# On an aerofoil or circular cylinder why does the airflow stay attached after passing the point of maximum thickness?

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?

• Would this answer help in answering your question? In short: It is suction caused by local curvature and incidence. – Peter Kämpf Jul 31 '20 at 11:17
• @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. – David Jul 31 '20 at 12:01
• There is another answer on flow separation here. Separation only happens when the flow at the surface is reversing. – Peter Kämpf Jul 31 '20 at 18:06
• @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. – Jan Hudec Aug 13 '20 at 19:44