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-   -   sampling interval and sampling frequency (https://www.cfd-online.com/Forums/fluent/86995-sampling-interval-sampling-frequency.html)

beauty April 8, 2011 05:07

sampling interval and sampling frequency
 
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\div(time step size\timessampling interval)

In addtion, how can I set an appropriate Sampling Interval, according to sampling frequency in the experiment?

Thanks for your help!

beauty

enayath November 3, 2014 16:26

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

LuckyTran November 5, 2014 23:07

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)

enayath November 6, 2014 11:09

Thank you LuckyTran!

srv537 October 20, 2016 02:11

Quote:

Originally Posted by LuckyTran (Post 517595)
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

sbaffini October 20, 2016 04:19

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.

srv537 October 21, 2016 02:58

Quote:

Originally Posted by sbaffini (Post 622209)
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

sbaffini October 22, 2016 06:01

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.


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