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Old 08-01-2009, 04:23 PM   #34 (permalink)
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one study

[QUOTE=Bicycle Bob;119013]
Originally Posted by aerohead View Post
---- The template respects streamline flow with an economy of structure and literally guarantees attached flow,at least if you don't cheat the minimums.----------- YOU CAN make the body with higher fineness ratio,I just want everyone to understand that there will be a small penalty to extra skin friction.

I agree that it is good to avoid extra skin friction, but when only one study suggests that flow can stay attached to a shape at 2.5 : 1, and hundreds of tests at a more suitable Reynold's number disagree, I want to see some tuft tests. In the NASA tests for streamlining trucks, you can see a tailcone extension eliminating a Kammback, while increasing the angle a bit, and the tell-tales show separation there, at an angle that is still less than this "ideal."
I have been unable to post any attachments since I posted the template.I've asked for help each computer session since and so far no help,so I'm deadlocked as far as that goes.---------------- I had additional documents scanned and attempted to attach with the other 5-documents but EcoModder message came up saying I was "maxed-out".------------------------------- The DVL research was conducted at Re 3,000,000.The work is included in Fig.5.13,page 69,Aerodynamic Drag,by Hoerner,1951.Why Wolf Hucho chose to include it in his table would be a matter of conjecture on my part.The form does exhibit one of the lowest Cds for a 3-dimensional form and is of lower drag than airship bodies of greater fineness ratio,of which it shares great resemblance.The airship Los Angeles,with fineness ratio of 7.23:1 only increases to Cd 0.045,when fins,gondola,and engine nacelles are taken into account),a 12.5% increase,however a 25% increase in ground-effect.----------------- The study does not suggest that this is the only form that will support attached flow.It does claim that a body of revolution with a fineness ratio this small CAN have full attached flow.So I chose it as a minimum,as with smaller fineness ratio,the exit architecture exceeds Mair's 22-degree maximum for a boat tail,suffering separation and attendant rise in profile drag.----------------------------- Since it is Hucho that says only a long tapering tail can prevent separation,and Jaray says that a half-body of a streamline body of revolution is the lowest 3-dimensional form for a vehicle body,my pea-brain seized on The DVL 2.5:1 as the logical minimum.Just a starting place.Obviously,this isn't a one-size-fits-all solution.The data suggests that it could be a no-brainer go-no-go,or template for which to contemplate potential fuel savings from aft body modifications,which Hucho suggests is our only path for truly significant drag reduction.He also states that in the context of cars which are designed specifically for low drag,the science is good and you can lean hard on it.------------------------------------------------ With respect to the NASA van,I think the exit angle off the boat tail is around 20-degrees,same as for my VW.If this is the same photo you're referring to,then I can tell you that it works and got me an extra 4-mpg.The important thing is the gentle curvature leading into the tail as you will see in Mair's boat tail research.Without that,the flow immediately separates,with little gain.------------------------------------------- I have a tuft-test photograph of the boat-tailed and streamlined T-100.There is very little perturbation of the airstream and all tufts are straight back at the terminus.The trailer will be a follow-on to the boat tail,providing,through the use of the gap-fillers,a continuous boat tail taper the a point about 10-feet aft of the tailgate.She's a little longer than needs to be,but that's where I'll be "living" on my way to and from Bonneville and Battle Mountain.The rig comes in at about 5.3:1 on the ground and 2.65:1 in "mirror" so she's a little over my minimum but I'll welcome the extra space inside.-------------------------- If you can post the NASA image I'd appreciate it,and be able to compare to what I have.Mines the truncated tail,only a couple of feet,maybe a meter in length.Thanks!
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