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Old 03-12-2020, 07:02 PM   #1 (permalink)
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Taking air from a Low Pressure area

If this has been covered elsewhere, my apologies. So there are well known relevant existing solutions I will try to tie into the discussion. Best known, is the typical Nascar engine air intake at the base of the windshield reported to be a high pressure area and relatively ambient air temp, the mythbusters discovery of the PU tailgate up reducing drag by effectively creating a high pressure "bubble" in the bed and smoothing rear of the Cab airflow, and typical racing sedan with a rather vertical rear window disturbing rear airflow needed for down force on the trunk deck, by use of a spoiler or a rear wing.

So my problem starts with finding symmetrical suitable location for twin rear brake cooling inlets, without resorting to scoops. Nothing seemed to be acceptable. So the next acceptable solution is resorting to axial fans in the brake ductwork, but I still need to find an inlet mounting point, and if it can help with other issues, I'm all for it.

So it seems currently with high end racers, aero trumps almost everything else on the cars. Nascar has long used the windshield base as engine air inlet, but they are highly regulated, and this decision was made by Nascar way before aero entered the pic, in the mid 60's?

So when one views common wind tunnel smoke trails, does removing any air from this Nascar high pressure area distort the path, by removing/reducing the high pressure "bubble" in this area? If so, that would seem to be a negative regarding overall aero when at speed, because the now lowered air flow would attack the front of the windshield at a steeper angle, and then tumble rearward.

I kinda base this off the above mentioned PU/Tailgate/Mythbusters revelation, where the bed "bubble" reduces drag.

So on my sedan racer track car, I am leaning strongly towards a rear wing for rear DF. Elevating the wing is very effective to help it enter "cleaner" air. I might also incorporate a very small lip spoiler on mine, to keep the rear deck in play for some added DF.

This brings me to my main question, can I use the two rear fan powered brake ducts properly positioned to help diminish the rear window turbulent low pressure (?) bubble, and help get the main over the roof after airflow lower, to better engage the rear wing, and maybe even the trunk deck lip spoiler?

There are seldom any rules limiting brake cooling fans, OK think Chaparral Sucker car, just kidding.


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Old 03-13-2020, 07:07 AM   #2 (permalink)
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I haven't done any math to support this yet, but my semi-educated opinion is that the current draw and subsequent engine drag from fans will be greater than the losses of scoops, for an equivalent volume of air.

Edit: At one point Formula One saw the use of axial fans underneath the cars to increase downforce, and it proved extremely effective. Brakes would be an obvious place to exhaust some of this air. I suppose it depends on just how much power you're willing to sacrifice in order to gain advantage in braking and traction in the corners.
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Old 03-13-2020, 08:52 AM   #3 (permalink)
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Quote:
Originally Posted by j-c-c View Post
So my problem starts with finding symmetrical suitable location for twin rear brake cooling inlets, without resorting to scoops
I would argue your problem starts further up than this. Begin at the beginning: are you having issues with rear brake temperature specifically? If so, have you figured out what the cause is?
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Old 03-13-2020, 09:25 AM   #4 (permalink)
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Fair question

The answer yes, and, no. Since this is a non competitive application, everything is to advance the hobby/my experience. With the trend towards "aero trumps everything", I'm jumping on the band wagon. My home track is rather high speed, heavy braking, I'm focusing on max DF, with a few aero extra tricks, and some restrict underbody air flow, and my thinking the high contribution of rear braking is rather useful at high speed. Lots of rear DF and sufficient tire choice and compound will likely tax the rear brakes. Fronts are already conventionally ducted. Before up-sizing/over-sizing the rear brakes, I'd rather adequately cool them, and see what their on track temp limits are.

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Old 03-13-2020, 02:29 PM   #5 (permalink)
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Fair enough! My next question was going to be what class rules you have to follow, but sounds like that's not a problem.

Similar to Ecky, I would be hesitant to jump immediately to a design with a fan. If you use a convergent duct you can get faster flow through the outlet at the brakes without the losses of a fan. I would not site the inlet at the rear window or trunk lid, however--you need to recover pressure there, not disrupt that recovery, as much as possible.

Just poking around online, there are companies that offer rear brake cooling ducts for cars like the C7:



Pull the air in just in front of the wheel housing, using as short a duct as possible.

Other options: The Gen 5 Viper and C7 take in air for differential coolers behind the B pillar, above the beltline on the Viper and on the fender of the C7:



My mistake--on the Viper, that's actually a brake cooling duct:



That might be a good design to mimic as a starting point.
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Old 03-13-2020, 09:22 PM   #6 (permalink)
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Ok, I should just get a Viper

Thanks for the significant pertinent reply.

"you need to recover pressure there, not disrupt that recovery, as much as possible."

I agree, if reduced drag was the primary objective. I'm trying to redirect the streamlines lower to better engage the wing, with the thinking "aero DF is everything".

