I assume FRIAM folks want to increase their knowledge - or mebbe not.
Credentials: I have supervised wind tunnel tests of vehicles in arrays at USC
and made extensive theoretical calcs with grad students on this subject , have
tested my own designs (the Sunraycer and GM Impact) in the Caltech tunnel and
the GM tunnel, and probed the wakes. I have driven instrumented test vehicles
in the wake of bluff bodies at a decommissioned airfield in CA, at our test
base at El Mirage Dry Lake, CA, and the GM Proving grounds in AZ. It's pretty
hairy. I hold the patents on two truck drag reduction airshields.
Here's the received knowledge, that I take to be correct:
There is NO SUCH THING AS A BOW WAVE in incompressible continuum flows. The
field equations are elli ptic, won't permit same, and Nature agrees! Bodies in
a fluid stream create a wake of low energy flow that trails behind ( but NOT as
a CYLINDER!). Statements that wak e pathlines are longer than in undisturbed
flow are correct. The idea that this somehow forces the flow to go faster is
VOODOO fluid mechanics that I didn't know was still accepted. Wake flows are
actually much slower than freestream. Said wake contains a lower energy flow,
and lotsa turbulence. It extends for about 12 scale lengths astern of the
body, until re-energzied by turbulent entrainment from the surrounding flow.
The drag of a body immersed in this wake is significantly reduced (but not the
drag coefficient). For bluff bodies like cars, bikes or peoples the velocity
deficit of the wake is very pronounced. The wake is influenced by ground effect
(unlike the prop turbine case), and is very turbulent, with eddies of about the
same as the body scale, especially when the body does no work on the flow, as
is the case with bikes etc. There are no lifting components here (at least
in the correct, nonFRIAM, use of the word), but severe crosswind disturbances
usually occur.
These wake effects are us ed to great advantage in peletons and in drafting for
formula race cars. And also by crazy people (like one of my professional
drivers, since killed !) for fun behind trailers on freeways.
There's no mystery about anything in this sort of array interference, except
the apparently eternal riddle of turbulence.
I'll be glad to answer furth er questions, if I know the answer!
Peter Lissaman, Da Vinci Ventures
Expertise is not knowing everything, but knowing what to look for.
1454 Miracerros Loop South, Santa Fe, New Mexico 87505,USA
tel:(505)983-7728
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