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----- Original Message -----
From: M Roberts <montyr2157@alltel.net>
Date: Friday, November 9, 2007 11:40 am
Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity????
> Well.. I haven't spent $$$ on it but SolidWorks now comes w/ a
> version of
>
> CosmosFloWorks in the package in 2008.. I've got the software so
> I may give it
>
> a whirl.. I was waiting to test it before I commented but.. since
> people are talking
>
> directly about it.. :-)
>
>
>
> Jarrett
>
>
>
> Jarrett,
>
> I'm not implying that CFD isn't fun to play with, and if you have
> CFD available go for it! Just don't expect to learn anything more
> than gross trends for design purposes. I have been involved in
> several internal flow designs where we used "expert" PhD types for
> analysis using high end programs like Fluent. In the end you still
> wind up in the lab with modeling clay to make things work right.
> Even if you know what you are doing and have good data to plug
> into the models (heat exchanger data, boundary layer conditions at
> the inlet and exit, pressure and temperature conditions, prop wake
> effects, etc) you are still limited by your fabrication
> capacities. Most of us don't have the ability to build an optimum
> heat exchanger. We have to use what is available off the shelf. So
> the system will be optimized for readily available exchangers, not
> the optimum. Then what level of fabrication accuracy is available
> for the duct work. Most of us carve some foam and slather some
> fiberglass on it. We don't use a 5 axis router to make a mold from
> a highly refined CFD model and vacuum bag some duct work. To get
> the 5% extra you might get from accurate CFD analysis is going to
> take a lot of testing to get data to "correct" the CFD model and
> then a lot of high end fabrication. All for....5-10% gain.....maybe.
>
> CFD has it's place. Primarily in very specialized areas of design
> engineering like turbo machinery. In these cases building and
> testing prototypes is so expensive, you can afford to hire the
> brain/computing power to develop the models and test/refine them.
> Each new real world test results in tweaking all the fudge factors
> for the models specific use....and there are a lot of fudge
> factors.
>
> In our application, you would be better off with a pencil, paper,
> calculator, and a copy of Kucheman and Webber, plus a thermo 1
> text book and a pile of old NACA reports. You are going to have to
> do this anyway as a gut check for your CFD models. If you don't
> you will be off down the rabbit hole with no trail of bread
> crumbs.......trust me.
>
> In the end you will still have to test to refine. Best to avoid
> analysis paralysis from the get go. One test can invalidate years
> of mental gymnastics.
>
> Our understanding of physics is still limited, especially non-
> steady state, non linear things like turbulence. Of course the
> people who sell the software will never admit this......but it is
> the truth. Models are models and reality trumps all.
>
> FWIW
>
> Monty
Monty, I'm totaly with you on this, I don't expect to get total and complete data from
this CFD, it's a dumbed down version of the real deal anyway.. but it will show some
data and might point out things that I'd have missed while designing something..
It's a good indication that things will be like this for a long time.. the military is still
doing testing on their designs before putting them into production.. we'll see this for
a long time.. I'm not sure we'll ever get to the point where we can design something
this complex via 3d model's and then just build it from there and have it work..
[ie.. w/ no proto types]. But then, that's half the fun! :-)
Also, I'm not your average builder in that I do have a full 3 Axis [5 axis is in the works as
I speak.. I've got to get my castings for the B/C head first] CNC mill. It has a 20 x 10 x 4 [all feet]
foot print and is able to machine all the molds I need for my vacuum bagged, composite
parts.. I intend do some fun stuff w/ this setup in the immediate future... [this winter]
I'm currently working on a mold set for a helicopter canopy [it's an RC] and I'm having a blast doing it..
Can't waite to tackle some neat & niffty stuff for my own projects..
I'm working on a website as well.. so I can promote my services to other builders who would
like to have access to this type of equipment and accuracy for their projects.
Lots of fun stuff to come in the future I'm sure!
All the best
Jarrett Johnson
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