# Obtaining negative pressure transients

 Register Blogs Members List Search Today's Posts Mark Forums Read

 May 7, 2020, 10:39 Obtaining negative pressure transients #1 New Member   John Applesmith Join Date: Feb 2020 Posts: 13 Rep Power: 6 Hello, I need to obtain dp/dt in a fluid domain so for each node I'd like to obtain how much the absolute pressure changes from the last time-step. I can then divide by the time-step to get dp/dt. Additionally, I would then like to index the maximum pressure transient over the volume at each time-step and plot the maximum obtained dp/dt against simulation time. There are moving immersed solids in my simulation if it makes a difference. I am simulating the flow across an aortic bi-leaflet valve. Thanks.

 May 7, 2020, 13:42 #2 Senior Member   Join Date: Jun 2009 Posts: 1,815 Rep Power: 32 In case it helps, you should be able to use (only in the solution step) Absolute Pressure.Time Derivative in any expression as well as the RHS of an algebraic Additional Variable. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

May 10, 2020, 04:04
#3
New Member

John Applesmith
Join Date: Feb 2020
Posts: 13
Rep Power: 6
Quote:
 Originally Posted by Opaque In case it helps, you should be able to use (only in the solution step) Absolute Pressure.Time Derivative in any expression as well as the RHS of an algebraic Additional Variable.
Hi, is this a Derived Variable to be added in the workspace during a run? I can't find this function in the documentation anywhere but sounds exactly what I need.

 May 10, 2020, 20:25 #4 Senior Member   Join Date: Jun 2009 Posts: 1,815 Rep Power: 32 What do you need to do with it? Use it in an expression? or copy it into another variable? For an algebraic additional variable, you can do Additional Variable Value = Absolute Pressure.Time Derivative or use as part of any expression: MyExpression = 2 * Absolute Pressure.Time Derivative __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

May 11, 2020, 02:46
#5
New Member

John Applesmith
Join Date: Feb 2020
Posts: 13
Rep Power: 6
Quote:
 Originally Posted by Opaque What do you need to do with it? Use it in an expression? or copy it into another variable? For an algebraic additional variable, you can do Additional Variable Value = Absolute Pressure.Time Derivative or use as part of any expression: MyExpression = 2 * Absolute Pressure.Time Derivative
Hi, I saw your message on a similar thread years ago and just added Absolute Pressure.Time Derivative as CEL in Set-Up: many thanks.

Is it possible to access the volume-maximum of this value at each time-step? So I could plot dp/dt against t and identify when in the simulation the maximum pressure transient occurs and then where using an isosurface? I'm unsure of how to access the domain maximum of a value at a time-step and have 0 experience with Fortran or User Routines.

 May 11, 2020, 05:34 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,728 Rep Power: 143 Most functions in CFX can be done using CEL and do not need user fortran. To get the maximum of this function at each timestep use the CEL expression: maxVal(MyExpression)@Domain Name Look in the CFX Reference manual for the other available CEL functions and expressions. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 May 11, 2020, 07:30 #7 Senior Member   Join Date: Jun 2009 Posts: 1,815 Rep Power: 32 What do you need to do with it? Use it in an expression? or copy it into another variable? For an algebraic additional variable, you can do Additional Variable Value = Absolute Pressure.Time Derivative or use as part of any expression: MyExpression = 2 * Absolute Pressure.Time Derivative __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

May 11, 2020, 09:40
#8
New Member

John Applesmith
Join Date: Feb 2020
Posts: 13
Rep Power: 6
Quote:
 Originally Posted by ghorrocks Most functions in CFX can be done using CEL and do not need user fortran. To get the maximum of this function at each timestep use the CEL expression: maxVal(MyExpression)@Domain Name Look in the CFX Reference manual for the other available CEL functions and expressions.
Hi, I tried your method and got the following error

"The function 'maxVal' referenced by parameter 'Expression Value' in object '/FLOW:Flow Analysis 1/OUTPUT CONTROL/MONITOR OBJECTS/MONITOR POINT:transPress' has an invalid argument, 'Absolute Pressure.Time Derivative'. The solver does not support the use of this operator for this variable when used as an argument for this function.

I get similar messages no matter where I try to create the function- Set-Up CEL, as Monitor Point or in Post. The formula I'm using- maxVal(Absolute Pressure.Time Derivative)@tubeFluid. Even when I define the pressure transient separately and pass it into maxVal. What does work is Absolute Pressure.Time Derivative on its own.

 May 11, 2020, 10:40 #9 Senior Member   Join Date: Jun 2009 Posts: 1,815 Rep Power: 32 As a workaround, you can always copy a valid variable into an Additional Variable (which usually works everywhere else), Create an additional variable: (unspecified) with units = [Pa s^-1] Activate the additional variable within a domain Select Algebraic Additional Variable Set it equal to = Absolute Pressure.Time Derivative Try using the Additional Variable in those places you need it, __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

May 14, 2020, 13:08
#10
New Member

John Applesmith
Join Date: Feb 2020
Posts: 13
Rep Power: 6
Quote:
 Originally Posted by Opaque As a workaround, you can always copy a valid variable into an Additional Variable (which usually works everywhere else), Create an additional variable: (unspecified) with units = [Pa s^-1] Activate the additional variable within a domain Select Algebraic Additional Variable Set it equal to = Absolute Pressure.Time Derivative Try using the Additional Variable in those places you need it,
Worked perfectly- thank you so much!

 Tags post, pressure transients, results