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:
- without the variable flap open;
- with the variable flap (the angle is approximately 25°).
A picture of the test setup is shown below:
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:
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:
- What could be the reason for that negative value?
- 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?
- 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?)