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. $\endgroup$ – Peter Kämpf Jul 31 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 Jul 31 at 12:01
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    $\begingroup$ There is another answer on flow separation here. Separation only happens when the flow at the surface is reversing. $\endgroup$ – Peter Kämpf Jul 31 at 18:06
  • $\begingroup$ @David, that's where viscosity enters the picture. Viscosity prevents steep velocity gradients in the fluid, so the fast flowing air over the top of the wing forces the air over the receding part of the wing to also flow even if at slightly slower speed. Only when the angle becomes too steep for viscosity to keep the air moving, then can a pocket of stagnant air form near the surface and separate the main flow. $\endgroup$ – Jan Hudec 2 days ago

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.

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