Outer flow and boundary flow are often different.

Do tufts at the wing surface show boundary layer flow or outer flow?

Picture source.

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Paint shows boundary layer flow.

Picture source

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    $\begingroup$ It does show boundary layer flow, that's why it can indicate when the flow is seperated. also, these things stick right to the surface - they physically cannot show the "outer flow". (to show outer flow, they will have to be placed a few centimetres away from the surface.) $\endgroup$ Dec 26, 2022 at 23:35
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    $\begingroup$ @AdityaSharma Please post answers as Answers, and not as Comments. $\endgroup$
    – Ralph J
    Dec 27, 2022 at 1:02
  • $\begingroup$ @RalphJ In my opinion, it was not fit for an answer, so I posted it as a comment. $\endgroup$ Dec 27, 2022 at 2:20
  • $\begingroup$ If you really want to understand this topic, read Doug McLean Understanding Aerodynamics: Arguing from the Real Physics. This book will answer your question and just about any other question you can think of, including how to interpret flowviz and the differences between streaklines and flow outside the boundary layer. $\endgroup$ Dec 27, 2022 at 20:01

2 Answers 2


The boundary layer being the layer of flow with less velocity than the free stream, and which can be from nearly zero at a leading edge to an inch or two thick at a trailing edge, you can say that tufts are generally showing boundary layer flow except on or near leading edges where the flow is laminar and the boundary layer is so thin, the thickness of the tuft may put part of it in the free stream.

But as soon as the laminar part of the boundary layer ends and the boundary layer is thicker than the tuft itself, the tuft is pretty much immersed in the turbulent boundary layer. As you go farther aft where the turbulent boundary layer gets thicker and thicker, the tufts may lift off the surface a little bit, hinting at the increased thickness of the BL but not indicating how thick it actually is.

  • $\begingroup$ "part of it in the free stream" not exactly free stream - free stream flow is one that is unaffected by the presence of the aircraft. The flow that you are talking about is affected by aircraft's presence, just that it's not affected by the viscosity effects. Perhaps that's why the questioner used the term "outer flow". Also, boundary layer is not "flow with less velocity than free stream" - a more appropriate definition would be "flow affected by the viscosity effects". $\endgroup$ Dec 27, 2022 at 4:25
  • $\begingroup$ But at a leading edge where the flow is laminar the boundary layer can get down to .020" or so, and a tuft stuck there that is a piece of 1/16th inch yarn will be technically partly in the free stream. $\endgroup$
    – John K
    Dec 27, 2022 at 4:40
  • $\begingroup$ It will be out of the boundary layer, but not in free stream. The laminar, out of the boundary layer flow at the leading edge still cannot be considered "free stream" because it is affected by the presence of the wing (it is affected not by the viscosity effects, but by the pressure effects - it does not have the same static pressure, dynamic pressure and vector [of dynamic pressure] as it would in the absence of the wing). $\endgroup$ Dec 27, 2022 at 5:05
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    $\begingroup$ I'd also use the term "outer flow" instead of "freestream". Freestream is the flow free, i.e. not modified by the the presence of the aircraft. $\endgroup$
    – sophit
    Dec 27, 2022 at 11:04
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    $\begingroup$ @JohnK: I perfectly understand your point but confusing terminology doesn't help in a site like this one. And pitot tubes are not in the freestream, that's why they need to be calibrated according to where they are placed on the fuselage. The only pitot tube lying in the freestream that I can think about is this one. $\endgroup$
    – sophit
    Dec 27, 2022 at 17:53

Comments after @JohnK's answer prompt an observation: it depends on the Angle of Attack.

The whole point of having "tufts" in research is to explore areas in the flight envelope where we don't have these nice neat definitions of airflow properties.

Airflow by any name creates lower pressure over the top of the wing by constantly "wicking" air away from the surface (Coanda effect). This "vacuum pump" only works if air is taken away at an equal or faster rate than it can leak back in.

So we have a flow velocity gradient and a pressure gradient as one moves away from the wing surface which maintains the lifting force.

As John mentioned, the boundary layer thickens and becomes more turbulent towards the rear of the wing. Tufts can show this by "wiggling" a bit, but by maintaining their rearward orientation, they are showing the velocity gradient is still intact.

At stall, the pressure gradient breaks down and wing surface airflow becomes chaotic and/or reversed. What better way to show this than with a tuft flopping all over the place.


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