How can i calculate a mean velocity in a duct for LES simulation? Thank You

Hi,

You can do it as an average against time as the solver progresses with a transient statistics object, you can put this in CFX-Pre on the output tab. You can do it against length or volume in CFX-Post as post-processing.

Glenn Horrocks

Unfortunately, i didnt write transient statistics when i was setting up my problem in CFX-Pre. I have obtained the solution now! How can i find it now? Thank you for your answer.

Mann

You can write an expression for the quantity you want to solve in CFX-Post, and then plot it as a function of time in a Chart. This will only evaluate the expression at each timestep that you have saved as a .trn (or .bak) file though, so it won't be a true mean of the variable unless you saved that variable at every step (this is why it is beneficial to use the transient statistics instead). Also, it will take more time since Post will have to load each timestep and then evaluate the expression to create the chart. If you want the average of these values, you can export the plot data to a text file and find the mean with matlab or excel. Maybe taking the mean of a chart can be done internally in Post as well, but I have never tried. Understand this is a kludge and not a replacement for calculating the data during the run, but will work if rerunning is not an option.

But i need to plot my u-velocity as a function of vertical distance. Why do i need to plot it against time at all? Can i not plot my u-velocity against vertical distance in CFX-post and then export that plot data to excel to find mean velocity value (which is u/inlet velocity, i think). Then i can plot my mean velocity against vertical distance (Y). Is there something wrong with this method Alex? Thanks

Maan

Yup, that will work. As Glenn said, if you are trying to find the mean with respect to a distance, area, or volume that can be done many ways in Post. Since you replied that you didn't collect transient statistics, I thought you needed a mean with respect to time. You can also use the post functions like massflowave or areaave to find averages. Check the documentation for more help on those functions.

-Alex

Ok this should help. Thank you for your reply Alex and Glenn! BY the way Maan and Usman happens to be same person.

Regards Usman