Cheesy

Damn its almost impossible to come up with a new idea, I was thinking of experimenting with some vortex generators just before the opening on a half bridge port to see what sort of effect it would have, since its a pretty tight radius to keep flow attatched. The votrex generators would not have a huge effect on port velocity because the boundary layer will almost definitly be turbulant, even if the Reynolds number is low enough for laminar flow it is likely that the manifold joints and such will trip the boundary layer to a turbulant flow. The hardest part about it is the fact that the size of the vortex generators is so small

Lynn E. Hanover

But vortex generators don't all look the same. Use a junk front or rear iron on the flow bench. Drill through the runner and put a small hole in iron on the inside of the runner. Fill the access hole with clay and flow it.

Now try 4 holes side by side.



How about that?



If you had a dentists drill you could do that without drilling through the outside of the runner, yes?



Ever see a NASCAR head with a row of punch marks around the intake valve pocket?



A vortex generator could be a sharp edged groove. Or a punch mark. or a small hole.



Ideas, Its whats for dinner.





Lynn E. Hanover

BLUE TII

The thing I would like to know is why that woman is your girlfriend and not your wife!





We are both shy of marriage- seeing so many go bad. I am not letting her slip away though!



She was helping me stack my rebuild this weekend; lifting the e-shaft 1" (and not 1.5") to slip the internediate housing on.



She was a bit worried about getting it right and saw the the through hole in the e-shaft. "that looks about 1 inch down can you stick a bolt in it to I can't push too far up?"



1.25", turns out- works perfect w/ an allen head bolt.



Good woman!

Old Splatterhand

this may sound off topic but:

the bernoulli effect sais, that fluids near a wall, for example in a pipe, flow very slowly.

when you pour for example a can of oil in the engine, there is that little remaining oil which takes forever to flow into the engine because it sticks to the walls of the can: is this already a part of the bernoulli principle?



btw this is a very nice thread.



even a rough finish of the port runner could be a vortex generator, right?

should one rather polisch the whole runner like chrome and then make punchmarks to add vortex generators.

or better leave a slightly rough finish?

Lynn E. Hanover

Bernoulli's major contribution was to notice that a narrowed section of a stream had a higher flow rate than a wider part of the same stream. He then determined that the higher velocity flow had a lower pressure than the wider part of the same stream. It is that higer velocity lower pressure flow (from Bernoulli) that is seen in and in most cases poorly described in explaining how airplane wings work.



Fluids, and gasses flowing next to a surface have lower relative speeds than fluids or gasses at even a slightly greater distancefrom that surface. If we can look very closely at an airplane wing, you would see that even the best surface is going to have little imperfections, like paint lines, scratches, polishing sworls and rivet lines and so on.



Imagine that the plane flys from NY to London, and on that trip there is one molecule of air stuck by way of static charge in the bottom of a scratch. During that time a number of other molecules for short periods had attaced themselves to the original, but through the action of molecules passing close above these they were knocked free, and still others stopped in to visit.



So, the relative speed of molecules close to the surface will vary from zero (stuck in a scratch)

to close to 600 MPH just a micron or two above the wing. This thin layer of air is called the boundry layer.



A layer of air between the fast moving air, and air that is not moving at all. So it is air that is mixing on one side with faster moving air and stopping and starting on the surface of a structure.



The same situation exists in similar situations. The very highest velocity flow will be found in the center of a tube, or pipe, or carb, or throttle body, because the fluid closer to the surface is mixing with the boundry layer.



The oil analogy has more to do with surface tension and pour quality and boundry layer, than it has to do with flow velocity.



It would seem obvious that the less smooth a surface, the thicker the boundry layer. The thicker boundry layer contains a large supply of molecules moving about at differing speeds and in slightly differing directions.



Where the boundry layer under review is on the top of a wing, there is a need to have flow remain uniform from the front to the trailing edge. The thicker the boundry layer the more difficult it becomes to maintain this situation.



What's the big deal?



There are two forms of lift generated by the wing. One form is quite simple to understand if you have had your hand out the window of a car. You can generate quite a bit of lift and make your hand swoop and dive by changing its angle of attack. Just like the airplane wing. Just like a water ski.



This kind of lift is just a simple force vector, and can be produced by a flat wing or piece of plywood, or a mattress lifting of the top of a VW bug. So a big wing causes a huge amount of air to change direction and in doing so the Opposite reaction (Newton) for the force vector then lifts the airplane.



