how to set the input as a variable?

IronLyon · March 27, 2019, 14:48

I want to use the mass flow rate as the input, but not constant. Like in the first 1 s, it is 0.03 kg/s, then 0.02 kg/s for the next 3 s, then 0.01 kg/s for the last 1 s. The total time is 5 s. Is there a simple way to do that?
(As I can image, I will connect three modules and try to set the constant mass flow rate in each like 0.03, 0.02, 0.01 kg/s. Then use the result of the last module as the initial condition of the next phase. But it may not work.)

Opaque · March 27, 2019, 16:03

You can write an expression using "step" function

step (arg) = 0 if arg < 0, 0.5 if arg = 0, 1 arg > 0

tfac = 1 [s]
massflow = Value1 * step( (t1 - t) / tfac) + Value2 * (step ((t2-t) / tfac) - step( (t1-t) / tfac) )

Recall you can plot the expression in CFX-Pre, Expressions panel. So you can try to see if the expression works as expected

March 27, 2019, 14:48	how to set the input as a variable?	#1
IronLyon Member Join Date: Feb 2019 Posts: 30 Rep Power: 7	I want to use the mass flow rate as the input, but not constant. Like in the first 1 s, it is 0.03 kg/s, then 0.02 kg/s for the next 3 s, then 0.01 kg/s for the last 1 s. The total time is 5 s. Is there a simple way to do that? (As I can image, I will connect three modules and try to set the constant mass flow rate in each like 0.03, 0.02, 0.01 kg/s. Then use the result of the last module as the initial condition of the next phase. But it may not work.)

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March 27, 2019, 16:03		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	You can write an expression using "step" function step (arg) = 0 if arg < 0, 0.5 if arg = 0, 1 arg > 0 tfac = 1 [s] massflow = Value1 * step( (t1 - t) / tfac) + Value2 * (step ((t2-t) / tfac) - step( (t1-t) / tfac) ) Recall you can plot the expression in CFX-Pre, Expressions panel. So you can try to see if the expression works as expected