Oversettelse - Brasilsk portugisisk-Engelsk - Com o intuito de compreender melhor o escoamento...

Looking good!
Two small things:

I'd say 'flow lines (..), which originate' rather than 'those originate'

and maybe 'the first half from the base' rather than 'starting at the base'?

Hi Lein, thanks for your input.

The structure of the sentence does not allow the use of "which".
When we build a relative clause starting by "which", this refers to the noun immediately before it. In this sentence it would be "cyclone" and not "lines", therefore the meaning would be wrong.
For the last sentence, in the original we have the verb "partindo" which means "start", we can't take it out of the sentence, can we?

I'm not sure I agree... I frequently see sentences like this in British or American journals (science in this case), written by native English speakers.

'Unlike the DNA in the nucleus of the cell, which comes from both egg and sperm, an organism's mtDNA comes only from the mother's egg.'

(with 'which' referring to the DNA in the nucleus of a cell, not just to the cell).

If you want to use 'those' (or 'these') I think in English you'd need to start a new sentence.

As for the next remark, I think this is a matter of how this expression is used in Portuguese (partindo da base) versus English (first half from the base, or bottom, is a more commonly used expression).

I'm not saying it's wrong, just slightly unnatural
(and voted in favour)

Well, it's true that it is sometimes used (by convention) in that way, and it could be used here, I don't think it's a big grammatical issue, after all.
However I think the last sentence should stay as it is.

What do you say?

CC: Lein

I think it sounds a lot more natural this way.
And I agree the second sentence is a more literal translation of the original text and perfectly understandable so may well be better than my suggested alternative.
Everyone happy!