Time averaged values dont match manual calculations

- September 10, 2019 at 11:14 am
  
  BeginerModel
  Subscriber
  
  I have a repeating BC with a period of 0.8s and want to time-average the wall shear stress (WSS) over this period (Time step is 0.005s). I can set this up in fluent with a period of 160 time steps, and I assume it sums the instantaneos WSS over 160 steps and then divides by 160?
  
  I save the overall results for my model every 0.1s, giving me 8 time steps of instaneous data to view later. If I time average the WSS for these results the same way, I get a significantly different answer. I know I have less data points to average, but the values of WSS differ by ~40%. Is there some explanation for this?
- September 10, 2019 at 12:33 pm
  
  DrAmine
  Ansys Employee
  
  The mean value is calculated as sum of (variable X time step size) from index 0 (start) to n-1(end) /Sum of all time steps size. I am not sure if your averaging is doing the same but if you want to verify you need to make a table and have value say at t0=0, t1=..,.. till t_final then you multiply the variable value with delta time. For point "0" you get zero till point N-1 and then you make the sum

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