You will use two parameters to control the time integration of the particle trajectory equations:
Each of these parameters is set in the Discrete Phase Model panel (Figure 22.11.2) under Tracking Parameters in the Tracking tab.
Define Models Discrete Phase...
where is the Length Scale that you define. As defined by Equation 22.11-1, is proportional to the integration time step and is equivalent to the distance that the particle will travel before its motion equations are solved again and its trajectory is updated. A smaller value for the Length Scale increases the accuracy of the trajectory and heat/mass transfer calculations for the discrete phase.
(Note that particle positions are always computed when particles enter/leave a cell; even if you specify a very large length scale, the time step used for integration will be such that the cell is traversed in one step.)
Length Scale will appear in the Discrete Phase Model panel when the Specify Length Scale option is on.
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 , FLUENT chooses a time step as
where is the Step Length Factor. As defined by Equation 22.11-2, is inversely proportional to the integration time step and is roughly equivalent to the number of time steps required to traverse the current continuous phase control volume. A larger value for the Step Length Factor decreases the discrete phase integration time step. The default value for the Step Length Factor is 5. Step Length Factor will appear in the Discrete Phase Model panel when the Specify Length Scale option is off (the default setting).
One simple rule of thumb to follow when setting the parameters above is that if you want the particles to advance through a domain consisting of grid cells into the main flow direction, the Step Length Factor times should be approximately equal to the Max. Number Of Steps.