Tripping the boundary layer why?

[aerojunkie]

Tripping the boundary layer...why??

I don't understand why we have these "vortex generators" to trip the boundary layer into becoming turbulent...i've seen this a lot on F1 cars. On a regular car you can trip the boundary layer in front of your windshield...does the boundary layer somehow reattach at the top of the hood? Can anyone shed some helpful information on boundary layers/ inducing drag/ and tripping boundary layers?

 

[ank_gl]

No boundary layer doesn't ATTACH later, but pushes down the separating BL downstream, or it delays the major BL separation.
One needs to understand the basics of drag to justify the use of vortex generators. Drag is made up of two parts, viscous drag(skin friction) & pressure drag(form drag). Adding a vortex generator does increase the viscous drag due to transition to turbulent flow, but reduces pressure drag due to delay in BL separation.

Use of any device like vortex generator or trip wire follows an extensive analysis of drag mechanism on the surface in context.

 

[Topher925]

I don't understand why we have these "vortex generators" to trip the boundary layer into becoming turbulent...

You do know that the boundary layer doesn't actually exist right? So there is no sense in referencing it to actual physical phenomenon.

Anyway, vortex generators are mainly used for two reasons. One being to generate turbulent flow and prevent separation as said above. You see this done a lot on older aircraft wings in order to maintain the proper pressure distribution across an airfoil. The second is to prevent transition. Either keeping a turbulent flow turbulent or to transition from laminar to turbulent as quickly as possible in an attempt to avoid/decrease transition. As I am sure you know, when it comes to transition we have no clue on how to predict it or even understand the physics of it so rather than deal with it engineers just like to try to avoid it all together.

 

[mshinavar]

a turbulent boundary layer is more robust (less likely to separate) than a laminar one. as such, F1 cars, airplanes, etc... use it to trip the BL to turbulent just before an area where the flow may otherwise separate. as mentioned above, the separation point is difficult to control, it is better to fix the separation point, usually with a sharp edge.

the boundary layer doesn't actually exist? now i think I've heard everything
VG's on older airplanes to control pressure distribution? sources?
-or because airfoil theory was not much more than flat plate theory back then. at higher angles of attack the wing would experience flow separation, stall, and the plane would crash. keeping a turbulent BL attached at high AOA increased the stall AOA and led to less crashes.

 

[ank_gl]

You do know that the boundary layer doesn't actually exist right?

??
what does that mean?? of course it does exist.

 

[minger]

I had a professor that didn't "believe" in the boundary layer as well. He said that the flow shouldn't need to be split into two regimes. He would ask where the boundary layer "ended" if it did exist. Why not 99.99%, 99.999%?

 

[Topher925]

I had a professor that didn't "believe" in the boundary layer as well. He said that the flow shouldn't need to be split into two regimes. He would ask where the boundary layer "ended" if it did exist. Why not 99.99%, 99.999%?

That's because your professor knew what he was talking about, probably why he's a professor.

Prantdl created boundary layer theory (1904) in order to solve fluid mechanics problems that couldn't be solved with previous methods. The boundary layer is purely a mathematical assumption that is made in order to determine behavior of fluid flows past a submerged object, it doesn't physically exist.

Next time you go down to the fluids lab, try to find the boundary layer around object and take a picture of it for me. I would love to see it.

Vortex generators delay flow separation and aerodynamic stalling; they improve the effectiveness of control surfaces

http://en.wikipedia.org/wiki/Vortex_generator

 

[stewartcs]

the boundary layer doesn't actually exist? now i think I've heard everything

??
what does that mean?? of course it does exist.

The boundary layer is a concept, not a physical object as Topher925 pointed out.

The boundary layer is the thin region near the surface of a body in which viscous effects are important.

CS

 

[FredGarvin]

I think that's a too literal of an interpretation of the term. Take a look at any flow visualization collection. You can clearly see boundary layers in schlieren visuals. We use BL rakes for taking BL measurements in certain tests. If the BL were just a mathematical device, there would be no distinction between a laminar and turbulent BL. If it is just a maathematical construct, why is there a "space" that does have a measureable/visible velocity gradient?

http://history.nasa.gov/SP-4103/app-f.htm

http://www.qcif.edu.au/research/CompressibleFlows.htm
http://media.efluids.com/galleries/boundary?medium=198
http://media.efluids.com/galleries/boundary?medium=65

 

[ank_gl]

That's because your professor knew what he was talking about, probably why he's a professor.