The really neat kind of lift is this: Induced, by the Bernulli effect at the top of the wing a slight increase in the velocity of the air passing over the upper surface will be at a slightly lower pressure than the air passing under the wing. A very slight pressure difference per square inch, to be sure, but a 747 has millions of those to push on. But wait, there is more.



What about rotation?



The venturi effect, where we squirt some high velocity air through the center of a venturi (Bernulli again) and a much higher volume of air (or any fluid) will be forced to flow into the low pressure area in that high velocity flow. Never heard of this? Ever use a bug bomb to paint anything? Or a real spray gun?



Also used in ships to move hugh amounts of fuel and water from tank to tank. Called Eductors.



So the air leaving the trailing edge is headed in a downward direction at some velocity and it acts as an eductor for air leaving the top of the wing, and pulls most of it down along with the air leaving the bottom of the wing. This air is not going straight down but at an angle based on the angle of attack of the wing.



If the boundry lay looses energy and begins to thicken, it in effect, changes the shape of the wing a bit and less eductor effect is possible. Thus less lift is generated. When this is done at low speed where excess lift is not available just by increasing angle of attack the wing can stall. This where the boundry layer becomes too thick and starts to tumble and normal flow off the trailing edge is destroyed. The aircraft begins to decend and will do so until energy is added to the flow in the form of relative velocity increase across the wing.



Reducing the angle of attack reattachs flow, and velocity increase (from pointing the nose at the ground) calms the boundry layer. One airspeed and lift is regained the plane is leveled. This stuff you practice when learning to fly.



But that boundry layer is thicker where there is a rough surface, so.................... Did I see a light come on?



Thicker is bad, because it makes fixed diameter unduction systems look smaller to the flow than they actually are.



So in most cases, shiny like chrome is the goal.



If the diameters involved are already adequate, or in most cases more than adequate for the power required, then polishing like chrome will make little difference.



But it won,t hurt anything on the outside of turns of in straight runs, and I like it, and it looks good.

In a street car there would be a slight increase in droplet formation at very low speeds but just not much of a problem.



The idea of leaving a slightly rough surface in the inside of a turn comes from the other outcome, which is, I made it smooth and it didn't help. If flow is separating already, smoother isn't going to help much, and rougher won't hurt much. Rougher is faster, so you get done quicker.



This is where you work on a VG.





Lynn E. Hanover

Cheesy

Adding a litttle bit more, the size of the VG can be well within the boundary layer and still be effective if it is close to the seperation point. If the VG can keep flow attatched to the surface there will also be a large drop in drag compared a detatched flow.



A question/or comment on the polished runners; if the runners are matched and polished allowing a laminar boundary layer, the flow is more likely to detatch from the surface than a turbulant boundary layer, this would then require VGs on or near the inside radius to trip the boundary layer to a turbulant flow

Old Splatterhand

thanks a lot for the very nice explanation, Lynn!

But something bothers me:

a while ago there was some hype about "shark-skin" for aeroplanes. the idea was to copy sharkskin, which has very low resistance and allows the shark to swim relativly fast with relativ small effort. the interesting thing is, that it was explictly stated that sharkskin is pretty rough. also divers have such suits which mimic sharkskin.



this may sound stupid, but i gotta start somewhere, right?



Cheesy, is there a way to calculate the point of flow separation?

i see the problem with our intakeports: a more or less uniform runner with uniform velocity goes into a very weird shape (the intake port shape) which obviously causes some problems.

BLUE TII

Micro turbulence?



I remember something about that from the early '90s. Could it be to break up surface tension between surface and boundry layer so molecules in boundry layer are more mobile? Would make sense denser fluids would be affected more.



A clueless guess on my part.

Old Splatterhand

yes some micro turbulence thingie. pretty popular in the 90s, yes.

but i don't know any more about it, or if it ever really worked.

Cheesy

to find the seperation point you would probably have to use a CFD package because of the unsteady flow, if it was a steady flow (such as an aircraft wing) the seperation point can be predectied reasonably accuratly. So without CAD models, a few thousand dollars worth of software and alot of time, taking an educated guess is the best bet. Although if someone did this it is likely that the results would show what some of the better engine builders solutions would be close to optimal.



Another thing with the laminar boundary layer is that although it is thinner than a turbulent bounadary layer it cannot stay indefinitly attatched to the surface (even without any adverse pressure gradients such as bends) so it is benificial to trip the boundary layer to a turbulent boundary layer before it seperates.