Can transition on an airfoil occur without prior separation?

i.e. can transition occur without the presence of a laminar separation bubble? If yes, what would this look like in a pressure distribution?

Here, a clear laminar separation bubble can be seen.

The Reynolds number has been increased by a factor of 10. The transition point is still visible, but it is not so clear anymore whether there is a separation bubble. Is there a possible scenario where a laminar boundary layer transitions without first separating?

EDIT: I'm not asking about how to force transition in XFoil or in real life (using turbulators), but rather if "pure" transition can occur naturally on an airfoil.

• use a forced transition. Feb 21 '20 at 20:17
• Good reference separation bubble. Reynolds Number indeed seems to be a critical factor, but laminar bubbles may be controlled with leading edge slats at higher AOA and/or with type of airfoil you use. As far as controlling transition, as Peter said ... Feb 22 '20 at 0:30
• And one for models by googling "laminar separation bubbles" by MH-AeroTools. Feb 22 '20 at 1:15
• @Daniel what scale are you working on? Full scale aircraft will be on the order of 2 million up to over 50 million. Slow hand thrown gliders are around 20,000. Feb 22 '20 at 12:47
• Yes, Daniel, a bubble-free transition is completely normal. Only then the transition is forced by roughness instead of increasing Tollmien-Schlichting waves. Again, a forced transition. Feb 22 '20 at 14:16

can transition occur without the presence of a laminar separation bubble?

In short YES.
To elaborate, there are usually 3 mechanisms causing the boundary layer to go turbulent.

1. Natural transition
2. Forced transition
3. Bypass transition

Because the question is about the Natural transition; this is usually caused by sufficiently amplified Tollmien–Schlichting waves. The reason why you see a transition followed by separation in low Reynolds number flow is because in the separated flow, the amplification factor is around 4 times as much as for the attached flow.

If yes, what would this look like in a pressure distribution?

Here, a clear laminar separation bubble can be seen. The Reynolds number has been increased by a factor of 10. The transition point is still visible, but it is not so clear anymore whether there is a separation bubble. Is there a possible scenario where a laminar boundary layer transitions without first separating?

As indicated above, it is possible for a boundary layer flow to make the transition to turbulent without separation. N-factor growth rate scales as $$\sqrt{RE_\infty}$$ for attached laminar flow and for sufficiently enough $$RE$$ you can easily see the transition undergoing without any separation.

Also, it's best to inspect the boundary layer parameters through the VPLO menu which gives a clear indication on whether the flow is actually attached or separated.

A very good reference for this is Prof Mark Drela's Flight Vehicle Aerodynamics. - Chapter 4.

• Let's see Reynolds $10×10^6$ and Alpha 6, 8, and 12 degrees, please. Feb 24 '20 at 5:48
• Great, thanks! And what's bypass transition? Feb 24 '20 at 9:26
• @Daniel Bypass transition is the transition occurring due to the amplification of freestream turbulence carried over to the BL. This is applicable for airfoils working in very noisy environments, such as turbomachinery blades or noisy wind tunnels. Also could be happening if the aeroacoustic frequencies happen to be in resonance with the amplification factors.
– user46017
Feb 27 '20 at 23:22
• @RobertDiGiovanni Not very sure why that suggestion. at 10 Mil in all 3 alfas, it should not appear to force the flow the separate. a good approximation could be come up with if one considers the self similarities. Please check out "Subsonic Airfoil Synthesis" from Prof Wortmann. has lot of insight in that paper
– user46017
Feb 27 '20 at 23:28