6 Series Airfoils were developed to enhance the laminar flow region over the airfoil. But, the Reynolds number at which planes fly are high for the flow to be Laminar. Then what's point of introducing 6 series airfoils when laminar flow is not possible?
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Flow at the forward stagnation point of an airfoil is laminar, because the local velocity will be zero. The boundary layer originating form the stagnation point will usually not remain laminar for long, but some airfoils like the NACA 6 series were optimized to conserve this laminar flow as far down the chord as possible.
A laminar boundary layer is thinner and offers less drag, so it is desirable for aerodynamic efficiency. One of the more famous laminar wing designs was the P-51 Mustang, which used the NACA 66-(1.8)12 (a=0.6) airfoil at the wing roots.
In reality the boundary layer will transition at some point before reaching the trailing edge, and become turbulent. In the case of the P-51, a shot down example was analyzed by H. Schlichting. He found that the transition point was at the 50% chord point for $Re=4\cdot10^6$ and as far forward as 20% of the chord for $Re=7.5\cdot10^6$, which suggests that the Mustangs performance had more to do with the polished finish and flush rivets of the wings than with the airfoil used.
answered Feb 6, 2020 at 12:45
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$\begingroup$ Thanks for replying to my question. Here is my question : how local velocity being zero makes boundary layer velocity zero ? $\endgroup$– PavanCommented Feb 6, 2020 at 16:54
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$\begingroup$ @Pavan what non-zero velocity value would you expect? You can prove that it must be zero at the stagnation point by taking the limits from above and below the stagnation point. Otherwise I am not fully understanding your question. $\endgroup$ Commented Feb 6, 2020 at 19:11