How can I get values of standard deviation in CFX Post?

- June 21, 2021 at 11:52 am
  
  ckuestner
  Subscriber
  
  Hey,
  I'm looking for a way to access the standard deviation values of monitor statistics from SolverManager (see https://www.youtube.com/watch?v=KLv-By0PKiM) in CFX Post for the last iteration interval of the calculation. Does anybody know if this is possible?
  Regards,
  Chris
- June 22, 2021 at 7:05 am
  
  DrAmine
  Ansys Employee
  
  You might access the values of the Statistics after following steps in CFX-Pre.
- June 22, 2021 at 9:22 am
  
  ckuestner
  Subscriber
  
  Thanks for your replie. Yes I tried to create a CFX-Pre expression for the Standard Deviation of the "Efficiency" expression with "probe(Expression Value.Standard Deviation)@Efficiency" but sadly this doesn't work. Do you have any other suggestion?
- June 22, 2021 at 11:00 am
  
  DrAmine
  Ansys Employee
  
  What do you mean it did not work?
- June 22, 2021 at 11:55 am
  
  ckuestner
  Subscriber
  
  Sorry, to be more precisly:
  First I created an expression "100 * massFlowAve(Isentropic Compression Efficiency)@Row1 Outlet" named "efficiency" in CFX-Pre. This expression is monitored in the SolverManager by a monitor point named "Efficiency". Additionally, for this monitor point the Standard Deviation is set as an SolverManager output using the monitor statistics option in CFX-Pre. Up to here everything is fine. Both, the expression and their standard deviation is shown in the SolverManager.
  Now I want to get the value of the standard deviation of the efficiency expression for the last iteration in CFX-Post. For this purpose, I tried "probe(Expression Value.Standard Deviation)@Efficiency" and "probe(Expression Value.Standard Deviation)@efficiency", but I keep getting the error message:
  "ERROR The following unrecognised names were referenced: Efficiency, Expression Value.Standard Deviation (on 'Efficiency')." or
  "ERROR The following unrecognised names were referenced: Expression Value.Standard Deviation (on 'efficiency'), efficiency."
  I hope it has become clearer now.
- June 22, 2021 at 12:37 pm
  
  DrAmine
  Ansys Employee
  
  Sure you won' get it in that way. In CFD-Post you can however create a chart based on that Monitored Data. To have the quantity as variable for post-processing you might require additional variable with algebraic expression to be solved.
- June 22, 2021 at 1:29 pm
  
  ckuestner
  Subscriber
  
  Okay, I got the plot data and this would be a workaround, but lets say I want to calculate the standard deviation of an expression and use it for post-processing. According to your answer I need to calculate it manually with an algebraic expression. For this purpose, I would need the expression values from previous time steps. I wonder how can I access them?
- June 22, 2021 at 2:07 pm
  
  DrAmine
  Ansys Employee
  
  You use Unspecified Additional Variable in CFX-Pre and assign it to the "Algebraic" expression. After solving again (or at least running a couple of iterations) you can post-process the variable in CFD-Post.
- June 22, 2021 at 3:47 pm
  
  ckuestner
  Subscriber
  
  Thanks a lot for your help, I got it! :)
- December 22, 2021 at 1:35 am
  
  Richard_Marx
  Subscriber
  
  Hi Ckuestner - It's the way you named your expression. You need to rename the expression that you're creating in your output and then call for the standard deviation data from that new expression. Not your original CFX-Pre expression.
  Hope that helps :)