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CFD-ACE+ User Tips

Using the STEP Function in CFD-ACE+

The Parametric Input feature in CFD-ACE-GUI is an excellent way to gain advanced control over boundary conditions. This feature can be found under the Tools -> Parametric Input choice on the menu bar. This note focuses on the STEP function and its use in controlling time dependent boundary conditions.

The principle of the STEP function is simple. It returns a value of 1 if the argument of the function is greater or equal to zero and it returns 0 if the argument is less than zero. Using STEP in combination with the built in variable "T" (for time) allows you to build advanced functions for time dependent boundary conditions.

Let's say you want the inlet pressure to be 10000 for 4 seconds and then be set to 0 after 4 seconds. To do this you would select Tools -> Parametric Input and add a parameter variable called P with a value of 10000 * STEP(4-T). Then to use the "P" parameter, go to the BC section of the GUI and select the desired inlet to set the pressure evaluation method to be Parametric with a value of "P". When the transient simulation is made the solver will evaluate your parameter every timestep and thus will set the inlet pressure to 10000 for time less than or equal to 4 seconds and 0 for time greater than 4 seconds.

The STEP function can also be used to create parametric values that increase or decrease over a period of time. An example of this would be setting the parameter P to 1000*T*STEP(4-T). In this case the velocity would increase from 0.0 at time equal 0 seconds to 4000 at time equal 4 seconds, after which the value would revert back to 0.

A more complex example is shown with the following set of parameters. Note how the previously defined parameters (P, A, B, and C in this case) can be used in any parametric expression (PRESS in this case).

Variable	Value
P	1000
A	1
B	3
C	10

PRESS

(P/A*T-STEP(T-A)*P/A*(T-A)-STEP(T-B)*P/(C-B)*(T-B))*STEP(C-T)

In the above example the pressure will increase from 0.0 to 1000.0 in 1 second, stay at 1000 until 3 seconds, and then ramp back to 0.0 by 10 seconds.

Please keep the parametric input feature in mind the next time you have a need for advanced time dependent boundary conditions. It could help you avoid writing a user subroutine. Additional information about all of the parametric input features can be found in the on-line help system by selecting the "Help" button on the Parametric Input dialog box.

If you have any questions about this feature or would like us to discuss some other topic in the future, please let us know.

Perry Daley
Group Leader
CFDRC Customer Support

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