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To obtain the half width, export the X coordinate, Y coordinate, and Axial Velocity by going to File-Export to CFD Post (described at the bottom of Turbulent Jet - Turbulent Setup and Solution). Alternately, go to File-Export-Solution Data. Select 'X-coordinate', 'Y-coordinate', and 'Axial Velocity' as values to export. Export them along the centerline, and various X locations (like X=10D) as either an Ascii file to copy into Excel, or use a Matlab script and this function to read in ASCII files at the specified X locations.
Using the Matlab script and plotting Y-coordinate approximate locations at which U=U_center/2 - determined by examining the values read in to Matlab - gives the half width variation:
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