Airflow -port taper

texasSlick said:

Agree with all stated. Regarding last sentence:

The numbers you cite are AVERAGE fps....sonic choke can occur if velocity on a particular streamline approaches sonic. For example, the velocity profile of laminar flow in a round tube is a parabola of revolution. In this case, the velocity of the stream tube on the axial centerline is twice the average mass velocity. Thus with your numbers, the centerline velocity would be 1080 fps....definitely near sonic (depends on temperature). As I stated earlier, once the flow approaches a choked condition, things happen to accelerate the process....which you also allude to above. Maintaining laminar flow in such a flow condition is doubtful. The laminar profile will disrupt to a turbulent, dropping the centerline velocity to near average mass flow velocity, which removes the sonic condition, but at the expense of greater skin friction. It is useful to keep in mind that nature ALWAYS takes the path of least resistance....in this example, nature eliminates the sonic shock wave which is a big energy hog.

In that case, the entrance and exit effects are part of the experiment. A radius at each end minimizes the effects. I do not think these effects are the principal reason why flow is different both ways....that is my professional opinion...I'm done.

Slick

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Thanks, that does help confirm what has been my suspicion.

I have done port maps of the flow tube in both directions some years ago using a wand that paints a picture according to airflow. When the tube is flowing from the small end there is very little boundary layer along the walls mid tube. However when the tube is reversed the boundary layer is very thick at the halfway point. The slow-moving air along the walls take up probably half of the area of the tube. Jim

texasSlick said:

Dances with Shrapnel said:

In this thread a simple experiment was presented and the difference in air flow results is due to the shock loss, yes shock loss, well below the speed of sound. This may not be a term many people are familiar with outside of fluid dynamics and ventilation systems. Judging by comnoz data (30-35mm dia ports and 100-140cfm), it is well below supersonic so lets not confuse things here.

Simply stated, in the experiment presented by comnoz, when a fluid exits a pipe, tube or conductor, there is a pressure loss due to not being able to convert the remaining kinetic energy into potential energy. Simple energy balance. The taper of the tube has nothing to do with the phenomena, the air velocity at exit has everything to do with the loss.

Normally the experiment would be run both ways at constant flow rate and a measure of inches water gauge would indicate the relative difference in pressure (pressure loss) to achieve the flows.[/quote

" It is well below supersonic.....etc". Exactly....so keep shock waves and shock losses out of it. Call it something else it you insist. Shock waves do not exist in subsonic flow. Shock loss is a term this aerodynamicist is not familiar with in subsonic flow.

"Normally the experiment....etc" Comnoz kept the pressure differential constant and measured flow variance both ways....one can interpret the effects either way.

Entance and exit effects are not part of the experiment if comnoz's pressure taps were BETWEEN the tube ends.....So just where were they? Until this can be clarified, I'll stand that skin friction is the dominant mechanism governing the pressure drop at constant flow, or flow drop at constant pressure.

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Well Tex, you need to understand that pressure taps were not part of the experiment (not on the taper) thus the entrance and exit shock losses are part of the experiment and observations.

Not going to spend too much time on terminology here but as an example, see page ten of the below reference for context.

http://web.mst.edu/~tien/218/Chap5.pdf

Also:

From the Second edition of Mine Ventilation and Air Conditioning by Hartman, Mutmansky & Wang:

"Friction losses represent head losses in flow through ducts of constant area, whereas shock losses are losses resulting from changes in direction of flow or area of duct. Shock losses also occur at the inlet or discharge of a system, at splits, junctions of two or more currents of air, and at obstruction in airways"

and

"However, when shock losses occur because of change of duct area, some velocity head of the fluid is converted to static head (if the area increases) or static head to velocity head (if the area decreases). Thus static and velocity are mutually convertible, although the conversion is accompanied by shock loss."

Again, all fundamental fluid dynamics.

So yes, Shock Loss is an appropriate term although not universally accepted by aerodynamicist and yes, the entrance and exit velocities have an impact on the amount of air that will flow for a fixed head or pressure and no this thread/experiment has nothing to do with Supersonic conditions. As I have suggested before, conducting this test with two tapers small end to small end should be telling and provide some enlightenment to the doubters. Basically I am asserting that adding a proper "evase" to the smaller diameter to reduce shock loss and convert some of the velocity head (pressure) into static pressure.

