Laminar flow to reduce intake turbulence?
 

Ever seen water fountain displays where the water flows in a perfect stream with no turbulence? That's an example of laminar flow. You can eliminate turbulence and get that smooth water stream by bundling coffee stir sticks (the hollow round ones) and directing the water through that.

I wonder if you could do the same thing in the RX-8's intake? Air isn't as viscous as water so I don't know if the same principles would apply. I also don't know if stir sticks would hold up to the heat if placed just before the MAF sensor. But if it does, wouldn't that smooth out the air stream?
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In certain homemade flowbenchs, people use straws in the same way to establish laminar flow before the flow calibration disk or pitot tubes. I decided against it though in mine. It's got it's advantages and disadvatages. Since the intake twists and turns, splits up, and has reversion pulses constantly flowing through it, I'm not too sure about the effectiveness in this situation.

I'm thinking more to reduce idle fluctuations caused by turbulent airflow at the MAF sensor. So you're saying it could work in theory.
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Keep in mind that the straws are also causing a restriction.
You'd be better off if you somehow make the MAF less restrictive, so that this thing doesn't generate turbulence in the first place (which I'd argue is not a big issue compared to the turbulence occuring in the manifold and throttlebody).

Globi, I think you misunderstand the problem laminar airflow is meant to solve. Turbulent airflow going past the MAF sensor causes it to give false readings, resulting in rough idle. This is more a problem with aftermarket intakes. If the straws are placed before the MAF sensor it could result in a smooth airflow past the MAF, hence resulting in more accurate readings by the sensor and therefore more accurate air/fuel mixtures. Given the aftermarket intakes are supposed to be less restrictive than stock, presumably the restriction caused by the straws is cancelled out.

This is not intended to improve airflow going into the engine, just fix the problem with false MAF sensor readings.
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I see, well in that case I would use an aluminum honeycomb structure instead which would be less restrictive than straws.

The best thing to use is a nice bell radius inside a filter such as the stock airbox. Just don't use one that doesn't have one.

the logic that a less restrictive intake would balance out the restriction is a bit flawed, not to mention the question of what the point of the whole exercise is if you're adding one restriction to the intake path as a remedy to reducing another restriction - which maybe puts you back where you started, just lighter a few hundred dollars....

bgreene, which part of the sentence below do you not understand?
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RG and I will soon (read this weekend) be testing a couple of intake modifications, so well see what changes they have on idle, as I hav a rough idle as well. we will report back this.

hmm a nice chemical engineering thread.
Re = rho*v*D/mu
rho = density of air
mu = viscosity of air
v = velocity, D = pipe diameter
Use your straw diameter for D, caclulate v based volumetric flow of air divided by cross sectional area of tube. Keep Re under 2300 and you are theoretically in the laminar regime.

I suck at math. Zootx8, what unit of measurement is volumetric flow based on? Here's what I got so far:

Assumptions:

• 3" intake pipe diameter, 1/4" straws
• ~126 straws fit inside the 3" pipe
• sea level at 15 degrees centigrade
• rho = 1.229 kg/m^3
• mu = 1.73 * 10^-5 N-s/m^2
• v = 1.3 liters per revolution * 750 rpm / 60 seconds / 126 straws or 0.13 liters per second per straw
• D = 0.00635 m

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NRG,

I understood your reason for looking the idea.

I simply meant that addressing the side effects of a less restrictive intake system (turbulent flow at the MAF) by reducing the net area of the intake may well get you back to the same level of total intake restriction, and therefore a negligible gain in intake flow, at which point you're just as well off with the stock intake (unless you have the turbulent flow issues with the stock airbox as well as with an aftermarket system).

Sticking straws, or honeycomb, or whatever into the intake pipe to straighten the air in the pipe will also reduce the net area of the pipe through which air can flow. This, in my mind, constitutes a restriction. In your example of the 126 straws, the area loss in the intake pipe would be A = 126*pi*1/4"*(wall thickness of one straw) (this is approximately 1 sq. in., assuming a straw wall thickness of 0.01", which would put it at just over 10% of the total intake area)

A good rule of thumb is making sure you have a length to diameter ratio of 8 to 1 for your intake tube before it reaches your next plenum (say between the filter box and the MAF). This will make sure the flow has enough distance to fully develop and become laminar. As long as your fluid velocity is less than .3 mach you can assume incompressible flow which means air will behave like water. Unfortunately that means for a 3" diameter pipe you need 24" of distance for the creation of laminar flow.

