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In this answer, it states that the deceleration of air in the region of pressure rise over a wing acts in a direction perpendicular to the lines of relative chord, explaining how spanwise flow is formed.

To be precise: The acceleration and deceleration acts in a direction perpendicular to lines of the local relative chord.

My main question is does the air decelerate in the vertical axis too? Imagine taking the spanwise flow and flipping it 90 degrees so it was pointing up. In other words, I'm asking if there is a flow component facing upward in the region of pressure rise. (because the acceleration and deceleration of air acts in a direction perpendicular to the lines of relative chord, and the vertical axis would be perpendicular) Below is an image to convey what I mean about vertical flow component. (Feel free to ask for clarification in the comments, it's probably hard to understand what I'm asking.)

Here is an edit that I just did, because I now have another question : Does the air decelerating vertically contribute to stalls? (the air decelerating vertically towards the trailing edge)

enter image description here

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  • $\begingroup$ Actually fascinating that, what is usually referred to as "downward acceleration", is being referred to here as "upwards deceleration", but OK! Studying stalling onset of swept wings vs straight wings may help. Interesting that the airflow is also "bent" by the wingtip vorticies. $\endgroup$ Jan 9 at 10:37
  • $\begingroup$ @RobertDiGiovanni well I had assumed that if air decelerates sideways for spanwise flow, it would also decelerate upward. $\endgroup$
    – Wyatt
    Jan 9 at 15:42

2 Answers 2

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In this photo, the streamlines just above the airfoil near the trailing edge are spaced more widely. So, as long as there's no vacuum there (as there might be for hypersonic flow), the airflow there must be "moving up" relative to ambient, as the red arrow suggests. That air has to come from somewhere.

A chord length or two above the airfoil, the spacing returns to ambient, so this upward flow has vanished by then. Thus, the upward flow must eventually decelerate to zero. Similarly, because the upward flow has to start from somewhere, it must begin with an upward acceleration.

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  • $\begingroup$ Ah I see, thanks. I'm going to copy/paste question from comments on other answer to here just to get your point of view : Say you had a swept wing, that was experiencing spanwise flow. Would the vertical component of the flow that you described be less, because some of the energy needed to do that is being used for decelerating air sideways for spanwise flow? (if that doesn't make sense I'll happily clarify it) $\endgroup$
    – Wyatt
    Dec 3, 2023 at 3:06
  • $\begingroup$ So, given two wings, identical except that one is rectangular, the other swept, are you asking if the upward flow near the trailing edge is less for the swept one? (Never mind the reason; just measure it in a wind tunnel, or pummel it with Navier-Stokes simulation.) $\endgroup$ Dec 4, 2023 at 5:51
  • $\begingroup$ yes that is correct $\endgroup$
    – Wyatt
    Dec 4, 2023 at 18:12
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From the picture it can clearly be seen that air does accelerate vertically. If not, it would have just continued to travel on a perfectly horizontal line. Instead, due to the presence of the airfoil, it get pushed upward and downward i.e. it gets accelerated upward and downward.

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  • $\begingroup$ ah okay thanks! One small question that might be hard to understand : Say you had a swept wing, that was experiencing spanwise flow. Would the vertical component of the flow that you described be less, because some of the energy needed to do that is being used for decelerating air sideways for spanwise flow? (again, if that doesn't make sense I'll happily clarify it) $\endgroup$
    – Wyatt
    Dec 2, 2023 at 20:53
  • $\begingroup$ this is a late reply, but another little question popped into my head. Does the air decelerating vertically contribute to stalls? $\endgroup$
    – Wyatt
    Jan 15 at 3:57
  • $\begingroup$ Air decelerating along the rear portion of the airfoil is the one mainly contributing to the detachment of the boundary layer i.e. to the stall. $\endgroup$
    – sophit
    Jan 15 at 5:09
  • $\begingroup$ I see, thanks. I was thinking it might be the whole reason wings stall in the first place. $\endgroup$
    – Wyatt
    Jan 15 at 5:12

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