Prantdl created boundary layer theory (1904) in order to solve fluid mechanics problems that couldn't be solved with previous methods. The boundary layer is purely a mathematical assumption that is made in order to determine behavior of fluid flows past a submerged object, it doesn't physically exist.

true, one cannot actually calculate the lamina at which flow attains free stream velocity, but I don't think BL means a layer where velocity becomes .99 or .9999 or whatever times U. BL is a thin region where a velocity gradient exists & hence viscous forces are appreciable.

Next time you go down to the fluids lab, try to find the boundary layer around object and take a picture of it for me. I would love to see it.

what does a contour for velocity show in a CFD analysis of a pipe flow??

 

[prost22]

Pity some got hung up on whether BL is real or not. I agree, it's purely a mathematical construct, invented by Prandtl and his collaborators to make the fancy math in the Navier Stokes equations more palatable.

Nevertheless, here's my take on the OP. It might be easier to visualize by considering just an airfoil in a flowing fluid like air, say in a wind tunnel. The air flows around the airfoil, of course. The boundary layer, a mathematical construct yet a convenient way to visualize what's going on, can be laminar, or turbulent. If you were able to visualize how the particles were moving in that region close to the airfoil, in the BL, you'd see the particles smoothly following the contour of the airfoil, if the BL was laminar. The particles close to the airfoil stay close. If you traced the paths of these particles, the paths would look like 'lamina,' thin layers, that stack next to the airfoil. These paths would look like progressively larger versions of the same airfoil.

What happens if the BL is turbulent? If you traced those particles' paths, the paths wouldn't look quite so orderly; particles would appear to speed up, slow down, move up and down relative to the airfoil surface.

Why is this important? If you were to calculate total drag (as one poster said, two kinds of drag, friction and pressure--I am talking about all forms of drag on this one airfoil), you'd see the optimum case (minimum drag) is the case with a laminar BL--the flow stays 'attached' over the entire length of the airfoil. This is optimum, which you can sometimes achieve with very smooth airfoils. A worse case (higher drag) occurs when the BL is turbulent over the entire airfoil. Even the tiniest imperfection or dust particle or dead bug can 'trip' the BL, which is, cause the laminar BL to become turbulent.

Why would someone intentionally trip the BL? In reality, perfect laminar flow is almost impossible to achieve; you might have laminar flow over half the airfoil, but the BL trips, becomes turbulent, and the flow can detach from the airfoil--if you traced the particles close to the airfoil, before the detachment point, the particles would stay 'close' to the airfoil; after the detachment point, the particles either seem to move completely away from the airfoil, or the particles stay close to the airfoil, but appear to be going in circles, moving away from the airfoil, then back towards the airfoil. When that happens, there is a huge pressure increase in flow next to the airfoil after the detachment point. Because the pressure increases, the drag goes way up. Obviously very bad.

If we trip the BL right away, say near the leading edge of the airfoil, the BL is turbulent over the entire airfoil. This seems bad, however, because the turbulent BL is more energetic (some people like to think in terms of energy--others, momentum) than the laminar BL, the flow will stay attached to the airfoil for a longer distance over the airfoil, sometimes staying attached over the entire length of the airfoil, even under the same conditions which will cause a laminar BL to detach. Because the BL remains attached longer, that huge pressure bubble is smaller than the laminar case (both in physical size and pressure magnitude), so the total drag on the airfoil with a fully turbulent BL is lower than the total drag on the airfoil with a laminar BL over part of the airfoil surface, but with flow detached over the rest of the airfoil surface.

So you are balancing two effects, pressure drag and friction drag. The ideal case, laminar flow over 100% of the airfoil, minimizes total drag, and especially friction drag. The next higher drag occurs over an airfoil with 100% turbulent BL. The worst case (highest drag) occurs when the flow detaches from the airfoil. Back to the race car, the ideal case is impossible to obtain; the worst case is to be avoided, so the compromise is trip the BL, cause the BL to be turbulent, and hoping that the BL doesn't detach at all, but in any case, trying to keep that BL attached as far along the skin of the race car as possible. In your windshield example, if the BL was laminar right up the edge of the windshield and the roof, once you reach the roof, the flow would detach with no hope of ever reattaching. If the BL was turbulent, you MIGHT get the flow to reattach to the car after the flow separates at the windshield--roof edge.

 

[deepthishan]

vortex generators generate vortices which energize the boundary layer making it more resistant to separation and subsequent stalling of the vehicle.
note: as someone said before, the important concept here is that turbulent flow has more energy than laminar flow and is therefore more robust.