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This gradient in the velocity profile contributes to the force that a floc experiencing roll-up feels. The sharp slope of the velocity gradient in the tube will cause the particle to experience a much higher velocity on the side closest to the center of the tube. This discrepency between velocities on either side of the particle will cause the floc to being to roll, creating this floc roll-up phenomenon. Flocs actually begin to roll up when the velocity at their edge exposed to the flow exceeds the settling velocity that the floc particle experiences. The velocity experienced by the floc particles at the point at which they begin to roll up the tube rather than settle out is called the critical velocity. The settling velocity of a floc particle is dependent on floc diameter and floc density. Conversely, the critical velocity a floc experiences is dependent on floc diameter and the inner tube diameter.
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