Concentration band dynamics in free-surface Couette flow of a suspension

Abstract

Particle concentration banding has been studied by experiments in the flow driven by the inner cylinder of a partially filled concentric cylinder, or Couette, device. In this geometry, alternating bands of relatively concentrated and dilute particle fraction are observed along the axis. A small ratio of the inner to outer radius was used, with Ri/Ro=0.29, and Ri=0.64 cm, resulting in concentration bands which were confined to the vicinity of the inner cylinder. This work examines the dynamics of band formation and subsequent band motions for a range of filled fractions of the annular region, f, and angles of inclination of the Couette device relative to the horizontal, α. The majority of the experiments were performed at a bulk average particle volume fraction of φ=0.2, although banding was observed over the range of concentrations 0.01⩽φ⩽0.4. The focus of this work is on band dynamics, which have been analyzed by time-lapse video imaging and image analysis. At zero inclination angle, the concentrated bands at f<0.65 are narrow, and fluctuate both in position and in number; the intervening zones are dilute relative to the bands but only slightly less concentrated than the bulk average. At fill fractions above f=0.65 and zero inclination, the number of concentrated bands decreases while the bands become much longer in the axial direction, and the intervening regions are both narrow relative to the concentrated bands and very dilute, φ≈0. This change in banding behavior occurs over a narrow range around f=0.65, which corresponds to suspension just covering the inner cylinder in the absence of flow. For inclination angles in the range 0.4°<α<5.3°, concentrated bands form at regular intervals at the elevated end of the device (shallow depth) and move intact at a nearly constant speed down the cylinder axis.