But I'm not certain my thinking/plan will achieve this, thru even lower pressure.
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Old 03-14-2020, 12:17 PM   #7 (permalink)
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brake cooling

Quote:
Originally Posted by j-c-c View Post
If this has been covered elsewhere, my apologies. So there are well known relevant existing solutions I will try to tie into the discussion. Best known, is the typical Nascar engine air intake at the base of the windshield reported to be a high pressure area and relatively ambient air temp, the mythbusters discovery of the PU tailgate up reducing drag by effectively creating a high pressure "bubble" in the bed and smoothing rear of the Cab airflow, and typical racing sedan with a rather vertical rear window disturbing rear airflow needed for down force on the trunk deck, by use of a spoiler or a rear wing.

So my problem starts with finding symmetrical suitable location for twin rear brake cooling inlets, without resorting to scoops. Nothing seemed to be acceptable. So the next acceptable solution is resorting to axial fans in the brake ductwork, but I still need to find an inlet mounting point, and if it can help with other issues, I'm all for it.

So it seems currently with high end racers, aero trumps almost everything else on the cars. Nascar has long used the windshield base as engine air inlet, but they are highly regulated, and this decision was made by Nascar way before aero entered the pic, in the mid 60's?

So when one views common wind tunnel smoke trails, does removing any air from this Nascar high pressure area distort the path, by removing/reducing the high pressure "bubble" in this area? If so, that would seem to be a negative regarding overall aero when at speed, because the now lowered air flow would attack the front of the windshield at a steeper angle, and then tumble rearward.

I kinda base this off the above mentioned PU/Tailgate/Mythbusters revelation, where the bed "bubble" reduces drag.

So on my sedan racer track car, I am leaning strongly towards a rear wing for rear DF. Elevating the wing is very effective to help it enter "cleaner" air. I might also incorporate a very small lip spoiler on mine, to keep the rear deck in play for some added DF.

This brings me to my main question, can I use the two rear fan powered brake ducts properly positioned to help diminish the rear window turbulent low pressure (?) bubble, and help get the main over the roof after airflow lower, to better engage the rear wing, and maybe even the trunk deck lip spoiler?

There are seldom any rules limiting brake cooling fans, OK think Chaparral Sucker car, just kidding.
You're looking at $100,000 just to solve this single question.
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Old 03-14-2020, 05:11 PM   #8 (permalink)
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Inflation accounts for the increase from $64,000?

Seriously, you mean your cost estimate for wind tunnel analysis, CFD, and/or real world implementation?

I was hoping someone had an educated guess for what I was trying to do, and how likely I might gain satisfactory results.

I haven't seen or read about this concept except maybe in wind tunnel ground plane boundary air reduction.
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Old 03-15-2020, 05:38 PM   #9 (permalink)
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Quote:
The answer yes, and, no. Since this is a non competitive application, everything is to advance the hobby/my experience. With the trend towards "aero trumps everything", I'm jumping on the band wagon. My home track is rather high speed, heavy braking, I'm focusing on max DF, with a few aero extra tricks, and some restrict underbody air flow, and my thinking the high contribution of rear braking is rather useful at high
No sanctioning body?

It's an architectural question. What is the vehicle in question? Does it even have quarter-windows? Do you want to cut gaping holes in the wheelwell?
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Old 03-18-2020, 02:04 PM   #10 (permalink)
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$100,000

Quote:
Originally Posted by j-c-c View Post
Inflation accounts for the increase from $64,000?

Seriously, you mean your cost estimate for wind tunnel analysis, CFD, and/or real world implementation?

I was hoping someone had an educated guess for what I was trying to do, and how likely I might gain satisfactory results.

I haven't seen or read about this concept except maybe in wind tunnel ground plane boundary air reduction.
There have been articles about Ferrari doing things like this,and it required $100,000 at Pininfarina's or their own facility to nail down a solution.
You must know where you are to begin with.You must know every effect of every modification,and unforeseen consequences which may have never been anticipated.It's a reason why some racing teams are now $multibillion operations.
A full-scale clay model,drilled with static pressure ports all over the body,hooked to hundreds of manometers,to be photographed and captured simultaneously, would be necessary to determine present conditions.From that you'd begin to search for pressure differentials which would,while providing advantageous locations to 'tap,' also occur on the body where a NACA submerged duct,piping,and extractor could physically be cut into the body in order to take advantage of these sites.
After the modification is made,the car would now have to be tested to see if it were trending in the right direction,or if you'd introduced some deleterious complication.
It's an iterative process,step by step,potentially covering hundreds of hours in a wind tunnel.'Industry' tunnels used to cost $1,800 /hour (Lockheed Marietta,Georgia).You'd have to be an automaker or race team to even get in there.Typically,NASCAR teams have already reserved all available tunnel time,years in advance.I tried,as Independence Technological Works in 1991.They told me they'd let me in,but I'd have to wait years.And that was at $1,800/hr..I ended up paying CAR and DRIVER for testing at the Chrysler Proving Grounds.
A2 Wind Tunnel has a two-hour introductory rate of $480/hr, after which it becomes $500/hr..It all goes back to the story about the fella walking into the speed shop and asking the owner how fast he can make his car go,with the owner responding,'How much money do you have?'

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