I dont understand why we have these "vortex generators" to trip the boundary layer into becoming turbulent...i've seen this alot 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
Sep20-08, 02:49 AM
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
Sep21-08, 06:09 PM
I dont 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
Sep30-08, 03:53 PM
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 doesnt 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
Oct1-08, 05:30 AM
You do know that the boundary layer doesn't actually exist right?
??
what does that mean?? of course it does exist.
minger
Oct1-08, 09:54 AM
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
Oct1-08, 11:16 AM
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
Oct1-08, 12:28 PM
the boundary layer doesnt 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
Oct1-08, 12:35 PM
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?
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 dont 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??