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Old 01-29-2011, 03:14 PM   #184 (permalink)
aerohead
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can't figure

Quote:
Originally Posted by KamperBob View Post
I played around with Flow Illustrator. I can't figure out if I'm using it wrong or if it is really inaccurate. Here is a 256x128 mask of Phil's template.



The flow field is not hugging curvature at all. The tail is shedding a KV street.



Re = 2500000 (2.5e6)
dt = 0.1 (0.01 wasn't working)
length = 40s
cutoff = 10 (40 wasn't working)

Can anyone tweak FI to validate this case?
Bob,I think that Flow Illustrator represents only the centerline flow in 2-D flow and is not sophisticated enough to represent the aft-body of any 3-D structure.It's why I never gravitated towards these CFD programs.
Hucho is quite explicit about the limited usefulness of CFD,especially with respect to the wake.
Current CFD technology IS sophisticated enough to accurately model wake flow,however I believe it is still relegated only to big-budget super-computers.
The template is derived from empirical wind tunnel studies of actual 3-D structures,with appropriate Reynolds numbers.
Anything constructed to its architecture should be separation-free in ground proximity.
The 2.5:1 ground-reflection finess ratio,when cut for vehicle ground clearance produces the 5.0:1 automotive fineness ration Hucho suggests as a 'minimum' for drag,and just squeeks by with respect for Mair's limit of 22-degrees max tangent angle for separation-free aft-body flow.
The ground clearance 'cut line' actually ends at about 80 % of the reflected image.This 20% of tail would demonstrate phantom flow,and from fuselage research can be cut away with no appreciable increase in form drag.
Steeper aft-bodies violate Mair's 22-degree limit and suffer separation.
Longer aft-bodies suffer skin friction increase due to the greater wetted area,although I admit that with respect to vehicles this is almost laughable.
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