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I was wondering if all of the low pressure created by the airfoil gets recovered on the rear side of the wing. (I know during a stall or flow separation, this doesn’t happen, I think)

My main question is if you put a pressure sensor right behind the trailing edge of an airfoil, would the pressure be the same as ambient, higher, lower? How about if you put a pressure sensor in the wake of the wing, what would that say? Does AoA or speed affect anything?

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  • $\begingroup$ I put it in a comment because it doesn't answer all of your question. No, it doesn't recover completely because of the boundary layer that doesn't allow the airflow to close at the trailing edge. $\endgroup$
    – sophit
    Commented Nov 21, 2023 at 7:04

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[Does] all of the low pressure created by the airfoil gets recovered on the rear side of the wing?

Only in inviscid flow. Then we speak of a rear stagnation point.

In reality, viscous effects cause enough energy loss that the static pressure at the trailing edge rises only a bit above its ambient value from the confluence of upper and lower flow and then quickly drops to ambient pressure when you follow the streamline from the trailing edge into the wake. See below for the XFOIL pressure coefficient plot of the HQ-17 airfoil.

Pressure distribution around the HQ-17 at Re = 1 Mio.

If you put a pressure sensor right behind the trailing edge of an airfoil, would the pressure be the same as ambient, higher, lower?

That depends on the direction in which the sensor is pointing. Normally, those sensors are placed in flow direction so they measure the total pressure (sum of static and dynamic pressure). A rake of those is used to measure the pressure distribution over the height of the wake which is a very good way to determine the drag of airfoils. In the wake, total pressure is quite a bit below its freestream value due to the slowing down of flow speed in the boundary layer.

If you point the sensor in a direction orthogonal to flow speed, it should measure static pressure. With attached flow the pressure would be slightly higher than ambient and with separated flow it would drop below ambient pressure.

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  • $\begingroup$ thanks for your answer. “In the wake, total pressure is quite a bit below its freestream value due to the slowing down of flow speed in the boundary layer.” I thought the boundary layer was just a slowed down layer that experiences the effect of friction? Wouldn’t the pressure in the boundary layer still recover on the rear side of the airfoil? (My assumption was that the pressure in the boundary layer was higher than the outer flow, because of it not moving as fast, so higher pressure) $\endgroup$
    – Wyatt
    Commented Nov 21, 2023 at 17:45
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    $\begingroup$ @Wyatt there is a speed dent in the wake which gets broader and shallower downstream until it dissipates completely. A bit of the airplane speed continuously gets transferred into a big chunk of air. $\endgroup$
    – Peter Kämpf
    Commented Nov 21, 2023 at 17:54
  • $\begingroup$ I see. A little late to comment, but coming back to this answer I had a small question. So I understand how high pressure is made when the upper and lower surface flows confluence at the trailing edge, but how is that related to viscous effects? I thought viscous effects just means the friction between the fluid layers. In other words, how would inviscid flow make the high pressure not happen? $\endgroup$
    – Wyatt
    Commented May 1 at 2:16
  • $\begingroup$ @Wyatt Inviscid flow produces the rear stagnation point, so it does "make the high pressure happen". With viscous effects the energy in the flow is reduced so pressure recovery remains incomplete. $\endgroup$
    – Peter Kämpf
    Commented May 1 at 8:19
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    $\begingroup$ @Wyatt Remember what happens when flow separates. Here the deceleration in the boundary layer is less severe, so the air still flows downstream, but slower than ambient. At the trailing edge this remaining speed produces only a minor pressure maximum, and total pressure in the wake behind the wing will show a bucket where the ex-boundary layer is. $\endgroup$
    – Peter Kämpf
    Commented May 1 at 18:28

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