(re-sending because I forgot to delete the trailer)
Jim:
I read somewhere recently that aerodynamacists themselves are also still
wishing
for just such a program as you describe. The book (or article-- I don't
remember
the source) led me to believe that a really useful program of that type
(for
determining Cd from a complex, three-dimensional model using
computational fluid
dynamics) does not yet exist.
Russ Mack
"Waldron, James" wrote:
> Elon,
>
> Ah! Yes, you are correct. As some of this stuff is starting to come back,
> drag is the square of the speed, and power is the cube. The more power, the
> faster the acceleration.
>
> In a frictionless environment, a very small amount of power would
> *eventually* get you up to speed, more power would just get you there
> faster.
>
> As you indicate, air resistance builds as the square of speed. I seem to
> recall that all of the other frictions remain fairly constant. So, if you
> know the air resistance of a vehicle, you can plot power against speed and
> determine where horsepower equals air resistance (less the built in other
> frictions, rolling resistance, bearings, etc., but I recall that these will
> be very small compared to the air resistance) and determine your 'terminal'
> velocity.
>
> Since the track at Bonneville is of fixed length, I also seem to remember
> working out that with a small amount of power, even though you had a very
> slippery vehicle, you would not have enough time to reach top speed before
> you ran out of road (because your acceleration would be so slow - this
> sounds like a basic argument for a transmission. A 15 to 1 gear ratio
> doesn't give me 15 times more horsepower - but it does give me roughly 15
> times more torque(?) Or is it just a mechanism to allow me to couple the
> best power producing setting on my engine to the current speed/acceleration
> need?) <<It might be a good idea to sit down and think about this awhile
> before blurbing out these emails>>
>
> As far as the Reynolds numbers went (and I'm still reading some of the links
> kindly provided by this forum), they indicated the slippery-ness or part of
> the calculations for the air resistance of the body. The math around
> calculating the airflow around any but the simplest of bodies was daunting,
> and I recall my professor telling me, "most are determined in wind tunnels,
> so your best bet is take some numbers from some known shapes and
> approximate".
>
> I am still searching for is a computer program (all my previous work was all
> pre IBM-360 days) where I can put in the 3 dimensional body design, and let
> the computer do all that daunting math, and tell me the Reynolds numbers and
> air resistance. Then, of course, I'd like to be able to change the shape of
> the body on the fly and see what effect various changes had, and see what
> could be done to minimize the drag.
>
> I feel sure that the guys at Lockheed and Boeing have wonderful systems
> where they can fly their virtual airplanes around through lots of virtual
> air and determine all sorts of things. (Hey! I can bring an autocad file!)
> I also remember seeing or reading about some PC type systems that were used
> by the America's Cup sailing guys to study hull shapes, but haven't been
> able to relocate the information.
>
> There are probably some general rules of thumb as well, don't put anything
> flat on the front, or don't exceed a certain growth rate of length to
> diameter, that I would be happy to know. Heck, for all I know there could
> already be the perfect shape and it's down to who does the best job of
> getting the mechanical stuff right.
>
> Then again, I may be over thinking this whole thing. I continue to search,
> will be very happy to learn from others experience of knowledge, and will be
> happy to post back here anything interesting I find. Let me know if you
> have any leads!
>
> Thanks,
> Jim.
>
> P.S. As the day wears on and I got some research time.
>
> I have found several 3D 'numerical wind tunnel' programs, some military and
> not available outside DOD, and some from NASA - I have asked about
> availability but have nothing back yet.
>
> A very interesting paper at.
>
> http://www.mech.soton.ac.uk/jrc/ME312/Auto_Aerodynamics_Notes.doc
>
> Some excerpts:
>
> "Simulation of the whole car flow (CFD, "numerical wind tunnel") will come.
> It is currently possible on a rather simplified scale, but rapidly
> developing. Still problems with size/speed of computers and with the
> simulation of turbulence."
>
> "Typical passenger car: Frontal area 1.8 to 2.1 m2
> Weight 800 - 1800 kg
> Typical drag coefficients 0.35 - 0.45. This has changed over time: see
> diagram in lecture. Minimum at about 1950: main ideas of streamlining had
> been sorted out, but regulations (higher lights), performance (wider tyres)
> and styling (integral front wings, no running boards) worked in opposition."
>
> "There is also non-aerodynamic drag (rolling resistance) which also
> increases with speed, but nothing like so fast. Aerodynamic drag: 50% at 65
> km/h, up to 80% total at 100 km/h."
>
> "Drag reduction is very much in the region of diminishing returns and very
> close attention to many very minor details. To quote one of the books I
> looked up, if you want a CD of 0.5, just draw it and make it. For 0.4, do a
> few confirmatory wind tunnel tests on a model. For 0.3, spend millions on
> model and prototype testing."
>
> -----Original Message-----
> From: Elon Ormsby [mailto:ormsby1@llnl.gov]
> Sent: Thursday, January 24, 2002 4:10 PM
> To: land-speed@autox.team.net
> Subject: Reynolds Numbers
>
> James Waldron I think your answer is correct but by indicating drag is the
> "cube" of speed might not precisely define the problem. Isn't drag the
> square of the speed but power required is the cube of the speed? I think
> the additional exponent is for acceleration and power is need for that as
> well as overcoming drag. I mention this here because many times the
> significance for the "cube" of the power is lost. Many think the power
> required is only to overcome drag (a squaring function) and it ain't so. I
> look forward to many more of your technically interesting contributions.
> -Elon
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