So comnoz is only measuring static pressure and not measuring velocity pressure. To do that he would need to use a pitot tube. Furthermore, there are losses at the entrance and exits due to entrance and exit "shock" losses due to the shape and form of the entrances and exits. As an example, a proper bell mouth would improve air flow through the taper and a proper evase (mirrored exit taper) would improve flow through the taper. This is a very common practice on mine fan ventilation systems for underground mining. This probably applies to performance IC engines where there should be a proper radius on the intake and exhaust megaphones with the 7 degree taper.

'If you have a close ratio trans then you can put a narrower powerband and possibly a higher peak power to use. But if you have a normally spaced gearbox then a short peaky powerband isn't much good when you have to wait for the rpms to build back to the powerband on each shift. Jim'

I suggest the situation is that it you build a severe top end motor with a savage powerband, you might need at least a six speed CR box to live with it. If you are considering building a racer, always start by looking for the gearbox first. If you haven't got that, you can go nowhere. In our historic racing, there is a guy with a Z900 engine Moto Martin - still has the 5 speed box. The GPZ900 6 speed box and clutch will fit. If he does that he can then use a real race cam instead of the usual Yoshimura - which is relatively mild. The bike is set up like an old GP racer - it could be excellent.
What I find with my Seeley using the 4 speed CR box, is that there is no 'lag' on each up-change - it seems to rev up at the same rate whatever gear it is in - in fact TOO QUICK. The motor and pipe are set up for max torque. I've recently greatly increased the overall gearing, and I might have to buy another lower first gear for the clutch start in races. I suggest that once you have enlarged the inlet ports on a commando engine, you have made a choice which is not cheap and easy to reverse .

Hobot, a megaphone exhaust usually makes a bike much more difficult to ride well. The power usually comes on a fair bit higher than with a muffler fitted, and with much more of a savage kick. You certainly get more top end, however unless the circuit is very big with no really tight corners, you can be at a distinct disadvantage. - The difference between Phillip Island and Winton. I'd race at Winton anytime, however Phillip Island would have me thinking before I went there. I've been told that the Poms at the Island last weekend didn't like turn one - neither do I - it's possible to go around there in terror.

https://www.youtube.com/watch?v=JedNuDhkXaY

comnoz said:

there are 1000 different ideas -some good -some bad and I am sure there are some still to be learned. Jim

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For the love of god there, Dr. Frankenstein, you sure like kickin that bee hive around, don't ya?

comnoz said:

I have done port maps of the flow tube in both directions some years ago using a wand that paints a picture according to airflow. When the tube is flowing from the small end there is very little boundary layer along the walls mid tube. However when the tube is reversed the boundary layer is very thick at the halfway point. The slow-moving air along the walls take up probably half of the area of the tube. Jim

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Jim, for clarity, when you say "When the tube is flowing from the small end" is this with air flow direction from the small end to the big end?

Hm, ugh, Slick went and introduced Reynolds numbers vectors across the flow cross section with the bullet shaped speed profile, so dang it sonic choke could come into play in some Nortons, ugh.

I'm sure the clever ones in class are having a great time with this.

But I don't think it will change my carb / manifold plans much... At all! :?

Yes, this was a decent thread and then someone mentioned sonic waves, then we went supersonic, there was a BOOM, then someone added pictures, then blah blah blah and it all went pear shaped.

Point was completely missed, completely.

Buy em books, buy em books and all they do is chew on the covers and wipe their arses with the pages.

Apologies in advance for the negativity here.

