23.2.3 Tracking Parameters for the Discrete Phase Model

You will use two parameters to control the time integration of the particle trajectory equations:

• the maximum number of time steps

  This factor is used to abort trajectory calculations when the particle never exits the flow domain.

• the length scale/step length factor

  This factor is used to set the time step for integration within each control volume.

Each of these parameters is set in the Discrete Phase Model dialog box (Figure  23.2.1) under Tracking Parameters in the Tracking tab.

Models Discrete Phase Edit...

Figure 23.2.1: The Discrete Phase Model Dialog Box and the Tracking Parameters

Max. Number of Steps   is the maximum number of time steps used to compute a single particle trajectory via integration of this equation in the separate Theory Guide. When the maximum number of steps is exceeded, ANSYS FLUENT abandons the trajectory calculation for the current particle injection and reports the trajectory fate as "incomplete'' . The limit on the number of integration time steps eliminates the possibility of a particle being caught in a recirculating region of the continuous phase flow field and being tracked infinitely. Note that you may easily create problems in which the default value of 500 time steps is insufficient for completion of the trajectory calculation. In this case, when trajectories are reported as incomplete within the domain and the particles are not recirculating indefinitely, you can increase the maximum number of steps (up to a limit of $10^9$).

Length Scale   controls the integration time step size used to integrate the equations of motion for the particle. The integration time step is computed by ANSYS FLUENT based on a specified length scale $L$, and the velocity of the particle ( $u_p$) and of the continuous phase ( $u_c$):

where $L$ is the Length Scale that you define. As defined by Equation  23.2-1, $L$ 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 dialog box when the Specify Length Scale option is enabled.

Step Length Factor   also controls the time step size used to integrate the equations of motion for the particle. It differs from the Length Scale in that it allows ANSYS FLUENT to compute the time step in terms of the number of time steps required for a particle to traverse a computational cell. To set this parameter instead of the Length Scale, turn off the Specify Length Scale option.

The integration time step is computed by ANSYS 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 $\Delta t^*$, ANSYS FLUENT chooses a time step $\Delta t$ as

where $\lambda$ is the Step Length Factor. As defined by Equation  23.2-2, $\lambda$ 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 dialog box 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 $N$ mesh cells into the main flow direction, the Step Length Factor times $N$ should be approximately equal to the Max. Number of Steps.