous length scale δ ν to achieve adequate resolution of stratified wall-bounded turbulence.Show more - ItemOpen AccessDirect Simulations of Fluid-Particle Flow in Newtonian and non-Newtonian Fluids Using Coupled Lattice Boltzmann and Discrete Element Methods(2021-09) Guo, Junwei; Wong, Ron Chik-Kwong; Zhou, Qi; Jasso, Martin; Wan, Richard G
Show more Coupled lattice Boltzmann and discrete element methods are employed to investigate a suite of fluid-particle flow problems in both Newtonian and non-Newtonian fluids. First, the rheological properties of finer particle suspensions in a Newtonian fluid are investigated, as the particle shape and solid fraction vary. An increase of the relative viscosity is observed when the particles become more oblate, accompanied by an increase of particle contacts and contact distance. Particle reorientation is seen to occur systematically in denser oblate particle suspension subject to the shear flow. A connection between the micro-structure statistics and the suspension viscosity is proposed. Second, the shear rate effects on the finer oblate particle suspension viscosity are studied. The viscosity of the suspension is observed to decrease under a higher shearing rate due to the reduction of the inter-particle friction coefficient, resulting in the shear-thinning behavior of the suspension. Finally, the sedimentation of a granular particle cloud in shear-thinning suspensions is investigated, as the rheology of the suspension, Reynolds number, and particle cloud concentration vary. At higher Reynolds numbers, the particle cloud length grows in the direction of settling and reaches a quasi-steady state. The ratio of the particle cloud quasi-steady settling velocity to the single-particle terminal velocity in the same fluid, increases when the cloud settles in the shear-thinning suspensions. This velocity increase is more significant at a low Reynolds number. At even lower Reynolds numbers, the cloud loses its initial shape and disintegrates while settling, with particles escaping from the cloud due to differential particle settling velocities.Show more - ItemOpen AccessEffects of volume fraction and particle shape on the rheological properties of oblate spheroid suspensions(AIP Publishing, 2021-08-06) Guo, Junwei; Zhou, Qi; Wong, Ron Chik-Kwong
Show more Coupled lattice Boltzmann and discrete element methods were employed to investigate the rheological properties of oblate spheroid suspensions in a Newtonian fluid. The volume fraction of the particles is varied along with the particle aspect ratio. As the particle shape is varied from sphere to oblate, we observe an increase in the relative viscosity as well as an increase in the particle contacts and the contact distance. The more oblate particles in denser suspensions are observed to reorient systematically subject to the shear flow. We recast the viscosity data using the Krieger–Dougherty formula and report the modified Einstein coefficients.Show more - ItemOpen AccessEvolution of Rayleigh-Taylor instability at the interface between a granular suspension and a clear fluid(American Institute of Physics, 2022-06-19) Guo, Junwei; Zhou, Qi; Wong, Ron C.-K.
Show more We report the characteristics of Rayleigh-Taylor instabilities (RTI) occurring at the interface between a suspension of granular particles and a clear fluid. The time evolution of these instabilities is studied numerically using coupled lattice Boltzmann and discrete element methods with a focus on the overall growth rate (σ) of the instabilities and their average wave number (k). Special attention is paid to the effects of two parameters, the solid fraction (0.10{less than or equal to}φ{less than or equal to}0.40) of the granular suspension and the solid-to-fluid density ratio (1.5{less than or equal to}R{less than or equal to}2.7). Perturbations at the interface are observed to undergo a period of linear growth, the duration of which decreases with φ and scales with the particle shear time d/w∞, where d is the particle diameter and w∞ is the terminal velocity. For φ>0.10, the transition from linear to nonlinear growth occurs when the characteristic steepness of the perturbations is around 29%. At this transition, the average wave number is approximately 0.67d-1 for φ>0.10 and appears independent of R. For a given φ, the growth rate is found to be inversely proportional to the particle shear time, i.e., σ ∝(d/w∞)-1; at a given R, σ increases monotonically with φ, largely consistent with a linear stability analysis (LSA) in which the granular suspension is approximated as a continuum. These results reveal the relevance of the time scale d/w∞ to the evolution of interfacial granular RTI, highlight the various effects of φ and R on these instabilities, and demonstrate modest applicability of the continuum-based LSA for the particle-laden problem.Show more - ItemOpen AccessNumerical investigation of particle cloud sedimentation in power-law shear-thinning fluids for moderate Reynolds number(Elsevier, 2021-09-03) Guo, Junwei; Zhou, Qi; Wong, Ron Chik-Kwong
Show more A series of numerical simulations are performed for the sedimentation process of a particle cloud in shear-thinning fluids using lattice Boltzmann and discrete element methods. The initial particle concentration, , and the power-law index of the fluid, n, and Reynolds number, , are varied in these simulations. For , the particle cloud size grows in the longitudinal direction as the cloud settles, leading to reduced particle concentration and a quasi-steady settling velocity, . The velocity ratio, , where is the corresponding single-particle terminal velocity, is found to decrease with both n and . This velocity ratio is only weakly dependent on the initial concentration () due to particle dispersion. For , the cloud loses its initial shape and disintegrates while settling, with particles escaping from the cloud due to differential particle settling velocities.Show more