@vv 

I can not understand what it means to a method for determining the box as a surface to calculate the flow. How to define where located this box. For example, with a sphere understanding, are the coordinates of the center and radius. I'm confused.

@vv 

Thanks for the reply and the link.

@acer 

Thanks, but I mean, how to calculate numerically (approximately) integral that can not be evaluated in closed form.

I'm sorry if I expressed myself not clear enough.

Thank you all for your help and participate in the discussion of this topic.

Slightly changed the first component of the field. Flux command is now working, but the same result. What could be the reason?

Also tell me how to calculate it numerically (mentioned it rlopez)

flux32.mw

@tomleslie Hello. It was just an answer to a simple question of how to solve the problem using the 2-D input. I did not compare the efficiency, conciseness, and the like, the different input methods. I think everyone decides which input method the expressions he used.

@tomleslie Perhaps this is what you need

Download Answer_2-D.mw

@Markiyan Hirnyk Thank you

@acer

Thank you. Really works. 

@Kitonum Thanks for your reply.I score as you recommend and still not working. You can attach your file?

Supplement may be you have changed something in the definition of expression in the sequence u, and after your corrections still not working.

Thank you for your file. You replaced x__1 on x[1].
I still do not understand why can not use subscript notation. Like here

http://www.maplesoft.com/support/help/Maple/view.aspx?path=MaplePortal/Tutorial6

Hi Edgardo S. Cheb-Terrab.

Could you please send me the missing file (Download DivergenceOfATenso.mw) in this thread http://www.mapleprimes.com/questions/35714-Tensor-Vs-Vector-Operations

Hi Edgardo S. Cheb-Terrab. Thanks for your help. Try to understand.