Quote:
Originally Posted by kach22i
Thank you Aerohead for putting this all in one spot.
I assume all of these radius's are modeled on a 90 degree return angle on a flat plane and not some complex curve tapered teardrop form with a NACA foil number.
Let's say one takes a symmetrical cord foil and cuts the fish head off at the thickest point, then puts a 2 inch radius at corners.
How much would that alter the drag coefficient?
See attached study.
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I had to go back 99-years, but Paul Jaray essentially did this experiment around 1922.
The data is in SAE Paper 700035, 'The Time Tunnel- An Historical Survey of Automotive Aerodynamics, by Karl E. Ludvigsen, Mobility Systems Corporation, SAE Transactions, Volume-79, PART-I, 1970, page-102, Figure 4, by Paul Jaray et al. circa 1922.
* from a square-rectangular cylinder 'brick', Cd 0.902
* they put a 2-D version of a convex-hemispherical nose, and 2-D boat-tail on, achieving Cd 0.255 in 2D flow.
* then removing the nose, measure Cd 0.765
* a 300% drag increase.
* restoring the nose brings it back to Cd 0.255, a 66.6% drag reduction.
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* To predict what a 2-inch radii would do to the nose-less wind section, we'd have to specify the original model dimensions.
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* A clue is found with K. Frey's research of March, 1933, published in Forschung Ingenieurwesen, pages 67-74, a part of which appears in Hucho's 2nd-Edition in the chapter on commercial vehicles, and the full schematic in, Hoerner's Aerodynamic Drag, 1951
- Frey took an airfoil of L/ thickness = 4.8, Cd 0.26 in 2D flow.
- then removed the nose for Cd 0.71, a drag increase of 273%
NOTE: a condition of Frey's model was that its leading-edge included a 7%- thickness radius, rather than a sharp edge, so its Cd may have been more like Jaray's 0.765 without the softening. If so, that would mean a 294% drag increase.
- adding turning vanes, to help flow attachment, top and bottom, the Cd dropped to 0.26, similar to Jaray's 0.255.
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* There's some railroad data available online which shows a streamline locomotive/railcar/ boat-tailed tailcar, of Cd 0.059.
* When the streamline locomotive is removed the drag degrades to Cd 0.432, a 732% increase, in 3D flow.
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I looked at some other ' brick' model studies in 3D flow,( Moller, Lay, Hucho, FIAT/ D.M.Waters- Ahmed ) which showed remarkable drag increase when the nose lost its radii ( 178%, 200%, 244%, 265%, and 299%, respectively.
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It's obvious from the body of past research that, at some degree of softening, leading edges will have fully-attached flow, achieve 'saturation', and no further rounding will lower the Cd.
We'd just need to specify the original 'height' or 'thickness' of the symmetrical section.