# About rmse velocity and rms velocity

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 April 9, 2019, 06:02 About rmse velocity and rms velocity #1 New Member   none Join Date: Jan 2017 Posts: 7 Rep Power: 6 In the Fluent, we can plot the contour of RMSE velocity magnitude (not only velocity but also for other parameters) base on the sampled LES results. However, as I checked, RMSE and RMS are two different concepts. RMSE is short for root mean square error, which is defined as:http://statweb.stanford.edu/~susan/c...it/node60.html It is the root mean square of the instantaneous velocity subtract by the average velocity. RMS is not an error. It is the root mean square of the instantaneous velocity without subtracting anything. I checked the ANSYS Fluent Theory Guide. I only find RMS in instead of RMSE. And in our Forum, people are mostly talked about RMS. Therefore, I am confused. Is RMSE in Fluent is the root mean square error of velocity or root mean square of velocity? Thanks Best regards StarZ soheil_r7 likes this.

 April 9, 2019, 17:14 #2 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,279 Rep Power: 51 This question has been asked over and over and over again. As you state correctly, RMS is root mean square of instantaneous velocity without subtracting anything. RMSE is the root mean square error, which is equivalent to the RMS of the fluctuating velocity. For unsteady flows, it was presumed obvious that we are looking for RMS of fluctuating velocities. But then one day people realized that RMS of velocity is confusing. So Fluent changed the labeling from RMS to RMSE to make it clearer that it is calculating RMSE. Fluent has always calculated RMSE, but you will find it labeled RMS in the program or manual. For turbulent flows people are talking about the RMS of the fluctuating velocity (rms of u', v', or w'), which is the RMSE of the instantaneous velocity. A lot of people say RMS when they actually mean RMSE. Heck a lot of people call u',v', w' as "the rms velocity." soheil_r7, roi247, starz and 2 others like this.

 Tags definition, rms velocity, rmse, unsteady flow