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Old   October 15, 2008, 15:34
Default expression with variable domains
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I want to create an expression used to define a boundary condition.

The expression needs to be able to calculate the specified boundary condition at different time(s).

For example, the inlet velocity for one of my boundary conditions is defined as:

(3*t+5), for t<2

(11), for t>2

(-12t+14), for t>5

I tried messing aronud with the step function, but I don't believe this is the right approach. Any help would be greatly appreciated.
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Old   October 15, 2008, 20:33
Default Re: expression with variable domains
Glenn Horrocks
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Either use the step function or an interpolation function. There are no other options.

In CFX V12 you will be able to use an if function.

Glenn Horrocks
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Old   October 16, 2008, 01:50
Default Re: expression with variable domains
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(3*t+5)*step(2-t)+11*step(t-2)*step(5-t)+(14-12*t)*step(t-5) Becouse Step is dimensionless function you should use t/1[s] insted of t (if t=time, of course). All parts of expression must have Velosity units. So, finally expression is ((3*t+5[s])*step(2-t/1[s])+11[s]*step(t/1[s]-2)*step(5-t/1[s])+(14[s]-12*t)*step(t/1[s]-5))*1[m/s^2] Not so camfortable as "if" "else" but it works. I hope it help you.

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Old   November 14, 2008, 18:46
Default Re: expression with variable domains
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Thank you very very much!!!
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