pollished ports (supposedly not all their cracked up to be?)

[thechuckster]

mirror surface flows worse than smooth (but finely textured) surface.

search Laminar & boundary layer

Race magazine has had a few good articles about porting practices and surface airflow.

Ed's V8 site (name eludes me at moment) also has much on good porting practice.

[Mooro]

Haven't read the whole thread so maybe already said but my understanding is that a non polished surface is better for the reason of fuel atomisation, ie air fuel mixing.

[RAd28]

the idea of a "boundary layer" is to achieve a mirror like finish with out the work...

First, the boundary layer adds to the effective thickness of the body

hence a 38mm OD port with a rough finish will have a "choked" air passage, therefore <38mm effective area, whereas a 38OD port with a mirrored surface will have a clear air passage and =38mm effective area.


boundary layers are regions close to the solid body where the air flow velocity is (close to) 0, this is due to the imperfect nature of the surface. almost immediatly after this boundary layer, the air flow is moving close to the maximum for any port.

the only reference to boundary layers, and improved air flow, is "boundary layer suction". in the use of air craft (where this theory originated) the boundary layer (immediatly on the surface of a wing) provides a smooth surface for the air to pass over.

However the smooth flow is often disturbed by the boundary layer breaking away from the surface and creating a low pressure region immediately behind the airfoil This low pressure region results in increased overall drag.

so basically... yes the boundary layer assists flow on a rough surface, BUT with a smoother surface, that boundary layer is smaller, which means the air speed reaches maximum alot closer to the wall of the port giving better air flow due to a large effective air passage. this is not to say polishing your intake will give better performance, because it won't. a slighty rougher intake port will enable the fuel/air to mix better hence giving a more effecient burn and more power. HOWEVER polishing your exhaust port will increase power due to more effecient removal of waste gasses


no matter how you say it, rough surface is worse for air flow... capiche?

[tricky]

RAd, your last quote appears to be talking about boundary layer seperation in an exterior flow, ie aerodynamics. In a pipe flow it's a bit different because the air body is finite. You don't get boundary layer seperation, but you do get a turbulent boundary layer transition (flows develop over a finite distance, which is a function of the reynolds number). If you want to compare this to an exterior phenomenon, it is more analogous to vortex shedding. And a boundary layer isn't an idea- It's a phenomenon caused by viscosity The boundary layer is an area where the velocity is <=99% of the free stream velocity.

A quick lesson on the velocity profile of a boundary layer... For a laminar flow, the layer profile is parabolic (Poiseulle's Law). In a turbulent flow, the velocity profile is much flatter. That is, the transition from the no-slip boundary (zero velocity) to free stream velocity occurs in a much smaller portion of radius. This means that at transitional velocities where the flow regime can be laminar or turbulent, depending on roughness, the turbulent regime can actually flow more.

In reality, port velocities are turbulent, so the above argument doesn't apply. The friction factor of a flow is an indication of the losses, and it depends on the reynolds number and the surface roughness. Friction factor determination isn't analytical- It's usually pulled off a Moody diagram which was created empirically. So basically, rough walls flow less volume in fully developed flows, but if the flow rate is high enough, the difference is negligible anyway. So as mentioned by others the real reason for 'rough' walls (still can be very smooth) is to prevent port wetting with fuel.

[SillyCarS]

waiting for camera batteries to charge, some of you on the money some not, air craft painter is correct but thats at much higher fluid speed (ie sub mach) which varies hugely as the fluid starts to become compressed

will put up charts of drag coeff's and boundary layer roughness tonight

[tricky]

click me

Beat you to it! 'tis a Moody diagram.

[RAd28]

link to uber-hi res image?

[RAd28]

lol... now this has got me thinking about a way to keep air flow laminar over the throttle plates... or NOT throttle plates

[TheDarkRaver]

For a simple explanation just look at a golf ball, originally they were a smooth round ball and it was found that as the ball became older and more dinged up it would go further, this was because of the imperfections causing a small amount of turbulence (same as a ruff surface). So they created the golf balls we have today with dimples in them, the dimples induce turbulence in the layer of air next to the ball which creates a turbulent boundary reducing drag as it moves threw the air.

Just had to add that as it was the first thing I thought of when I originally read your post.

Marc,

[tricky]

Welllll... The best throttle design I've come across is the rolling cam design. It keeps a nice smooth radius and has no shaft in the middle. An iris type arrangement would also be good, but sealing would be impossible. Another potentially fantastic and relatively cheap throttle design would be a poly-sleeved pinch valve as used in heavy slurry applications... But that is probably best suited to another thread.

[Karma Supra]

I thought it was nothing to do with actual airflow, more air turbulance to keep the air fuel mix more consistant. The ports in my head work is smoothed but not polished, and that was done by an engine builder for Renault F1, so I'd hope he knows his stuff!

[RobertoX]

But that is probably best suited to another thread.

such as this one

Interesting thread by the way

[SillyCarS]

For a simple explanation just look at a golf ball, originally they were a smooth round ball and it was found that as the ball became older and more dinged up it would go further, this was because of the imperfections causing a small amount of turbulence (same as a ruff surface). So they created the golf balls we have today with dimples in them, the dimples induce turbulence in the layer of air next to the ball which creates a turbulent boundary reducing drag as it moves threw the air.

Just had to add that as it was the first thing I thought of when I originally read your post.

Marc,

how bout no scott

stops it from turning and shit, overall turbulance means lack of control, so goes where its hit, turbulance does not reduce drag

the flow in the head of your engine is going to be so rediculousy turbuance it ain't gunna matter

youre jumping to say 50000 rwkw's by porting alone, polishing maybe only 1000rwkw's, so personal preference i reckon as youve already got 95% of the gains

[SillyCarS]

ive gotta say this is why toymods pisses all over other forums, people actually know their shit

yeah great thread

[ed]

to consider port flow as laminar is plain wrong, and as such these considerations as to boundary layer thickness and its effects on flow become embarrasingly trivial. plenty of old wives tales about fuel atomisation and golf balls exist, but i believe they have exceedingly little to contribute to the realisty of intake dynamics

a few lines stolen from a thread here:
http://www.v-eight.com/tech_forum/viewtopic.php?t=23
and here
http://www.cheresources.com/colebrook1.shtml

Turbulent Flow
Under turbulent flow conditions the friction factor is a complex function of the Reynolds number and the relative roughness of the pipe bore as given by the Colebrook formula:

The Colebrook formula requires an iterative solution. and an equation of similar accuracy that allows direct calculation of the friction factor



Where:

f is the Friction Factor and is dimensionless
e is the Absolute Roughness and is in units of length
D is the Inside Diameter and, as these formulas are written, is in the same units as e.
R is the Reynolds Number and is dimensionless.
Note that e/D is the Relative Roughness and is dimensionless.

These three equations are referred to as “Implicit” Equations. “Implicit” means that “f”, the Friction Factor, is “Implied or understood though not directly expressed”2. Simply stated, the equations ARE NOT in the form of “f = ………”. These are sometimes referred to as “equivalent” but as we will see, the results will vary when calculated to the fourth significant digit.