Scott F. writes:
>>closer examination. I'd originally ignored the issue of turbulence
>>increasing with the cube of velocity, but the picture of the open
>>wing mounts in Carroll Smith's book kept haunting me. (For those who
>>haven't read Tune to Win, the picture I'm talking about is in the
>>chapter on aerodynamics, and outlines the flow disturbances caused
>>when you leave the mounting holes open on the top of a wing; basically,
>>you lose 20% of your wing surface due to turbulence screwing up the
>>In vehicle aerodynamics, turbulence is BAD and laminar flow is GOOD.
>>Laminar flow does things like push the car onto the track or slip
>>evenly with little drag over the body (and it's almost always OR,
>>not AND). Turbulence does things like build up air in front of the
>>car acting as a brake, or trying to pick the rear end of the car up
>>and make it come around (or sometimes over), or at the very least
>>reduce your car's grip, stability, and fuel economy (which is even
I really hate to dispute anyone who so obviously knows more
about cars than I do, but some of this aero stuff is a little off.
Turbulent flow over a body will cause higher drag when
compared with a corresponding laminar flow. The problem is that car
aerodynamics are dominated by separated flow regions. If you consider
the relatively sharp corners on the back of your car, you can imagine
that the air will have a pretty difficult time negotiating them. A
useful analogy is water flow around a rock. The flow behind the rock
is pretty disturbed - i.e. separated. Separated flow regions add to
what is called pressure drag. In general, turbulent flow is able to
stay attached to a body better than a laminar flow. So while it is
true that the turbulent flow will have a higher surface drag, the
reduction in total drag due to delaying separation more than offsets
it. This is why golf balls are dimpled. The dimples create a
turbulent flow which stays attached on more of the ball (so the wake
behind the ball is smaller). A dimpled ball will fly much farther
than a smooth one.
There are some obvious exceptions to this. For example, a car
with a vertical rear window like a pickup truck. The flow is going to
separate when it hits the edge no matter what. In this case it would
probably be best to have laminar flow over the roof. Also, turbulent
flows make a lot more noise than laminar flows. Perhaps you would
accept a higher drag on your Lexus if it was quieter.
I'm not going to add anything to the coolant flow in the
radiator discussion. Well, just one thing. One of the reasons you
are all coming up with such different conclusions, as far as heat
transfer is concerned, is that no one has really considered the
characteristic time scale associated with the flow in the radiator.
If you speed the flow up, the unsteady effects of heat transfer become
more important. When they are the same order as the steady effects,
all bets are off. You show me someone who says they have an
intuitive feel for unsteady heat transfer, and I'll show you either a
Cray owner or a fibber :-)
woodruff@caen.engin.umich.edu
And MY qualifications are that I'm the only one dumb enough to
disagree with Scott Fisher and admit it.
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