[Saurabh Kurmi]

Hello everyone!!
I am doing a transient analysis in Ansys fluent. I want to get the time averaged velocity contour. I have enabled the data sampling for time statistics but confused about what to put in sampling interval??

[Svetlana]

Sampling interval of 1 second means that the variable is measured every 1 second.


For example, with a time step of 0.1 seconds, you have these results in a point:


a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20


They correspond to these time values:


0.1s, 0.2s, 0.3s, 0.4s, 0.5s, 0.6s, 0.7s, 0.8s, 0.9s, 1s, 1.1s, 1.2s, 1.3s, 1.4s, 1.5s, 1.6s, 1.7s, 1.8s, 1.9s, 2s


With a sampling interval of 0.1s you will get the average as


sum(a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20) / 20


With a sampling interval of 0.2s it only takes every second value,


sum(a2, a4, a6, a8, a10, a12, a14, a16, a18, a20) / 10


With a sampling interval of 1s it only takes one value every second,


sum(a10, a20) / 2

[LuckyTran]

The sampling interval is not in seconds, it's time-steps.

[Svetlana]

Thank you for the correction, LuckyTran


In this case here is the updated example:


Sampling interval of 1 means that the variable is measured every 1 step.


For example, with a time step of 0.1 seconds, you have these results in a point:


a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20


They correspond to these time values:


0.1s, 0.2s, 0.3s, 0.4s, 0.5s, 0.6s, 0.7s, 0.8s, 0.9s, 1s, 1.1s, 1.2s, 1.3s, 1.4s, 1.5s, 1.6s, 1.7s, 1.8s, 1.9s, 2s


With a sampling interval of 1 you will get the average as a mean of all points:


sum(a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20) / 20


With a sampling interval of 2 it only takes every second value,


sum(a2, a4, a6, a8, a10, a12, a14, a16, a18, a20) / 10


With a sampling interval of 10 it only takes one value every ten steps meaning every second,


sum(a10, a20) / 2

[Saurabh Kurmi]

Thanks for all the replies.
I am performing the simulation of flow over a circular cylinder. I have to get the average velocity contour.I have given 1000 time steps with time step size 0.1 sec.What value of sampling intervel should i give??

[Svetlana]

1. "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." (link)


In my experience another non-1 sampling interval was useful once when I wanted to compare with experimental data in which the lab recordings were sampled with a different frequency than my time step in ansys fluent.


2. Please also understand that you need to time average your data after the flow has finished developing. After you start your simulation it may take a while to settle. Adding a monitor point and watching its behaviour can be a good way to know when to begin your averaging. There is a plot about this here, and in this example they need to average after 0.03s.