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Old 12-15-2008, 09:17 PM   #171 (permalink)
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Quote:
Originally Posted by cmroseberry View Post
Some cold water:
I would not expect to see the separation points move around much at all for such a modest difference in Reynold's number. Furthermore, an increase in velocity (increase in Re #) would tend to promote a turbulent boundary layer which would tend to adhere further downstream on the surface.
One overriding consideration is that Volkswagen has almost certainly tuned-up this model in the wind tunnel. These Germans know their stuff - Hucho himself may have fine-tuned the Jetta shape. VW engineers are masters in making hardedge, almost boxy shapes aerodynamically efficient through detail optimization. Accordingly, I would bet the flow separates right off the trailing edge of the trunk lid.
As for vortex generators, I made some comments on the business intro thread about how they may help in some situations where the pressure gradient is close to the critical point for separation. Knowing VW's reputation, I would not recommend vortex generators on any of their cars except for possibly a Beetle.


For a New Beetle, I'd actually recommend a full tapered rear, at least 6 more feet long.

That car is nearly perfect for the tear-drop shape.

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Old 12-15-2008, 09:48 PM   #172 (permalink)
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Originally Posted by Christ View Post
How would a single VGT create more airflow than a setup where one turbo is activating another one? VGT's don't serve that purpose... they are basically there to keep specific boost under varying conditions.

My idea involved spooling the first (smaller) turbo off the engine, and using it's spooled (compressed) air, obviously a much larger CFM than the exhaust flow, to spool another turbo's exhaust side, before going into the engine. The secondary turbo's compressed air would then be fed through ducting into the wake at the rear of the vehicle, hopefully eliminating part of the wake.

Through the feedback loop that standard turbos use, this isn't really possible though. The secondary turbo would never reach "full spool" and thus, wouldn't create the kind of airflow it could otherwise achieve.

Also, a turbo won't spool properly without more airflow than it's host engine can achieve, hence the reason for "turbo lag".
In terms of wake infill there isn't much that can be filled, so to speak, at reasonable speeds. And regardless of how many turbos you use, the maximal amount of infill will only be however much exhaust gas the engine is putting out.

If going for peak power a twin turbo (or twin charged) setup can be advantageous, but in terms of efficiency they'll still have inefficient portions of their compressor maps that they operate in. I suppose you could overlap those inefficient portions, but by then you might as well just go w/ a VGT in the first place and not add the extra weight/complexity.
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Old 12-15-2008, 09:53 PM   #173 (permalink)
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Thanks for clarifying what I said up there^^^.

Question though, b/c you state that "regardless of how many turbos you use, the maximal amount of infill will only be however much exhaust gas the engine is putting out"

Is that in terms of CFM? B/c if you're referring to it that way, I don't believe it's correct. Simply b/c of the feedback loop idea. The turbo needs to constantly be taking in more than the engine is putting out, otherwise there can't be a spool effect.

This effect can be multiplied by messing with the turbo's compressor angle and sizing relative to the turbine size. IOW, for 200CFM of exhaust, 250CFM of intake might be generated.

Is this correct?
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Old 12-15-2008, 10:14 PM   #174 (permalink)
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Not just CFM, since you can just reduce the temperature and you'll have less energy/wake infill, but volume at ambient (pressure/temp) per unit time. W/ a turbo we're using some of the energy from the exhaust to compress air so the energy that's left in terms of equivalent gas volume at ambient pressure/temperature would be less than what it would be w/ an uninterrupted exhaust stream, but either way it's a very small amount, something like a third of a cubic foot per second at low highway speeds IIRC.
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Old 12-15-2008, 10:15 PM   #175 (permalink)
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Ok, that makes more sense now.
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Old 12-15-2008, 11:06 PM   #176 (permalink)
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cmroseberry - Thanks for the cold shower!

