# sampling interval and sampling frequency

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 April 8, 2011, 06:07 sampling interval and sampling frequency #1 New Member   beauty Join Date: Feb 2010 Posts: 27 Blog Entries: 1 Rep Power: 12 dears, I am solving a time-dependent problem using fluent. Fluent help says "You can set the Sampling Interval such that Data Sampling for Time Statistics can be performed at the specified frequency". However, I am puzzled by the relation between sampling interval and sampling frequency. I wonder whether the following equation is right. sampling frequency=1(time step sizesampling interval) In addtion, how can I set an appropriate Sampling Interval, according to sampling frequency in the experiment? Thanks for your help! beauty

 November 3, 2014, 16:26 #2 Senior Member   Joe Join Date: Feb 2012 Location: Canada Posts: 109 Rep Power: 10 Hello Beauty! Did you find the answer of your question? I have the same question and wondering if can provide me the answer. Thank you, H

 November 5, 2014, 23:07 #3 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,041 Rep Power: 49 There is no sampling frequency option. There is only a sampling interval. Sampling Interval = 1 means sample every time-step Sampling Interval = 2 means sample every other time-step To achieve a certain sampling frequency (with units 1/s), you must choose the sampling interval and time-step size accordingly. The relation between frequency, sampling interval, and time-step size given was correct. frequency = 1/(step-size x sampling interval) dean993 likes this.

 November 6, 2014, 11:09 #4 Senior Member   Joe Join Date: Feb 2012 Location: Canada Posts: 109 Rep Power: 10 Thank you LuckyTran!

October 20, 2016, 03:11
#5
Member

Saurav Kumar
Join Date: Jul 2016
Posts: 76
Rep Power: 6
Quote:
 Originally Posted by LuckyTran There is no sampling frequency option. There is only a sampling interval. Sampling Interval = 1 means sample every time-step Sampling Interval = 2 means sample every other time-step To achieve a certain sampling frequency (with units 1/s), you must choose the sampling interval and time-step size accordingly. The relation between frequency, sampling interval, and time-step size given was correct. frequency = 1/(step-size x sampling interval)

sample interval = 200 means it will calculate average value of all 200 time steps.
is it right?
i am using delta_t = 10^-6, could you please suggest me what sampling interval should i use?
thank you

 October 20, 2016, 05:19 #6 Senior Member     Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 1,384 Blog Entries: 19 Rep Power: 30 It means it will collect data each 200 time steps. Hence, after the first 200 time steps, your statistics will coincide with the instantaneous field saved at the 200th time steps. To do what you wrote you need sample interval = 1. This, btw, is always your best choice if you don't know what you're doing.

October 21, 2016, 03:58
#7
Member

Saurav Kumar
Join Date: Jul 2016
Posts: 76
Rep Power: 6
Quote:
 Originally Posted by sbaffini It means it will collect data each 200 time steps. Hence, after the first 200 time steps, your statistics will coincide with the instantaneous field saved at the 200th time steps. To do what you wrote you need sample interval = 1. This, btw, is always your best choice if you don't know what you're doing.

if i select select sample interval =1 or 10 then i should save Data and case file for every 1 or 10 time step respectively.
if sample interval is 3 and autosave of data and case file is 100 then i wont get u', v' and w' at any time step.
am i right? if not then please explain.

thank you

 October 22, 2016, 07:01 #8 Senior Member     Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 1,384 Blog Entries: 19 Rep Power: 30 I never tried but i suspect that it simply works such that when you save you just get the last set of collected samples. For example: Autosave each 100 time steps Sample interval 30 time steps Then, i guess, at the first autosave after 100 dt the statistics saved in your file are relative to: Time 0 (not sure here) Time 0 + 30 dt Time 0 + 60 dt Time 0 + 90 dt Still, obviously, i can't see any reason to do something like this. In theory, the sample interval should be the minimum to achieve sufficient statistical independence between the samples. In practice, 1 works just great if you collect enough samples.