# use equations

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 September 4, 2009, 05:57 use equations #1 New Member   Tuffy Join Date: Jun 2009 Posts: 28 Rep Power: 9 Hi everyone! I must use an equation in CFX to set a condition about the voltage. The idea is to create something like this: if the temperature is lower than 90° use a voltage of 60 V. If not decrease the voltage until the temperature is not lower than 90°. do you know exactly which is the meaning of the following equation? step((maxVal(Temperature)@REGION:B64-90 [K] )/1[K])*60[V] how much is the value? 60 V if? please, help me!!!! thanks, Ema

 September 4, 2009, 06:07 #2 New Member   Tuffy Join Date: Jun 2009 Posts: 28 Rep Power: 9 i've also tried to implement the following expression: volt= 60[V]*(1/(1+e^(5*(maxVal(Temperature)@REGION:B97-90[K])/1[K]))) but my update failed...

 September 4, 2009, 06:28 #3 Senior Member     George Join Date: Mar 2009 Location: Birmingham, UK Posts: 257 Rep Power: 10 your equation doesnt make any sence step function returns 0 or 1 and your conditions should be non dimensional (thats why you divide the temperature by 1/[K] ) if you multiply by 60[V] then your expression has value either 0*60[V] or 1*60[V] if you want to to create some feedback control search in the cfx manual for "Example: Feedback to Control Inlet Temperature" __________________ Top 4 tips 1. Knowledge is everything and Ignorance is dangerous. 2. Understand your limitations and try to eliminate them. 3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window. 4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 September 4, 2009, 06:54 #4 Member   Join Date: Mar 2009 Posts: 44 Rep Power: 9 You don't need to define the unit [K] after your maxVal(Temperature)@.... statement, as the value returned already has unit.

September 4, 2009, 07:59
#5
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George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
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Quote:
 Originally Posted by Timon You don't need to define the unit [K] after your maxVal(Temperature)@.... statement, as the value returned already has unit.
the units are for the 90[K]. the step function can look something like:
step( (T - 90 [K]) / 1[K] ) or
step( (T/1[K] - 90 )
__________________
Top 4 tips
1. Knowledge is everything and Ignorance is dangerous.
2. Understand your limitations and try to eliminate them.
3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window.
4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

 September 4, 2009, 08:02 #6 New Member   Tuffy Join Date: Jun 2009 Posts: 28 Rep Power: 9 exactly. Otherwise CFX will say you that the values are not correct. What about my equation? why no sense at all? i was thinking to implement like a sigm function to multiply with the voltage...

September 4, 2009, 08:02
#7
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Quote:
 Originally Posted by ckleanth the units are for the 90[K]. the step function can look something like: step( (T - 90 [K]) / 1[K] ) or step( (T/1[K] - 90 )
True, disregard my previous post...

September 4, 2009, 08:08
#8
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George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 10
Quote:
 Originally Posted by Ema What about my equation? why no sense at all? i was thinking to implement like a sigm function to multiply with the voltage...
well you said you wanted to do some feedback. I replied that if you want to to create some feedback control search in the cfx manual for "Example: Feedback to Control Inlet Temperature". your equations as it stands dont do that
__________________
Top 4 tips
1. Knowledge is everything and Ignorance is dangerous.
2. Understand your limitations and try to eliminate them.
3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window.
4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials

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