[DeTomaso] Kurt Busch Pantera

Sat Nov 18 20:06:58 EST 2017

I "borrowed" this equation (hope it is correct) that provides the "ram" pressure increase for a perfect scoop.
I don’t think I could understand Bernoulli now to verify. 

-----Original Message-----
From: DeTomaso [mailto:detomaso-bounces at server.detomasolist.com] On Behalf Of Daniel C Jones
Sent: Saturday, November 18, 2017 19:31 PM
Cc: detomaso
Subject: Re: [DeTomaso] Kurt Busch Pantera

> It's been my understanding that the laminar flow of air along the 
> sides
of the Pantera require enormous "elephant ear" style scoops in the rear quarter window area to catch it because it's so far off the car.

Not exactly. There are laminar and turbulent boundary layers which are distinct from pressure separation.  If the flow remains attached to the surface of the vehicle, the layer on the surface will have essentially zero air speed.  Each adjacent layer will have a higher airspeed until the boundary layer reaches the free stream velocity (equal to the speed of the vehicle if you assume zero wind).  Note the boundary layer can be either laminar or turbulent.  It's generally difficult to maintain a laminar boundary layer and the farther flow goes over a surface, the more likely it is to transition to turbulent.  The point at which an undisturbed boundary layer transitions from laminar to turbulent is determined by the Reynolds number.  Note that while pressure separation (where the flow detaches from a surface) is generally bad, a turbulent boundary layer can be a good thing as it adheres to shape changes better (or hangs onto a shape longer) than a laminar boundary layer.  We measured the boundary layer thickness over the roof of a fox body Mustang in an attempt to get flow to remain attached over the hatch to improve the rear wing effectiveness but the flow was already turbulent (vortex generators made no change) and still detached at the transition of the roof to the hatch so the majority of the flow over the wing was from the sides of the car.

Boundary layer thickness can be measured using an inexpensive hang glider airspeed indicator and, for the speeds involved, the boundary layer thickness is small (a few inches).  The scoop only needs to be raised off the surface by the the thickness of the boundary layer to get to free stream air.  If flow pressure separates, then the distance will be larger to reach free stream air.

> So.... the simple rectangular openings in the sail panels of the rear
deck lid would seem to me, to be -wrong-.

An engine power increase from a duct or scoop comes from two sources:
colder/denser air (compared to the engine compartment temperature) and/or from a static pressure increase.  It appears they are using the ducting to feed cool air to the engine, not to pressurize it.  To provide a static pressure increase, the air flow needs to be slowed.  For speeds under 100 MPH, even slowing the free stream air to zero velocity has a minor pressure increase and the dominant effect is from the cooler air.  There is also a drag penalty associated with hanging a scoop out in free stream air and, when air is compressed, there is an attendant temperature increase.

> A couple pics from the Ford Motorsports weekly email Supposedly in the 
> next episode they put it in a wind tunnel....

It will be interesting to see if they show any flow visualization in the wind tunnel.  It looks like most of the air over the rear wing will come from the sides of the car since flow will detach over the sugar scoop.  I also don't see anything in the way of structural reinforcement of the wing (more of a flap really) that would be necessary if the aero loads were very high.  Presumably they will run without side windows and the location of the duct opening looks like some/most of the flow will come in from the roof and top of the door.  The front aero looks functional with a flow through nose and there are side skirts but I don't see a rear diffuser.
I'd expect some belly pans underneath to smooth things out.

> those large dive plane elements they added to the ends of the air dam
will send vortexes along the sides of the car and 'clean' the boundary layers to a greater or lesser extent.

The upward swept portion of the air dam on the sides looks like it would direct flow to mix with the air coming over the hood and around the doors.
Do you think a vortex will be created at the top edge of the fender or maybe somewhere else?  Vortices off the flat lip are too low and in front of the spinning tires to do much.

> And shaving the rain channels off the a-pillars will help skinny up 
> the
side boundary layers as well

Those were likely removed to narrow the frontal area (drag is directly proportional to frontal area) and they  wouldn't have provided any sort of aero benefit since, even if they had tripped the boundary layer, the window area is open just behind them.

Dan Jones

On Sat, Nov 18, 2017 at 6:20 PM, Mike Drew via DeTomaso < detomaso at server.detomasolist.com> wrote:

>    In a message dated 11/18/17 14 52 19, pantera at vtc.net writes:
>
>      /The simple answer is that as speed increases, the sealed air box
>      becomes a ram air system. The only place for the air to go is into
>      the
>      engine. I never bothered to test the design above 100 mph, but I
>      never
>      saw an inlet air temperature increase at any speed.//The only time
>      the
>      engine inlet air temperature went up is when the ambient temperature
>      increased./
>
>    >>>I think the question was raised because over the years, loads of
>    armchair aerodynamicists, armed with facts/experience/knowledge/BS in
>    varying degrees, have postulated that as the speed increases, the
>    laminar boundary layer seperates from the skin of the car (and thus the
>    scoops), leaving a turbulent boundary layer behind.  It is a well-known
>    "fact" that air scoops that don't stick out very far have zero effect
>    because they are in 'dead air' with no actual flow, and you need the
>    giant Hall 'elephant ear' scoops in order to accomplish anything at
>    triple-digit speeds.
>    I neither subscribe to this theory nor refute it--I freely admit I lack
>    the requisite facts/experience/knowledge to weigh in on the argument,
>    other than to clarify the argument itself....
>    Mike
>
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