The external flow over the louver should certainly induce air to
exhaust through the louver but the next question is, where is that air
really coming from.
There are four paths for air to get INTO or OUT OF the engine
compartment:
1. Directly through the radiator (some flow due to velocity head
(ram air), some due to the fan (Triumph and/or electric fan)).
2. Upward from below the engine (relieving pressure buildup of air
going below the front bumper)
3. Through the louvers (most likely exhausting due to induced flow
over the outside)
4. Through the gap between the bonnet and the fenders (my gap is
~1/2" wide along the entire length of the bonnet) - not a major path
but technically it contributes.
I submit (without benefit of real supporting data) that a fair portion
of the air exhausting through the louvers is probably coming INTO the
engine compartment from below the engine rather than through the
radiator. The radiator flow (without benefit of auxilliary fan) is
probably signifigantly less than the air flowing up from below the
engine.
If the bottom of the engine compartment were sealed to prevent airflow
upward, the pressure under the front would increase (unless an airdam
is added) and the front end would feel lighter while the airflow out
through the louvers would be lessened and hotter (no mixing/diffusion
from the cooler underbody air, less overall air due to the limited
radiator flow).
On the other hand, putting an airdam below the front bumper should
drop the pressure below the engine so much that the source air from
the radiator may not be enough and it might draw air back through some
of the louvers in order to balance the pressures. Under these
circumstances I would guess the louvers toward the back would be more
susceptible to backflow since they are in the turbulent shadow of the
leading edge of the car.
So does anyone have some grant money to go study this phenomenon in
detail? It would require use of several vehicles, a modest
instrumentation budget (thermocouples, anemometer, a tuft of yarn on
the end of a stick), workspace, and nice weather... I volunteer for
data collection (unless it involves crawling over the bonnet while at
speed ala Frank Costin).
"Vandergraaf, Chuck" wrote:
>
> Dave,
>
> Wait no longer; see my e-mail dated yesterday, copied below:
>
> Len,
>
> Not just the +8, but also the 4/4 and the +4 (but, apparently not the new
> Aero 8) have louvres. I'm not an engineer (aerospace or otherwise) but I
> suspect that, when the car is in forward motion, the louvres force the air
> rushing over the bonnet away from the bonnet and create a partial vacuum to
> help suck the air out. The only other way for air to escape is through the
> bottom of the engine compartment. I agree, the louvres look cool but are
> hard on the finger tips when polishing the car. But, "no pain, no gain."
>
> Chuck Vandergraaf
> Pinawa, MB
> '52 +4 (complete with louvres)
>
> Chuck (radiochemistry; not physics)
> ----------
> From: David McCoy[SMTP:gdm_1419@yahoo.com]
> Sent: Thursday June 01, 2000 11:57 AM
> To: morgans@autox.team.net
> Subject: Re: Louvers in the Bonnet
>
> --- "Michael D. Miles, PE" <mdmiles@mdmpe.com> wrote:
> > Not exactly. The theory is that a certain amount of
> > air enters the
> > GRILL but its 'path of least resistance' is actually
> > downward through
> > the opening to the ground between the grill and
> > radiator. This
> > reduces the amount of air rammed through the
> > radiator signifigantly
> > and therefor allows backflow (recirculation) from
> > the engine
> > compartment to percolate forward through some of the
> > radiator, thereby
> > reducing its efficiency in removing heat.
>
> Waiting patiently for "bernouli" to surface...IR
> airflow over and across louvers pulling air from
> engine area... OR perhaps louver as "wing" ....
>
> Dave (C in physics) McCoy
>
> __________________________________________________
> Send instant messages & get email alerts with Yahoo! Messenger.
>
--
"Entropy Happens!"
Michael D. Miles, PE Consulting Design Engineer
(503) 292-1234, FAX: (503) 292-1105
email: mdmiles@mdmpe.com
http://www.mdmpe.com/
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