[akbar.mech]

hello everybody ,
I am trying to simulate particle deposition in a 90 bend and I have problem with choosing a proper integration time step. according to fluent 6.3 help:

"The integration time step is computed by Fluent based on a characteristic time that is related to an estimate of the time required for the particle to traverse the current continuous phase control volume. If this estimated transit time is defined as (delta t star) , then fluent chooses a time step (delta t ) as :
(delta t star/landa)"

the continuous phase has been solved steadily and the particles will be tracked unsteadily. what i want to know is that : fluent chooses a time step based on the equation above and on the other hand we determine a time step in the DPM window in the unsteady particle tracking section. i want to know what is the difference between the discretized time step that we determine and the time step that fluent chooses ?
I appreciate your attention .