Discharge of elongated grains from silo with rotating bottom

Abstract

We study the flow of elongated grains (wooden pegs of length L = 20 mm with circular cross section of diameter d(c) = 6 and 8 mm) from a silo with a rotating bottom and a circular orifice of diameter D. In the small orifice range (D/d < 5) clogs are mostly broken by the rotating base, and the flow is intermittent with avalanches and temporary clogs. Here d (3/2d(c)(2)L)(1/3) is the effective grain diameter. Unlike for spherical grains, for rods the flow rate W clearly deviates from the power law dependence W proportional to (D - kd)(2.5) at lower orifice sizes in the intermittent regime, where W is measured in between temporary clogs only. Instead, below about D/d < 3 an exponential dependence W proportional to e(kappa D) is detected. Here k and kappa are constants of order unity. Even more importantly, rotating the silo base leads to a strong-more than 50%-decrease of the flow rate, which otherwise does not depend significantly on the value of omega in the continuous flow regime. In the intermittent regime, W(omega) appears to follow a nonmonotonic trend, although with considerable noise. A simple picture, in terms of the switching from funnel flow to mass flow and the alignment of the pegs due to rotation, is proposed to explain the observed difference between spherical and elongated grains. We also observe shear-induced orientational ordering of the pegs at the bottom such that their long axes in average are oriented at a small angle <theta > approximate to 15 degrees to the motion of the bottom.