Based on the CFD from tre and your insight, what in your opinion are good aerodynamic mods for the highly tuned 99-04 Jetta's?
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Old 12-16-2008, 12:53 AM   #177 (permalink)
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I give VW a lot of praise, because Hucho literally wrote the book on detail optimization. But this is not something particular to VW, most every car gets a thorough work-over in the wind tunnel. If there is a little feature that can be altered for better aerodynamics that won't have much of a styling impact, any of the major automakers are likely to jump on it. Furthermore, automakers spend about as much wind tunnel time reducing aerodynamic noise as they do reducing drag. So in many cases, it may be best to leave things "stock". I also suspect that it is still true that almost all aftermarket 'aero' add-ons are never wind tunnel tested and thereby as likely to detract from as improve the Cd.
So having just looked at some Jetta photos, and knowing the subtle and easy stuff is already squared away - I would recommend seeing if you can do without the side mirrors. Next, I would consider fairing-in the back wheel wells. Not having a Jetta around to look over, my only other suggestion would be a boat tail to try to reduce the area that the wake projects. Note that these are changes that automakers normally shy-away from because they worry about customer acceptance. I hesitate to recommend a belly pan because I am not sure how it would effect the heat transfer from the exhaust pipes (I would defer to those ecomodders who have tried it). If I had a Jetta to look at, I could evaluate the cooling air flow path and make suggestions. One of these days, I will try to write a general guide to improving the cooling air flow.
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Old 12-16-2008, 12:58 AM   #178 (permalink)
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You really don't want to cool the exhaust flow, as more heat - higher pressure - faster velocity - more scavenging from engine - better Volumetric Efficiency.

Therefore, insulating the exhaust from cross-flow and direct air contact would be a good idea.
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Old 12-16-2008, 01:12 PM   #179 (permalink)
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Let me clarify what I meant. In mentioning the "cooling airflow path" I had jumped to a different topic - the airflow to and from the radiator and condenser. If I could look at a Jetta, then I might be able to make some authoritative suggestions based on my research for my MSME. For example. looking at my own cars, I would say my Jaguar X-type has a more efficient flow path than my Taurus wagon.

In regard to the heat transfer from the exhaust system, I was concerned what the increased temperatures might do to the pipes and surrounding structure. It may or may not be a problem; I would want to hear from someone who has tried a bellypan on something similar to a Jetta. If I were to install a belly pan on one of my cars, I would put dabs of temperature indicating lacquer on the pipes and surrounding structure to see if I had introduced any severe hot spots. (templac is available from omega.com)

Seeing that it has been brought-up, I do not agree that hotter exhaust would be helpful (if it makes much of a difference at all). Flow resistance is proportional to the square of velocity.
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Old 12-16-2008, 02:54 PM   #180 (permalink)
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separation point

Quote:
Originally Posted by Dieselguru View Post
Seeing how this is my first post, I'll try not to overstep my welcome.

Tre - It would be very interesting to see some of the runs you have done at a little higher speed, say 75 mph (the speed I drive my 110 mile round trip to work every day). If there are gains to be had, they they will show up faster at those speeds. Especially things like VG's.

As a side note: probably the reason most people don't see much improvement using VG's is that
1. VG effectiveness increases with speed (need a seperation point, otherwise its just drag)
2. VG placement probably depends very closely on speed (location of seperation point)

Look very closely at the Mitsu report on VG's. In the test setup section (page 2), they mention the baseline speed of 50 m/s (111 mph). if tre's settup is correct, will the seperation point move forward with increasing velocity to somewhat match that of the Lancer (wont be exactly the same, but simular), and will the VG's will become more effective?

Tre - if you still taking requests, through higher speeds into the mix and see what happens to some of your results.
I believe that once the car hits a Reynolds Number of 500,000 the boundary layer goes turbulent,the Cd is established and remains a constant,and what ever flow patterns exist,exist for the remainder of the velocity spectrum until the car reaches about about 250-mph,where compressibility becomes an issue.I don't believe the separation point moves at all.Power saved by the VGs will very as the cube of the velocity,so certainly,as the velocity increases,their benefit becomes more apparent.

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