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This gradient in the velocity profile contributes to the force that a floc experiencing roll-up feels. Flocs actually begin to roll up when the velocity at their edge exposed to the flow exceeds some critical value the settling velocity that the floc particle experiences. The settling velocity of a floc particle is highly dependent on floc diameter and floc density. Conversely, the critical velocity a floc experiences is dependent on floc diameter and the inner tube diameter.and the capture velocity of the system, among other things.
Theoretical Analysis of the Velocity Gradient
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