Numerical modeling of fluid-particle interaction in granular media

Fluid-particle interaction underpins important behavior of granular media. Particle-scale simulation may help to provide key microscopic information governing the interaction and offer better understanding of granular media as a whole. This paper presents a coupled computational fluid dynamics and discrete element method （CFD-DEM） approach for this purpose. The granular particle system is modeled by DEM, while the fluid flow is simulated by solving the locally averaged Navier-Stokes equation with CFD. The coupling is considered by exchanging such interaction forces as drag force and buoyancy force between the DEM and CFD. The approach is benchmarked by two classic geomechanics problems for which analytical solutions are available, and is further applied to the prediction of sand heap formation in water through hopper flow. It is demonstrated that the key characteristic of granular materials interacting with pore water can be successfully captured by the proposed method.

ROTATIONAL RESISTANCE AND SHEAR-INDUCED ANISOTROPY IN GRANULAR MEDIA

This paper presents a micromechanical study on the behavior of granular materials under confined shear using a three-dimensional Discrete Element Method （DEM）. We consider rotational resistance among spherical particles in the DEM code as an approximate way to account for the effect of particle shape. Under undrained shear, it is found rotational resistance may help to increase the shear strength of a granular system and to enhance its resistance to liquefaction. The evolution of internal structure and anisotropy in granular systems with different initial conditions depict a clear bimodal character which distinguishes two contact subnetworks. In the presence of rotational resistance, a good correlation is found between an analytical stress-force-fabric relation and the DEM results, in which the normal force anisotropy plays a dominant role. The unique properties of critical state and liquefaction state in relation to granular anisotropy are also explored and discussed.

Density-induced social stress alters oxytocin and vasopressin activities in the brain of a small rodent species

It is known that social stress could alter oxytocin(OT)and arginine-vasopressin(AVP)expression in specific regions of brains which regulate the aggressive behavior of small rodents,but the effects of density-induced social stress are still unknown.Brandt’s voles(Lasiopodomys brandtii)are small herbivores in the grassland of China,but the underlying neurological mechanism of population regulation is still unknown.We tested the effects of housing density of Brandt’s voles on OT/AVP system with physical contact(allowing aggression)and without physical contact(not allowing aggression)under laboratory conditions.Then,we tested the effects of paired-aggression(no density effect)of Brandt’s voles on OT/AVP system under laboratory conditions.We hypothesized that high density would increase aggression among animals which would then increase AVP but reduce OT in brains of animals.Our results showed that high housing density induced more aggressive behavior.We found high-densityinduced social stress(with or without physical contact)and direct aggression significantly increased expression of mRNA and protein of AVP and its receptor,but decreased expression of mRNA and protein of OT and its receptor in specific brain regions of voles.The results suggest that density-dependent change of OT/AVP systems may play a significant role in the population regulation of small rodents by altering density-dependent aggressive behavior.