Alright,
V=3.9 liters/rev @ 1000 rpm = 3900 liters/min = 3.9 m^3/min
D=0.0762m
v=14.25 m/sec
velocity through the intake tube would be the same as through the straws - only volumetric flow would change
So at idle with the 3 inch intake (no straws)
Re=1.229*14.25*0.0762/1.73E-5 = 77157, which is pretty high if all our numbers are right
Now then if you use the straws
Re = 6430, which is significantly lower, however not laminar, but much less turbulent (a paper airplane has an Re of about 50,000)
As shaun mentioned you should use a minimum 8 to 1 ratio for fully developed flow, however it will still not be laminar as the Re is calculated for the fully developed case, however, you use the diameter of the straw, and not the intake tube, so really you just need 2 inches worth of straw to get the full affect.
I assumed 1000 rpm at idle - your straws shouldn't really provide much resistance to flow so I don't think it would at all be detrimental, and it's definitely worth a try.

You know I did that per rotor revolution, so really V=1.3 liters/rev of the eccentric shaft
So Re = 25719 for intake
Re = 2143 for straws which would be laminar. :)

This thread is giving me a thermodynamics flashback.....uugghhhh...

the flowbenches i used in our fluids lab always had about a 4" long honeycomb section in to straighten out the flow. However, there is a huge pressure drop across those things (lots of restriction).

Sorry to jump in so late here; I guess the real question is, is it more important to eliminate as much turbulence as possible, or to have as little restriction as possible? As is often the case in engineering, I guess we can't have everything... Well, we could use that 24" long intake, but then the hood would look funny. But if you're most interested in eliminating turbulence, then I'd see what kind of results you get with the straws, then try optimizing toward higher volumetric flow by changing the length of the honeycombed section, or the straw diameter. Maybe you'll hit a sweet spot that gives you better flow with negligible (or tolerable) turbulence.

Assuming the MAF is a simple "hot-wire anemometer" type of sensor, you don't necessary need to have fully developed laminar flow. Since the sensor is probably giving you an average flow over its surface, the shape of the velocity profile across the tube probably doesn't matter much, as long as the stream across the sensor isn't turbulent.

Oh, and leave it to a ChemE to reduce everything to a dimensionless group! :)

Hey everything we deal with is just correlations anyhow - cracks me up how applied science is just finding an equation that works most of the time - I know way too many dimensionless numbers.

just offhand it appears the pressure drop would only increase by about 0.007 psi - not much, also i have a feeling the straws are probably a smoother material which would have less friction and create less of a difference since the intake tube (I believe) is a rougher material.

I thought the more air turbulence you have the better.

It mixes the fuel and atomizes it better in the chamber so it burns more efficently.

We're not mixing fuel at the mass air flow sensor or the air filter.

I'm not sure that this is as simple a calculation as one might expect. Air doesn't exactly flow into the engine as much as it pulses into the engine (although it may pulse so quickly that you might as well call it flow /shrug). And with the harmonic tuning involved you can get more than 3900 L/min/1000rpm of air into the engine. I don't know, seems like one of those things that would look good on paper and leave you scratching your head once you implemented it. Of course I could be way off base and fusing a bunch of knowledge, but that's how I understand it. :p

given that a rotary engine is basically always feeding the engine with air I think continuous flow is a pretty good estimate, if you think about the rotor faces as a "syringe" that is sucking in air as it rotates around once one "syringe" is full the apex seal passes and the next one begins sucking in air, take into account there are two rotors doing this and they are exactly out of phase and I would say a rotary engine has a much more continuous intake feed than a piston one would - which is very much a series of pulses. i suppose the take home message to this problem is whether or not laminar flow is established all the time, it would be less turbulent by a factor of 10, which is definitely significant and certainly could cause the MAF sensor to read more accurately.