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Old   April 8, 2011, 05:07
Default sampling interval and sampling frequency
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beauty
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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!

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Old   November 3, 2014, 16:26
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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,
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Old   November 5, 2014, 23:07
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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)
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Old   November 6, 2014, 11:09
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Thank you LuckyTran!
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Old   October 20, 2016, 02:11
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Quote:
Originally Posted by LuckyTran View Post
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
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Old   October 20, 2016, 04:19
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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.
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Old   October 21, 2016, 02:58
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Quote:
Originally Posted by sbaffini View Post
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
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Old   October 22, 2016, 06:01
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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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