Measuring transition on aerofoil

[eng1234]

Hi,

Does anyone have any suggestions on how I could measure where the boundary layer transition is over an aerofoil. Preferably by flow visualization techniques. The flow will take place in a low speed wind tunnel.

thanks

 

[boneh3ad]

There are two methods you can use that I have done before using only visualization techniques. There are others that you can do with measurements but they don't seem to fit your request.

Naphthalene sublimation
One method I've used is to dissolve naphthalene in acetone, put it in a paint sprayer, and then lightly coat your airfoil. The acetone evaporates quickly, leaving a thin layer of naphthalene behind. Naphthalene sublimates very near normal room conditions, so in regions of higher shear stress, the slight heating causes it to sublimate faster, so if you let the tunnel run for a few minutes, you will start to see the transition pattern as well as many other shear-stress-related features such as streaks from crossflow vortices on a swept wing. There is quite a bit of literature on this that you can look up. Check Journal of Fluid Mechanics, Physics of Fluids, AIAA Journal, and the various AIAA conference proceedings. If you go this route, make sure to wear protective clothing and some sort of chemical mask, because naphthalene is pretty nasty.

Infrared thermography
If your airfoil is either made out of a nonmetal or else can be painted/powder coated or covered with some kind of insulative, smooth coating, then you can look at the surface with a IR camera and see similar patterns to what naphthalene reveals. Probably the best paper I know of that discusses this method is by Crawford et al. (2014) from the AIAA SciTech meeting.

 

[Henryk]

Other methods

1. Use a fog generator (buy it in a Halloween store)
2. Resistive wire wetted with oil. Oil will form droplets on the wire. When you apply enough current to the wire it will start burning the oil and you can see the smoke very easily.

 

[boneh3ad]

Here's the thing, though: putting smoke generating devices in the flow path can affect the transition location.

 

[OrangeDog]

What about pressure sensitive paint? I believe Boeing used to use that on their models.

 

[boneh3ad]

Pressure doesn't change appreciably when a boundary layer transitions so PSP wouldn't help. Temperature-sensitive paint would a better approach. It would essentially be equivalent to the IR approach.

 

[OrangeDog]

True, but I thought PSP was pretty reliable for locating separation points. Also, why would temperature change significantly? I could see how the conduction and convection coefficients would change but if there is no temperature gradient then isn't this a moot point?

 

[boneh3ad]

It is effective at locating separation points because the pressure inside a separation bubble is dramatically different than in an attached boundary layer. This isn't true with a simple transition from a laminar boundary layer to a turbulent one. The pressure would change slightly owing to a change in displacement thickness (and therefore the effective shape of the airfoil), but it generally isn't very much.

Temperature may or may not be much different, but you can control that. You can, for example, heat (or cool) the model slightly so that there is still heat transfer to (from) the surface, and then when the boundary layer transitions, that heat transfer increases and it will be noticeable. With both IR and TSP, as long as the surface is not too conductive of heat, this allows a temperature gradient between the laminar and turbulent regions to exist and be visualized. In the case of a swept wing, you can even see temperature gradients that exist as a result of differential heat transfer resulting from crossflow vortices in the laminar region as well (see the paper I linked above, for example).

 

[OrangeDog]

Ok, so how does one account for free convection? I would imagine it would be negligible if the freestream velocity is significantly higher than what would be expected from free convection alone.

 

[boneh3ad]

You don't need to get the surface glowing red hot or anything. Just 1 K different from the free stream would probably work. Even a few K would be better without causing any issues. Sure, if the temperature difference was large enough, you might see an appreciable effect on the boundary layer thickness that you may be able to attribute to "free convection", but it would still be a fairly small effect, especially compared to the effect of the free-stream velocity.