Transition from laminar to turbulent flow ?

[stiwari98]

hey guys i have read in numerous books that if u have an airfoil with greater camber the transition from laminar to turbulent flow takes place closer to leading edge than normal as compared 2 airfoils with lesser camber .why is this so ???

[HerkS]

Transition from laminar to turbulent boundary layer is influenced by a lot of different factors. I would think camber as a factor would be at the end of the list if it's on there at all. Below critical RN, increasing the camber of a specific airfoil is as likely to promote separation as transition.

I don't believe I've ever seen a book or article that made this kind of statement.

More important, can you be more specific about your interest in boundary layer transition and what kind of aircraft application are you thinking of?

[stiwari98]

hey herk thanks fr ur reply .......well 2 put it differently i wud love 2 noe the factors that affect the shift in the critical point of transition of flow from laminar to turbulent flow ......if i m nt wrong 1 such factor is the skin friction drag ...greater it is the closer the transition takes place to the leading edge ....

[HerkS]

stiwari98 -- Welcome to the groups.

Drag of an aircraft comes from two sources. The first is the sum of the pressure distribution over the entire surface. Second is the sum of the shear forces between the skin and the airflow.

That second item is the one you are talking about. The nature of the boundary layer affects the shear forces - with generally lower shear forces where the flow is laminar and higher where the flow is turbulent.

However it's not nearly that simple. A laminar boundary layer separates easily from curved surfaces like the top of an airfoil. When the boundary layer separates, the flow goes with it. Turbulent boundary layer is more "sticky" and stays attached to the surface longer than a laminar boundary layer under the same conditions. When the flow separates from the top surface of an airfoil, lift takes a big hit and pressure drag increases pretty dramatically.

So, for small light aircraft like models, we often force transition with turbulator devices to help delay separation - because the drag (and lift) penalty from separation is way bigger than the drag increase from the turbulent boundary layer.

On a full scale aircraft like a P-51 the designers chose airfoils that had extended laminar boundary layer flow within a fairly narrow lift range. This does reduce drag when the wing is operating within that range - often referred to as the drag bucket. This would be useful on a model only if the model's primary purpose was very high speed flight.

I think I've written an article that talks to this in more detail. If I can dig it up, I'll post it for you. If you have a question, put it up here -- Herk

[BMatthews]

Stiwari98, the bottom line is do NOT expect a model to have laminar flow for any significant portion of the airfoil ever. There's far more to it than the camber and thickness. But mostly it's due to the fact that our normal model construction techniques cannot produce laminar flow for more than perhaps 20% of the airfoil under excellent conditions. To make it last longer will require higher reynolds numbers and accuracy that is just not practical for regular model building techniques. You'd be looking at female CNC cut molds and full composite construction to guarantee laminar flow for even 50 to 60% of the chord at most model sizes

[Montag DP]

Stiwari98, the bottom line is do NOT expect a model to have laminar flow for any significant portion of the airfoil ever. There's far more to it than the camber and thickness. But mostly it's due to the fact that our normal model construction techniques cannot produce laminar flow for more than perhaps 20% of the airfoil under excellent conditions. To make it last longer will require higher reynolds numbers and accuracy that is just not practical for regular model building techniques. You'd be looking at female CNC cut molds and full composite construction to guarantee laminar flow for even 50 to 60% of the chord at most model sizesI'm a little confused about this - why do you say you'd need higher reynold's number for more laminar flow? Doesn't higher Re make for more turbulent flow?

[HerkS]

Here are a couple of columns I've written on the subject.

They are somewhat old, but the dynamics of airflow haven't changed - so the info is still relevant. :rolleyes:

[stiwari98]

hey herk thanks a ton fr the document u posted .....u really seem 2 b a experienced man in aeronautics ......but what really baffled me is the althaus and volker results regarding separation ......at lower angle of attack the fluid sticks not so easily as compared 2 higher angles of attack...what abt the pressure gradients formed at higher angles f attack dat promote flow separation (r vortex formation)...

[bwalt822]

hey herk thanks fr ur reply .......well 2 put it differently i wud love 2 noe the factors that affect the shift in the critical point of transition of flow from laminar to turbulent flow ......if i m nt wrong 1 such factor is the skin friction drag ...greater it is the closer the transition takes place to the leading edge ....


