CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Disturbances in drag monitors after switching on 'Data Sampling for Time statistics'

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 25, 2017, 02:39
Default Disturbances in drag monitors after switching on 'Data Sampling for Time statistics'
  #1
New Member
 
Shashank
Join Date: Sep 2014
Posts: 28
Rep Power: 8
shashanktiwari619 is on a distinguished road
I am running a LES simulation for a flow over bluff body where in I monitor drag and lift coefficient w.r.t time. I switch on the 'Data Sampling for Time statistics' after my converge fairly well which is at around 1 secs (my time step is 1e-5). I have observed that the value of drag coefficient monitor show a lot of oscillation around the previously obtained time data of drag coefficient after I switch on the 'Data Sampling'. why is it so?? I have attached the time vs drag coefficient plot where I switch on the time statics after around 1.8 secs and the oscillations are clearly visible.
I get that switching on data statistics gives RMS and mean values of all variables for every time step specified. But why should it effect the drag monitor values??Am I missing out in understanding something here??
Attached Images
File Type: jpg drag.jpg (26.6 KB, 16 views)
shashanktiwari619 is offline   Reply With Quote

Old   August 25, 2017, 15:01
Default
  #2
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 4,144
Rep Power: 50
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
There shouldn't be any relation. Probably your problem was going to oscillate anyway. Can you prove it was caused by switching on data sampling? I.e. if you turn data sampling off does it go away? If you had not turned on data sampling, what result do you get? Did you also do something else?
LuckyTran is offline   Reply With Quote

Old   August 26, 2017, 04:58
Default
  #3
New Member
 
Shashank
Join Date: Sep 2014
Posts: 28
Rep Power: 8
shashanktiwari619 is on a distinguished road
I tried running the simulations by turning off the statistics and attached are the plots that I have got.
The only two changes I did after initial run were 1. switching on the Data Sampling for time statistic and 2. Switching on Extrapolate variables.
Attached Files
File Type: docx a.docx (37.4 KB, 3 views)
shashanktiwari619 is offline   Reply With Quote

Old   August 26, 2017, 07:53
Default
  #4
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 4,144
Rep Power: 50
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
I don't know the exact procedure but when you use extrapolate variables, Fuent predicts the values of the time-step (extrapolates) and then it uses that as the initial guess. Normally, the final values from the previous time-step are used as an initial guess.

Extrapolate variables is a separate option from time sampling and it does affect the solution.

Next time, be honest to yourself about what you did.
LuckyTran is offline   Reply With Quote

Old   August 27, 2017, 02:34
Default
  #5
New Member
 
Shashank
Join Date: Sep 2014
Posts: 28
Rep Power: 8
shashanktiwari619 is on a distinguished road
Thanks for your reply LuckyTran.
I got it that this is because of the Extrapolate Variables that I have switched on.
My problem is as the plots change due to switching on the extrapolate variables, an FFT of the time domain signal shows that the frequency changes too.
In terms of Strouhal Number for Re 300 initially I get a value of 0.07 which does not match with reported value of 0.137 in literature. Whereas when I do the FFT analysis of the time data I obtain after switching on the Extrapolate Variables option gives me a value of 0.134 which is very much in agreement to previously reported values in literature. Why is there such a difference in St?
Which one of the two St should I consider and why?
shashanktiwari619 is offline   Reply With Quote

Reply

Tags
data sampling, les, mean velocity, rms velocity, statistics

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Parachute drag data Abuzar Adid Main CFD Forum 3 July 27, 2013 07:33
"Data sampling for Time Statistics" Madhukar Rapaka FLUENT 2 June 20, 2011 11:28
Data sampling for time statistics Muhammad Shakaib FLUENT 0 October 2, 2006 23:33
Sampling of statistical data Boerge Main CFD Forum 0 July 25, 2006 04:40
Data sampling for time statistics with RANS Flav FLUENT 0 June 6, 2001 04:32


All times are GMT -4. The time now is 03:38.