Is it ok to say "the flow structure of wave-driven flow is much different from single-direction flow" or must I say "the flow structure of wave-driven flow is much different from that of single-direction flow"
Either is valid.
The choice depends partly on user preference, as writers tend to be maximalists, minimalists, or somewhere between. If the sentence is very long and complex, removing such phrases may make the text more understandable. Conversely, if the sentence is very short, using such phrases may make the result sound 'more complete'.