You ask a good question. The reason that higher cambered wings would cause turbulent flow sooner is because higher camber means more lift becuase the air has been accelerated to faster speeds on the upper surface. Higher speeds means higher reynolds numbers. Reynolds number also increases with distance along the wing skin. When you hit a certain reynodls number the flow transitions from laminar to turbulent.

You can also trip the boundary layer into becoming turbulent by having a bump or rough spot on the wing.

[HerkS]

hey herk thanks a ton fr the document u posted .....u really seem 2 b a experienced man in aeronautics ......but what really baffled me is the althaus and volker results regarding separation ......at lower angle of attack the fluid sticks not so easily as compared 2 higher angles of attack...what abt the pressure gradients formed at higher angles f attack dat promote flow separation (r vortex formation)...

One of the things that promotes transition from laminar to turbulent flow is anything that produces rotation of the air in the boundary layer. That is essentially what a turbulator strip does.

At higher angles of attack, the flow around the leading edge does the same thing. The dividing point between the air that goes over the top and that which goes over the bottom of the airfoil is called the "stagnation point." At high angle of attack the stagnation point is actually below the leading edge. So the flow going over the top of the airfoil takes a tight turn right at the leading edge.

That turn introduces rotation in the boundary layer that promotes transition. When you look at the drag polar charts you can see that the low RN flow separates at mid-lift causing high drag, but then re-attaches at higher lift - lowering the drag. Highly effective airfoils for very low RN conditions usually have a small leading edge radius which helps promote transition throughout the lift range.

[stiwari98]

hey herk,thanks once again .of all the books and sites which i have gone through till date have talked abt boundary layer separation in terms f skin friction drag and wake turbulence and dat flow starts separating when the velocity of the fluid layer part of boundary layer reverses when compared 2 free stream velocity....ur theory seems 2 deal with it in a different manner which is very much interesting me ....cud u pls suggest me books r links which could give me in depth analysis of this theory ... cheers!!!

[HerkS]

stiwari98 -- This is a really technical subject - but if you dig into it, you can develop a pretty good understanding of what's going on.

You might try this website as a starting place

http://www.mh-aerotools.de/airfoils/

You might also go to my website and download SoarTech 3. It contains an article on low speed airfoil design by Michael Selig. (starting about page 29).
Here is the link:

http://www.soartech-aero.com/SoarTech-Online.htm

[nmasters]

Herk, thanks for the articls.

stiwari98, You can see the things Herk is talking about yourself with software like XFLR5 (http://www.rcgroups.com/forums/showthread.php?t=751591)or Profili (http://www.profili2.com/eng/default.htm). I've marked the stagnation point and laminar separation bubble on this plot of the pressure distribution from Profili. Since pressure can only exert a force perpendicular to the surface that it acts upon you can always find is on the pressure distribution chart by measuring the vectors to find the longest one. I believe the angle of this vector is also the upwash angle.

--Norm

[banktoturn]

hey guys i have read in numerous books that if u have an airfoil with greater camber the transition from laminar to turbulent flow takes place closer to leading edge than normal as compared 2 airfoils with lesser camber .why is this so ???

Hello Stiwari,

There are 3 primary factors that influence where the boundary layer becomes turbulent: surface quality, Reynolds number, and the pressure gradient. A smoother, less wavy surface tends to delay the transition. A higher Reynolds number tends to encourage the transition. An "adverse pressure gradient" tends to cause transition. The phrase "adverse pressure gradient" describes the situation in which the air is flowing from a low pressure region to a high pressure region. Since the lowest pressure on the surface of the wing tends to be near the point of the wing's maximum thickness, the flow tends to encounter an adverse pressure gradient near that location.

The Reynolds number depends on the airspeed and dimensions of the wing, and model aircraft have much lower Reynolds numbers than full sized aircraft. In principle, this should make it easier for a model-sized wing to maintain a laminar boundary layer. The surface quality is determined by the building technique, and as someone else pointed out, it is difficult to build model wings with really good surface quality. As I mentioned above, the boundary layer flow will experience an adverse pressure gradient from approximately the thickest part of the wing to the trailing edge. This is why airfoils that are intended to maintain laminar flow usually have the thickest point further back than conventional airfoils.

banktoturn