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I know that turbulent flow has more energy than laminar flow and hence can sustain adverse pressure gradient for a longer period of time. It basically delays flows separation.

But referring to this source from aviationchief.com, turbulent flow encourages flow separation. I'm not exactly sure whether statement is true so can anyone give me an explanation behind this concept

This is what I think - Since turbulent flow is known for having skin friction drag, it slows down air after the suction peak. This slow down of air (due to pressure rise and skin friction caused due to turbulent flow) is gonna cause flow separation.

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  • $\begingroup$ Yeah the dimpled golf ball, right? $\endgroup$
    – Koyovis
    Commented Jan 9, 2020 at 10:47
  • $\begingroup$ yeah that's right $\endgroup$
    – Johnson
    Commented Jan 9, 2020 at 11:10
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    $\begingroup$ Could you provide a reference to the claim regarding separation? It is true that turbulence increases skin drag, but it also true that the separation point is the one where skin drag disappears. And the turbulent layer is also better att distributing momentum downwards from the free flow to the skin. $\endgroup$
    – Mats Lind
    Commented Jan 9, 2020 at 12:05
  • $\begingroup$ Yes it is true that turbulent flow creates skin friction drag. $\endgroup$
    – Johnson
    Commented Jan 9, 2020 at 12:44
  • $\begingroup$ I haven't heard of turbulent flow encouraging flow separation. Do you mean to say that separated flow usually has turbulent characteristics? $\endgroup$
    – JZYL
    Commented Jan 9, 2020 at 13:04

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Turbulence delays flow separation. This is why we attempt vortex generators and turbulators in aerodynamic design when local flow separation is an issue.

A. Adverse Pressure Gradient

Fundamental to the generation of subsonic lift, as airflow accelerates over the leading edge, its pressure drops, resulting in net suction. At some point, usually corresponding to the point of re-converging airfoil geometry, the flow begins to decelerate as its pressure rises, eventually equating to the flow pressure on the lower surface at the trailing edge. This process is called pressure recovery, and the increasing pressure profile along the upper surface is called adverse pressure gradient.

enter image description here

Image modified from: https://conself.com/blog/calculate-lift-drag-with-paraview/

B. Laminar Boundary Layer

Within the boundary layer, which is a thin envelope flow close to the airfoil surface, an adverse pressure gradient in the external flow can slow the flow close to the surface (which is already much slower than the external flow due to no-slip condition) to a standstill. At that point, the separation occurs. This is more or less the story with laminar boundary layer where the flow is smooth.

enter image description here

Image ref: https://en.wikipedia.org/wiki/Flow_separation#/media/File:Boundary_layer_separation.svg

C. Turbulent Boundary Layer

Now in the case of a turbulent boundary layer, there is a lot more mixing going on within the boundary layer due to eddies. As a result, there is continued energy injection into the bottom layer flows, which delays the flow separation that would normally occur in a laminar boundary layer.

enter image description here

Image ref: http://www.bakker.org/dartmouth06/engs150/11-bl.pdf

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  • $\begingroup$ In general a correct answer. But still, there are cases where a laminar boundary layer postpones separation. Every PIK-20 or Janus pilot knows this. With a clean wing, laminar flow provides for a thinner turbulent boundary layer in the region of pressure rise. Once that laminar flow is disturbed by bugs or rain, the stall speed of both those gliders goes up by 20-30%. $\endgroup$
    – Peter Kämpf
    Commented Jun 8, 2020 at 20:12
  • $\begingroup$ The PIK 20 is said to use the Wortmann FX 67 K-170 airfoil, a top lifter, which seems to be right at the edge of the Reynolds number range where top lift works, especially at lower "stall" speeds. This might make it perfect to study flow separation effects, as well as the benefits of implementing slats. There may not be an easy answer to the question, as wing types and airspeed vary greatly. $\endgroup$
    – Robert DiGiovanni
    Commented Jun 9, 2020 at 1:04

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