Hey! No cheating here, either! That was without books and tables. The
wife has banned them from the house, and with good reason! Can't remember
my phone number, but the Cp of ethylene glycol is in my head. Not good.
You are right - straightness is not necessarily laminar indication. I was
over-simplifying a bit. We call the flow adjacent to the body "boundary
layer" flow, and it is indeed laminar as you say. My job is to cool stuff
with air Cycles and vapor cycles, and we do a lot of scoop design to get
air across those heat exchangers we've discussed ad nauseum. The boundary
layer is a funny place, and we try to keep scoops out of it, and things
like antennas and radomes in it. You can't recover much pressure there.
On our airplane the boundary layer can be many inches thick. The problem
is that to get good recovery pressure in a scoop you therefore suffer a
drag penalty, unless you do a flush NACA duct (triangle shaped), but that
takes up a lot of surface area. Can't win. Our biggest hassle is in the
transition area around Re=3000, the math gets unreliable (even CFD), and
you must test.
Enough with this technical crap, isn't there a good stock broker on the
list so we can make all some money?
----------
> From: Mike Gigante <mikeg@vicnet.net.au>
> To: Ron Soave <redlotus@spacey.net>; Daniel1312@aol.com
> Cc: spridgets@autox.team.net
> Subject: Re: Wind tunnel access minor math ahead
> Date: Monday, December 14, 1998 1:58 PM
>
> Thanks Ron! I couldn't remember the formula and I didn't have my
> books/tables handy.
>
> On the 2nd point, actually, laminar flow is not the same as
"straightness".
> Laminar flow refers to non-turbulent flow immediately adjacent to the
> bodies' surface whereas the "straightness" I was referring to was for the
> overall
> airflow in the tunnel. You always want a smooth straight airflow in the
> tunnel.
>
> For low drag, you try to maintain laminar flow over the body, whereas for
> high lift applications, you often need to artificially induce turbulent
> airflow
> immediately adjacent to the lifting body so as to avoid airflow
separation.
>
> For a car, you want laminar flow over the (upper) body, but you *may*
want
> non-laminar flow over the rear wing. Whether you do or not will depend on
> the track - high speed (Spa) or high downforce (Monaco)
>
> Mike
>
> -----Original Message-----
> From: Ron Soave <redlotus@spacey.net>
> To: Mike Gigante <mikeg@vicnet.net.au>; Daniel1312@aol.com
> <Daniel1312@aol.com>
> Cc: spridgets@autox.team.net <spridgets@autox.team.net>
> Date: Sunday, December 13, 1998 3:43 PM
> Subject: Re: Wind tunnel access minor math ahead
>
>
> >Last math for 1998, I promise.
> >
> >1) Mike's first point - The Reynolds Number is:
> >
> >Re = {(rho) * (v) * d} / (mu)
> >
> >where
> >rho is density of the air ( = to 2.7 * P/T, with pressure in psia and
temp
> >in degrees rankine {deg R = deg F +460})
> >v = velocity
> >mu = kinematic viscosity of the air (varies slightly with temp)
> >
> >2) Mike's 2nd point - "Straightness" = laminar flow, which is typically
> >considered to be Reynold's number less than 3,000.
> >
> >You really want to make sure your tunnel provides a repeatable source
flow
> >of air with evenly distributed lines in the flow field (representing a
nice
> >even pressure delivery).
> >
> >----------
> >> From: Mike Gigante <mikeg@vicnet.net.au>
> >> To: David Ramsey <dwramsey@worldnet.att.net>; Daniel1312@aol.com
> >> Cc: spridgets@autox.team.net
> >> Subject: Re: Wind tunnel access
> >> Date: Monday, December 14, 1998 12:47 PM
> >>
> >> If you do decide to use scale models and a home built tunnel,
> >> there are a couple of important things to take care of.
> >>
> >> 1) Reynolds Number (size, density, airspeed, viscosity all combine
> >> to make a scale model act the same (aerodynamically) as the full
> >> size.
> >>
> >> 2) "straightness" of the air. Baffles and honeycomb (in-line) are used
to
> >> reduce the turbulence and vorticity of the air.
> >>
> >> If you want to be *really* fancy, you could include a rolling road as
> >part
> >> of the tunnel.
> >>
> >> Of course if you just want a faster sprite, why not buy the sebring
coupe
> >> replica kit from Brian Archer. It's aerodynamics would be a lot better
> >> than stock and it is a purdy little thing.
> >>
> >> Mike
> >>
> >
>
|