Air velocity and stall angle...

[alipourzand01]

Hello;

I'm testing an aerofoil in a wind tunnel to see the stall angle and effect of vortex generators.

1 - I'm using a NACA 6313 aerofoil, when testing it in my company's low-speed when tunnel (when it's plain i.e. no VG's on it) at low air velocity (25 m/s) the aerofoil stall at 13 degrees angle of attack which seems correct however when increasing the air velocity to 45 m/s the aerofoil doesn't stall untill 18 degrees AoA.

I know that the stall angle is not dependent on the air velocity so can anyone think of anything that could explain this behaviour.

2- The second issue I've got is with lower air velocity. As the aerofoil stall, at 15 degrees AoA onwards the lift start increasing again (with a very small margin), based on my understanding after the stall the lift should keep decreasing. what could be the reason for this?

PS1: All test have been done multiple times to ensure the accuracy and same result came out in both cases.

PS2: The wind tunnel is designed for automotive engineers and the software that output the data is specifically written for automotive engineers. But I'm not sure HOW it could effect my results.

Any help is highly appreciated.

Ali

 

[crysta1c1ear]

I know that the stall angle is not dependent on the air velocity ...

From my paragliding lessons, which I took in Swiss Germany, applying the brakes for too long can cause a "Sackflug". The brake lines are attached to the back of the wing and applying them basically increases the angle of attack.

So I would say that stall angle is dependent on air velocity. Going fast the paraglider flies. After about 10 seconds of full braking it starts to drop a bit like a parachute. The speed of decent is much faster and you could easily break legs etc if you hit the ground while dropping like a parachute in a "Sackflug".

I scared my instructors by going into a Sackflug once. The transition from flying to dropping is a smooth one. The wind noise disappears gradually as the chute ceases to fly and the calmness of the fall lulls many people into a false sense of security. Indeed the first I really knew aout it was a violent tug upwards during a later sharp turn, where the chute started to fly again.

=

The same I think is true of planes. From my flying lessons, if you fly slower and slower eventually the plane stalls; that is when going fast the stall angle is larger than the actual angle of the wing and the plane flys; go too slow and the stall angle is less than the cuurent angle of attack and the plane drops.

As the aerofoil stall, at 15 degrees AoA onwards the lift start increasing again (with a very small margin), based on my understanding after the stall the lift should keep decreasing. what could be the reason for this?

My understanding of stalling is that the airflow over the top of the wing becomes turbulent if the angle of attack is too large and this results in a loss of "suction" above the wing.

Air under the wing is still hitting it at an angle and pushing the wing upwards. Increase the angle and the air hits at a bigger angle giving more lift.

(Play a fine cut with a snooker ball and the cue ball will keep most of its energy. The thicker you cut your shot, the more energy the cue imparts into the object ball.)

A greater angle of attack should give more lift and more drag, up to a point, then lift should drop off as the angle of attack approaches a flat board aainst the wind with just drag and no lift.

=

I hope that helps, but this is from someone who has flown a few times, not a wing designer!

 

[alipourzand01]

Thanks CrystalClear

I will do some more research on relationship between a Stall angle and the air velocity, but what you saying does make sence to me.

about my second issue. I'm not sure exactly what you mean.
Do you mean at higher AoA the air hitting the bottom of the aerofoil would actually push it up thus increasing the lift a little bit?

I appreciate some other comments regarding these issues.

 

[Disgruntleddave]

Hi,

I'm not a professional - I'm currently in my last year of mech. eng. undergraduate, however I'm involved in a decent bit of airfoil stuff and have a couple projects involving them. One has me doing a lot of CFD analysis for airfoils through extreme angles of attack (and the normal ones too).

The general lift and drag characteristics of an airfoil do not depend on wind velocity. Plotting coefficients associated would yield (theoretically) identical results for different airspeeds, until reaching stall angle.

Around stall the airflow becomes turbulent, and essentially stops following the contour of the airflow (on the top). Near stall angle, the point where the air first detaches from the top of the airfoil moves up from near the back of the airfoil to near the front, which will result in your stall. The position of where the smooth airflow detaches is the key thing in the formation of a stall.

If you increase the velocity the slope of lift vs. angle of attack will be the same as at lower angles, but it will not stall until a higher angle of attack. The mechanics of the process are unknown to me, but that's how it is.

As well, once you pass stall angle, your lift will decrease a decent bit and your drag will shoot up, but further increasing the angle of attack will give you coefficients of lift higher than those at regular flying angles. In the case of a symmetric airfoil, it will keep on increasing until about 45 degrees just due to the deflection of air downwards, and will likely exceed the values you can get while flying with regular angles of attack. The drag being huge past the stall angle will also increase until it reaches something similar to that of a flat plate in the wind, which has a drag coefficient of just under 2, as mentioned above.

 

[AminG]

Hello,

Have you verified that the airflow does not change angular direction with airspeed? Another way of saying this is "are you sure that everything else is constant except for airspeed and angle of attack.

Did some wind tunnel testing years ago and found that the wind tunnel was not properly calibrated. There was a flow deflection by as much as 5 degrees at the test section.

