# How to set up transient BC?

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 March 17, 2005, 06:11 How to set up transient BC? #1 Sam Guest   Posts: n/a Hi, I've a problem setting up the transient BC. What I wish to get is: 1-2 sec, function1(t) 2-4 sec, function2(t) 4-6 sec, function1(t) 6-8 sec, function2(t) ...... Anyone know how to implement this expression in CFX-Pre? Thanks, Sam

 March 17, 2005, 10:10 Re: How to set up transient BC? #2 Robin Guest   Posts: n/a Hi Sam, Create expressions for each of your functions:function1 = ... function2 = ... ...Then create an expression that includes all the functions and multiplies each by a corresponding step function:allFunctions = function1*step((2 [s]-t)/1[s]) + function2*step((t-2 [s])/1[s])*step((4 [s]-t)/1[s]) + function3*step((t-4 [s])/1[s])*step((6 [s]-t)/1[s]) + function2*step((t-6 [s])/1[s])*step((8 [s]-t)/1[s]) + ...A step function takes a dimensionless argument (hence dividing by 1 [s]). If the argument is negative, it returns zero, positive returns 1 and if it is equal to zero, it returns 1/2. By combining your functions this way, you can turn them on and off as needed. Regards, Robin

 March 17, 2005, 22:09 Thank you. #3 Sam Guest   Posts: n/a Thanks, robin. It works eventhough it's lengthy. I've to simulate over 40 sec & allfunction becomes a giant equation.

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