I know there are standing waves both in the inlet tract and exhaust, because you can hear a note. The combination of the wave and mass transfer of gasses doesn't necessarily equate lo laminar flow situations, and looking for advantage by using a flow bench might be good for very low RPM situations only. I've often wondered about the effects of skinny as opposed to fat exhaust pipes . I always used the skinny ones because I believe the pulses occur at higher pressure. You can just as easily take the opposite stance and claim that fatter pipes must flow more gas. I also believe that the tail pipe dimensions on two into one exhausts are very important. The system runs with back pressure, and the tail pipe must resonate at twice the frequency of one header pipe, so the pulse pressure is important. It is not as simple as gas simply flowing down two pipes into a collector and exiting. When two waves meet in the collector the result is interference effects. The header pipes on a 360 degree twin are precisely out of phase by 180 degrees ? So the tailpipe length should reinforce the pressure pulses over the desired rev range. - Combination of timings and pipe dimensions.
Do you believe that bellmouths on carburetors are only about 'ram' ?

Dances with Shrapnel said:

comnoz said:

I have done port maps of the flow tube in both directions some years ago using a wand that paints a picture according to airflow. When the tube is flowing from the small end there is very little boundary layer along the walls mid tube. However when the tube is reversed the boundary layer is very thick at the halfway point. The slow-moving air along the walls take up probably half of the area of the tube. Jim

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Jim, for clarity, when you say "When the tube is flowing from the small end" is this with air flow direction from the small end to the big end?

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Yes, the air is entering the small end for the highest flow readings. Jim

Yes, this all makes sense.

The air is decelerating when flowing from the small opening to the larger opening.

When the air is flowing from the larger opening to the smaller opening the air is accelerating.

comnoz said:

Yes, the air is entering the small end for the highest flow readings. Jim

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Ok. Time for a few questions on how this pertains to the masses.

Is any of this saying what is optimal for combustion?

Is this to say a slower moving, higher volume mixture is better than a faster moving lower volume mixture?

What percentage of horse power increase/decrease can be expected at what level and type of taper(increasing or decreasing in the direction of flow)?

Or is this a matter of efficiency rather than power, that is to say, will atomization increase or decrease across the board relative to any fuel delivery system and with what level and type of taper(increasing or decreasing in the direction of flow)?

pete.v said:

Ok. Time for a few questions on how this pertains to the masses.

Is any of this saying what is optimal for combustion?

Is this to say a slower moving, higher volume mixture is better than a faster moving lower volume mixture?

What percentage of horse power increase/decrease can be expected at what level and type of taper(increasing or decreasing in the direction of flow)?

Or is this a matter of efficiency rather than power, that is to say, will atomization increase or decrease across the board relative to any fuel delivery system and with what level and type of taper(increasing or decreasing in the direction of flow)?

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optimal for combustion? - laminar or undisturbed flow is going to carry atomized fuel most efficiently, turbulence causes fuel to drop out of suspension.

To have enough momentum to continue filling the cylinder after the piston has reached BDC the air must be moving very fast -the faster the better.
But since a port can not be a perfectly shaped flow tube, then there are places such as curves and the area around the guide and valve that the air needs to be slowed down -otherwise it would create turbulence that reduces the available port area and effectively limits how much mixture can get through.
The basic idea is to move the air fast where it is able to move fast without causing turbulence. IE -the straight section of the port. If that straight section is parallel walled or has an outward taper then there will be little induced turbulence.
If the walls are converging then there will be friction induced turbulence along the wall. This layer of turbulence grows thicker as the speed increases.

The original point to my post was to show that a long converging taper in an intake port will hinder the ability to flow mixture at high speed.
Of course if the ports are large enough then the velocity will be low and turbulence will not be a problem.

But if the velocity is low you are then missing out on a substantial increase in cylinder filling, or at least the ability to keep the mixture in the cylinder after BDC when the piston is rising with the intake valve still open. Jim

This suggests that the 32 to 30 mm intake stubs used with a stock rh10 head might not be a great thing?

Glen

comnoz said:

pete.v said:

Ok. Time for a few questions on how this pertains to the masses.

Is any of this saying what is optimal for combustion?

Is this to say a slower moving, higher volume mixture is better than a faster moving lower volume mixture?

What percentage of horse power increase/decrease can be expected at what level and type of taper(increasing or decreasing in the direction of flow)?

Or is this a matter of efficiency rather than power, that is to say, will atomization increase or decrease across the board relative to any fuel delivery system and with what level and type of taper(increasing or decreasing in the direction of flow)?

