Fan Curve

Implicit approach

In the implicit approach, the solver implicitly calculates the volume flow rate at current iteration (n + 1) based on the pressure rise at the same iteration (n + 1) and the active coefficients of the linear system. The solver obtains the linear system coefficients from the fan curve derivative, based on the fan's pressure rise at previous iteration (n). Therefore, both the fan's pressure rise and fan's volume flow rate are the results of the linear system solution.

Explicit approach

In the explicit approach, the solver computes the volume flow rate at current iteration (n + 1) directly from the fan curve itself using the fan's pressure rise at previous iteration (n), and imposes the volume flow rate as an explicit boundary condition to form the linear system. The software then obtains the pressure rise at the current iteration (n + 1) by solving the linear system.

In the explicit approach, the pressure rise and volume flow rate must converge through explicit iterations. This could lead to strong fluctuations in the solution convergence based on the characteristics of the fan curve. You have to control these fluctuations using the relaxation techniques. The implicit approach suppresses these fluctuations by making the pressure rise and volume flow rate better converge at each iteration, through implicit coupling in the linear system.

Implicit vs explicit approach

The flow solver supports both implicit and explicit approaches to define the volume flow rate for all fan curve types, except for internal fan that always uses explicit approach.

By default, the flow solver uses the implicit approach. To switch to the explicit approach, you need to set the value of the USE EXPLICIT FAN CURVE advanced parameter to TRUE.