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As I’ve read in the book “Fluid Dynamics” by Yunus Cengel, The Pressure Drag decreases and the Skin Friction Drag increases when fluid flow over body transitions from laminar to turbulent thus, resulting in overall decrease in Drag Coefficient.

The Pressure Drag is reduced during the transition implying that the Normal Pressure Force on the body is reduced and as the Lift Force is mostly provided by the Normal Pressure Force thus, implying Decrement in Lift with transition from Laminar to Turbulent flow.

So which flow is preferred in case of aeroplanes?

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  • $\begingroup$ Preferred? You can't have laminar flow in case of aeroplanes (unless it is sitting still). $\endgroup$ – Deep Apr 18 '18 at 4:25
  • $\begingroup$ @Deep but if it’s assumed that laminar flow is possible than in which case we’ll get higher Cl? $\endgroup$ – Varun Patel Apr 18 '18 at 4:53
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Laminar flow is clearly preferred. But a turbulent one has its uses, too

Not only will a laminar boundary layer result in much less friction drag (the velocity gradient at the wall is much less steep than with turbulent flow), but for the same reason it will extract much less energy from the flow so its ability to endure the pressure rise later is better preserved. However, once the flow encounters this pressure rise, the boundary layer becomes unstable and will transition into a turbulent one if that had not already happened before. Because the energy transfer from the far wall to the near-wall layers is much greater in a turbulent boundary layer, it allows to sustain much steeper pressure rises, allowing to reach a higher angle of attack without separation.

Much depends on the speed and physical size of the craft: Even though a negative pressure gradient is stabilizing the boundary layer, large and fast airplanes have rarely any laminar flow left while slow and small gliders sport it on most of their wing's lower and half of its upper side.

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