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As I wrote last week, I'm doing a Benedek 10355 airfoil performance report. At this point of the work I am analyzing the stall of the profile at a fixed Reynolds number. The Benedek 10355 airfoil has a thickness of 10.1%. Figure 1 shows the polar calculated with Xfoil at Reynolds number = 500,000.

Figure 1

Analyzing this curve, and comparing it with this answer, it seems to me that we are in the presence of a trailing edge stall. In fact, after the stall angle ($\alpha_{stall} = 11.0°$), the decrease in the lift coefficient is gradual ... Then, however, at about $\alpha = 21°$ there is a sudden change in lift, as if a laminar bubble had "burst". Figure 2 shows the separated zone for $\alpha = \alpha_{stall} = 11.0°$.

Figure 2

Looking at the pressure coefficient, there appears to be a laminar bubble very close to the leading edge (Figure 3). As the incidence increases, this bubble moves more and more towards the leading edge and, when $\alpha = 22°$, it "disappears". This makes me think that maybe at low incidences there is a trailing edge stall and then, going further, a leading edge stall ... Maybe I'm missing something? Thanks.

Figure 3

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    $\begingroup$ So what you're asking is where flow separation's happening? $\endgroup$ – Camille Goudeseune Nov 21 at 18:15
  • $\begingroup$ @CamilleGoudeseune, yes I am asking where the laminar flow separation is happening and if we can talk about a combined leading edge-trailing edge stall. $\endgroup$ – leo95nf Nov 21 at 18:19
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The Benedek 10355 clearly stalls with separation starting from the trailing edge.

There is some discussion whether a laminar-to-turbulent transition due to rising pressure will always involve a laminar separation bubble, however short. But that bubble does not constitute a leading edge stall.

XFOIL is not meant for flow simulation deep into the stall region. It does amazingly well to capture the initial stall but cannot possibly model the extended separated flow well into the stall accurately. I am actually surprised that it still produces converging results at such high angles of attack.

In short, I would not expect the XFOIL results above 12° AoA to model accurately what is happening in reality. Especially that sharp drop in lift at around 21° looks very much like the code settling on another solution but not something that can be expected in real life.

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  • $\begingroup$ thank you for helping me this time too. What do you suggest is best for my report? Stop at 12-13° or show this "anomalous" Xfoil behavior? I have to do a post-stall study, that's why I'm interested in going beyond $\alpha_{stall}$. Thanks again $\endgroup$ – leo95nf Nov 22 at 10:54
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    $\begingroup$ @leo95nf: I wonder who told you to do a post-stall study with XFOIL. Of course you may describe what you found, but be sure to add a disclaimer that real results may very well be different. $\endgroup$ – Peter Kämpf Nov 22 at 16:02
  • $\begingroup$ Thank you again! $\endgroup$ – leo95nf Nov 22 at 18:31

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