Figure 6. Schematic illustrating the direction of forces acting on particles at typical positions in a rectangular cross-section spiral channel.

Particle (a) is at the minimum lift force line, which is a stable balance position. Particle (b) and (c) have a small offset from the minimum lift force line where the lift force will drive them to the equilibrium point. Although the lift and drag forces on particle (d) are in opposite directions at the center line of the depth and could potentially lead to force balancing, whose position is not a stable equilibrium point. And a small offset from the center line as illustrated by particle (e) would result in a significant deviation of the particle from the center line by the lift force. For particles at outer half of the channel, the two forces will always be acting in the same direction, as illustrated by Particle (f) and (g), and will prevent the force balancing or equilibration. White cones indicate the direction and relative magnitude of simulated Dean velocity as well as Dean drag.