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optimal for combustion? - laminar or undisturbed flow is going to carry atomized fuel most efficiently, turbulence causes fuel to drop out of suspension.

To have enough momentum to continue filling the cylinder after the piston has reached BDC the air must be moving very fast -the faster the better.
But since a port can not be a perfectly shaped flow tube, then there are places such as curves and the area around the guide and valve that the air needs to be slowed down -otherwise it would create turbulence that reduces the available port area and effectively limits how much mixture can get through.
The basic idea is to move the air fast where it is able to move fast without causing turbulence. IE -the straight section of the port. If that straight section is parallel walled or has an outward taper then there will be little induced turbulence.
If the walls are converging then there will be friction induced turbulence along the wall. This layer of turbulence grows thicker as the speed increases.

The original point to my post was to show that a long converging taper in an intake port will hinder the ability to flow mixture at high speed.
Of course if the ports are large enough then the velocity will be low and turbulence will not be a problem.

But if the velocity is low you are then missing out on a substantial increase in cylinder filling, or at least the ability to keep the mixture in the cylinder after BDC when the piston is rising with the intake valve still open. Jim

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Sounds good to me.

worntorn said:

This suggests that the 32 to 30 mm intake stubs used with a stock rh10 head might not be a great thing?

Glen

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There is going to be turbulence in a curved manifold and it will reduce the flow around 10 CFM on the flowbench.

A 30 mm manifold is going to have the most turbulence and a 32 mm manifold will show less as the velocity is lower at the same rate of flow.

Since there is turbulence anyway -the taper in a 32 to 30 manifold does not seem to make a big difference but I have found slightly higher flow by using a 32mm manifold and then using a 3/16 th inch radius on the head to match it to the 32mm manifold. Jim

Dances with Shrapnel said:

Yes, this was a decent thread and then someone mentioned sonic waves, then we went supersonic, there was a BOOM, then someone added pictures, then blah blah blah and it all went pear shaped.

Point was completely missed, completely.

Buy em books, buy em books and all they do is chew on the covers and wipe their arses with the pages.

Apologies in advance for the negativity here.

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Hobot posting diagrams/pictures regarding Reynold's Numbers and flow is completely relevant to the thread. Reynold's numbers suggest the characteristics of flow, in pipes for example, dependent on the surface roughness amongst other factors.
If you kept your references to automotive instead of civil engineering you might have got your point across.

comnoz said:

[quote="Rohan"
Rawlins/Baker sure got some good speed/power out of that 850 with big ports. ?
143 mph average 2 way over the flying kilometre (mile ?) isn't hanging about...
And was apparently still quite strong for average street riding - being registered and on the road.

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I would not say that you can not get good peak power with large ports. Peak power is what counts when you are looking for max MPH.

What I look at when building a motor for street or roadracing is "power under the curve" That is the area under the horsepower line over a rpm range that is going to be used when accelerating. [determined by the rpm drop when shifting]
If you have a close ratio trans then you can put a narrower powerband and possibly a higher peak power to use. But if you have a normally spaced gearbox then a short peaky powerband isn't much good when you have to wait for the rpms to build back to the powerband on each shift. Jim[/quote]

Baker was fond of the large port. The Baker head I had had its ports opened to 34mm, and ran 36mm carbs, smaller carbs killed it a bit. It was good top end motor and had good mid range being an 850 with a good ex Thruxton cam designed by PW to be similar to a 4S but with more mid range so I guess that mean't closer lobe centres. OK on my lighter weight race bike. Not so good for all applications I think, and I am not building a replica, but a smaller ported Full Auto....I expect good results as long as I can get the cam I really wan't to work with it.....

Al-otment said:

If you kept your references to automotive instead of civil engineering you might have got your point across.

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Civil, really?

SteveA said:

Baker was fond of the large port. The Baker head I had had its ports opened to 34mm, and ran 36mm carbs, smaller carbs killed it a bit.

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So here we have an instance of converging walls to a port improving performance. Do you recall whether the carbs (36mm and smaller) were smooth bores?