DEM study on effect of particle roundness on biaxial shearing of sand

In this study,discrete element method(DEM)simulations of a biaxial test were used to examine the effect of particle roundness on the mechanical behavior of sands at both the macro and micro scales.First,a stack of microcomputed tomography images were binarized,segmented,and labeled using advanced image processing and analysis techniques.Second,a spherical harmonic(SH)analysis,which involves a complete set of orthogonal functions,was implemented to rebuild the natural particle shape.Then,five templates of virtual particles were built in a DEM simulation,four of which were obtained from SH degrees of 3,8,12,and 15,and one template was an elementary sphere.A flexible membrane was numerically generated to allow the material to deform freely under a prescribed confining stress.Finally,the effect of particle roundness on the mechanical properties of granular materials was investigated and discussed.Two shear bands were found to intersect,forming an X shape in both the rotation and displacement fields.Moreover,a lower particle roundness results in higher deviatoric stress and stronger dilation in the volumetric change.A decrease in particle roundness leads to less rotation of particles despite a higher displacement value.In addition,a larger SH degree leads to smaller normalized contact forces of the particles.This implies that decreasing the roundness results in higher anisotropy of the contact forces.

A Case of New Variant COVID-19 First Emerging in South Africa Detected in a Security Guard at the Isolation Point—Shenzhen,China,January 23,2021

On January 22,2021,a 34-year-old male,who served as a security guard at the isolation point in Shenzhen,tested positive for coronavirus disease 2019(COVID-19)during the every-three-day routine test.After the nasopharyngeal swab was further confirmed as positive for COVID-19 by Shenzhen CDC,the patient was transferred to the Third People’s Hospital of Shenzhen and was diagnosed as a COVID-19 asymptomatic infection.On January 25,2021,Shenzhen CDC identified the 20H/501.Y.V2(B.1.351)variant,which was a variant of COVID-19 virus first emerging in the South Africa.