Why does it follow the curvature rather than separating? Its not due to the coanda effect as the accelerated flow near the surface of the aerofoil can't be considered a jet. It's not due to surface tension because there is no interface between two fluids. I know the flow of a fluid is due to pressure, inertial and viscous effects.

  1. Is it due to viscous effects on fluid particles in the boundary layer?
  2. Is is due to the pressure normal to the surface of any body submerged in a fluid?
  3. Another reason?

Thanks for your answers!

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    $\begingroup$ Would this answer help in answering your question? In short: It is suction caused by local curvature and incidence. $\endgroup$ Commented Jul 31, 2020 at 11:17
  • $\begingroup$ @PeterKämpf - Yes that does help, especially the part about viscosity meaning that air molecules, because of their oscillation, tend to assume the speed and direction of their neighbors. When combined with the velocity profile in the boundary layer this could explain the continued attached flow. I'm not so sure about your statement that if the flow didn't follow the curvature then a vacuum would form and so it reluctantly follows the curvature. When flow separation does occur a vacuum isn't formed, we have a turbulent wake. $\endgroup$
    – David
    Commented Jul 31, 2020 at 12:01
  • 1
    $\begingroup$ There is another answer on flow separation here. Separation only happens when the flow at the surface is reversing. $\endgroup$ Commented Jul 31, 2020 at 18:06
  • $\begingroup$ @David, there can't be stagnant air near the receding surface because that would mean higher pressure, so it would get sucked out. So there can either be recirculation, or flow along the surface. Because viscosity prevents the air from turning too sharply, the recirculation is only possible at high angles (stall) while at low angles the flow stays attached. $\endgroup$
    – Jan Hudec
    Commented Aug 13, 2020 at 19:44

1 Answer 1


This is due to the Coandă effect.


In the 1930s Henri Coandă, a Romanian scientist, described what is now known as the Coandă effect, a major contribution to fluidic technology. He observed that as a free jet emerges from a jet nozzle the stream will tend to follow a nearby curved or inclined surface. It also “attaches” itself to and flows along this surface if the curvature or angle of inclination is not too sharp. Coandă explained this tendency as being caused by the jet stream’s entraining (picking up) nearby fluid molecules. When the supply of these molecules is limited by an adjacent surface, a partial vacuum develops between the jet and the surface. If the pressure on the other side of the jet remains constant, the partial vacuum, which is a lower pressure region, will force the jet to bend and attach itself to the wall.

  • $\begingroup$ This is only partially true. The Coandă effect is for a jet in otherwise stagnant fluid, but in this case all the fluid is moving, so it is not applicable as defined. The entraining mechanism is there, and prevents pocket of stagnant air from forming near the surface. This still allows both attached flow and a flow from behind filling in, the later being what happens in stall. Viscosity prevents the later at lower angles – viscosity prevents the air from taking sharp turns, which is what fixes the rear stagnation point to the trailing edge. $\endgroup$
    – Jan Hudec
    Commented Aug 13, 2020 at 19:48

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