" The quasi-coherent structures described above have been identified mainly through visualizations of low-Reynolds-number turbulent flows (either from experiments or from DNS). Such a subjective approach inevitably leads to controversy over the nature and significance of the structures. As a consequence, over the years, there have been many suggestions for objective eduction techniques, to identify structures and to quantify their importance." (quoted from Popes' Turbulent Flow, 2003)

I do not understand how objectivity can be achieved in eduction techniques. I supposed DNS is the most accurate solution to turbulent flow. If DNS is a subjective approach, how do eduction techniques achieve an objective solution? Does objective solution possible in science?

You have missed the point of the paragraph. The phrase 'subjective approach' refers to the fact that the structures have been identified from visualization rather than from any rigorous mathematical or physical approach (which are objective).

-S