 

[alipourzand01]

Hello,

Have you verified that the airflow does not change angular direction with airspeed? Another way of saying this is "are you sure that everything else is constant except for airspeed and angle of attack.

Did some wind tunnel testing years ago and found that the wind tunnel was not properly calibrated. There was a flow deflection by as much as 5 degrees at the test section.

Thanks Amin but the wind tunnel was calibrated 2 days before i take the test by a professional so I assume there was no flow deflection. However I keep that in mind and check it out again.

Disgruntleddave

Thanks for the reply it seems you are confirming my outcome regarding the increase in stall angle when increasing the air velocity but don't the reason.
At least assure me I've done the test properly.
But if you could get a bit of more info for me that would be highly appreciated.

Regards
Ali

 

[bitstar]

Hello Ali,

of course, there are relations between AoA and the velocity and at least the circulation of the airfoil. please search for Kutta-Joukowski! and u'll find the solution of your problems.

Best regards
bitstar

 

[crysta1c1ear]

Do you mean at higher AoA the air hitting the bottom of the aerofoil would actually push it up thus increasing the lift a little bit?

Yes, that is exactly what I am saying.

If you increase the velocity the slope of lift vs. angle of attack will be the same as at lower angles, but it will not stall until a higher angle of attack.

This seems to agree with my opinion that stall angle is velocity dependent.

 

[alipourzand01]

of course, there are relations between AoA and the velocity and at least the circulation of the airfoil. please search for Kutta-Joukowski! and u'll find the solution of your problems.

Thanks my friend, it seems everyone having a same opinion regarding the stall angle and velocity, but I still can't understand how is that possible.

I mean yeah lift is directly related to velocity but I can't see how this change in speed could change the stall angle.

It's happening and everyone can confirm it, but i don't see how any explaination is much appreciated.

crysta1c1ear

Thanks alot, it seems the answer for increase in lift was much easier to verify and understand.


any more input is appreciated.

 

[swearingen]

Stepping away from the mathematics and just trying to envision what is happening helps me in many engineering situations. If I focus on the area above a high AoA wing, just aft of the leading edge, and imagine a velocity of air that does not separate from the top of the wing, the low pressure region overcomes the inertia of the air and changes its direction down over the top of the wing. I can see that as the velocity would increase, the inertial mass of the air would try to keep it from changing direction and flowing down the back of the wing until finally the air separates and you have a stall. At that new velocity, to regain laminar flow, I would have to reduce the AoA.

Intuitively, if I think of it this way, it's clear to me that velocity would certainly have an effect on stall angle.

 

[bitstar]

hi ali,
actually the way to investigate the stall angle is to determine the polar curve ca(alpha) depending on Reynolds (velocity). and that curve is specific for each profile and also for your NACA6313. so the best thing u can do is to get this specific curve..

best regards
bitstar

 

[Disgruntleddave]

The problem with looking at the kutta-joukowski stuff is that it was only established and founded for working with low angles of attack. At least to the extend I learned about it, there may well be much more to the theory on top of the 'circulation at trailing edge = 0' thing.

I've been trying to think of the physical mechanics behind the dependence of stall angle on velocity. One thing I thougt about is maybe as reynolds number increases, it becomes more difficult for it to transition to turbulent flow and have the laminar flow detach from the airfoil surface. This is counterintuitive to me because higher reynolds numbers directly corresponds to the fluid being more succeptible to turbulent versus laminar flow.

Maybe as higher RE, the suction holding the airflow to the top surface of the airfoil (maintaining more or less laminar flow) increases at a rate larger than that which would prompt the flow to transition to turbulent. Maybe transition to turbulent flow is dependant to some degree on pressure, and the variation of pressure on top of the airfoil with RE high enough to keep the flow laminar until it becomes more difficult for the airflow to stay laminar (with further increased AOA).

Another option might have to do with the trailing vortices off of the end of the airfoil at higher AOAs. Apparently at stall speed there are oscillating sets of vortices (above and below the trailing edge of the wing). Maybe these play a role in prompting the air to detach and transition, and with higher wind velocity they act differently in such a way that they don't prompt transition until more extreme angles of attack.


All guesses, probably all, or mostly wrong. I'll ask some profs of mine and see if they know the mechanics behind it.

 

[btrueblood]

Close, Dave. Increasing Re pushes the laminar-to-turbulent boundary layer transition closer to the nose of the airfoil. Turbulent b.l. actually improves the stall angle of an airfoil, because with turbulent b.l. the flow stays "attached" under more adverse pressure gradients (higher AOA).

Take a look at drag characteristics of a cylinder or sphere in cross-flow in any good fluid mechanics book. There is a drag decrease around Re = 10,000 to 100,000 that occurs because the b.l. has transitioned to turbulent, the turbulent b.l. stays attached farther around the sphere, and the detached wake is decreased.

 

[alipourzand01]

Thanks everybody.

So unlike what i used to think the stall angle is actually dependent on air velocity, which proves my results were correct. (higher speed , the higher stall angle).

and RE is propotional to velocity so higher velocity the higher RE. is that correct?

 

[rb1957]

stall speed (ie Max CL) is slightly dependent on velocity (in that it is dependent on Reynolds number).

yes, reynolds number is proportional to velocity (amongst other things)