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A wind tunnel boundary layer experiment was done for a flat plate with 5 stagnation probes distributed horizontally (each probe has 5 vertical sensors) and a variable flap, for 2 different free stream velocities "V=20 m/s" & "V=25 m/s" and 2 different cases:

  1. without the variable flap open;
  2. with the variable flap (the angle is approximately 25°).

A picture of the test setup is shown below: enter image description here

A schematic of the setup: enter image description here

The pressure values over the plate were collected from the sensors, then the local velocity was calculated using $$\Delta P = \frac{1}{2} \rho v^2$$ and the 2 graphs below were plotted for each case: enter image description here enter image description here However, as you can see in the case of a flat plate with a variable flat, I got a negative pressure value at sensor 5, which in return gave an undefined velocity value (you could see the sudden drop).

My questions are:

  1. What could be the reason for that negative value?
  2. Is my velocity profile okay? Or is it necessary to look like we all used to see (approximately a parabolic shape) because the one we always see is a curved line towards the top when mine is curved towards the bottom?
  3. How can I tell whether I am on the laminar boundary layer or the turbulent boundary layer based on the graphs? (Or must the Reynolds number be calculated in order to find out?)
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    $\begingroup$ Do you have a picture or a schematic of the assembly? $\endgroup$
    – sophit
    May 23 at 7:31
  • $\begingroup$ @sophit for the moment I don't, but I can provide it, if that helps, the experiment was done in the TecQuipment AF1300 subsonic wind tunnel. $\endgroup$
    – I.ham
    May 23 at 12:49
  • $\begingroup$ A simple schematic is enough, just to understand the setup of the experiment $\endgroup$
    – sophit
    May 23 at 14:20
  • $\begingroup$ I couldn't really provide what you asked for since doing a schematic isn't my strong suit, so I hope this helps so you could help me, here're some pictures for the setup: ibb.co/y5H45R2 & ibb.co/r669FbF $\endgroup$
    – I.ham
    May 23 at 21:28
  • $\begingroup$ I would need a diagram of this experiment apparatus to see what’s going on. $\endgroup$
    – Romeo_4808N
    May 26 at 1:18

2 Answers 2

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1- what could be the reason for that negative value?

Probably a bad sensor. If it were me, I would re-run the experiment but swap two of the probes. If the negative value follows the probe, then it's definitely a bad probe. If it does not follow the probe, then maybe something else more interesting.

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  • $\begingroup$ This is lab work in college, so I can do it just once, but I still could check with the others; if you see both graphs, the one without a flap didn't have such a problem, so I think if the sensor was bad it would show in both graphs, correct? (because it was the same plate only a flap was added). $\endgroup$
    – I.ham
    May 23 at 6:18
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It seems we need to go back to $\Delta$P.

is this the difference in ambient P and lower pressure created by the freestream?

This is what would be expected from the "aspiration" effect of a moving fluid on a pressure sensor.

Your first plot doesn't look like the others. After checking the apparatus to make sure nothing is wrong with the sensors, you may have found ... an area of stagnation, made stronger by the flap.

Your $\Delta$P is based on the effects of freestream velocity. A "negative velocity" calculation is actually ... an area of compression, also known as a high pressure stagnation zone.

Is this not what you are looking for?

Data from the "no flap" run seems to show a reading of lower velocity at both 20 and 25 m/s. Make sure, in your calculations and presentation, that your "$\Delta$P" (and $\Delta$V) is based on either ambient pressure, or freestream pressure.

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  • $\begingroup$ Looking at the setup, my guess is that the probes are mounted on the protruding elements and are perpendicular to the flow, and are all measuring stagnation. The difference between total pressure and ambient would then be used to calculate the velocity $\endgroup$
    – ROIMaison
    May 26 at 11:40
  • $\begingroup$ It just seemed illogic for a stagnation zone to appear so soon, nearly at the leading edge, this is a sample of the data with no flap at 20m/s ibb.co/3CV3cSH the data was collected directly from the computer that's why I had no idea what was going on exactly and what went wrong? so I am just trying to have an explanation, and since I don't know how a correct experiment would go I couldn't make any comparison. $\endgroup$
    – I.ham
    May 27 at 17:35
  • $\begingroup$ @I.ham. so the "odd" data was from the front of the plate, 5 being closest to it. Angle of Attack (of the plate) will also have a lot to do with your results. $\endgroup$
    – Robert DiGiovanni
    May 27 at 19:52
  • $\begingroup$ @I.ham, as the probes are protruding out of the plate, the flow will collide with them and stagnate. $\endgroup$
    – ROIMaison
    May 30 at 6:20

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