Author Topic: Exhaust Flow and Aero  (Read 125100 times)

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Offline John Burk

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Re: Exhaust Flow and Aero
« Reply #15 on: June 28, 2011, 01:45:22 AM »
Drag improves as the temperature of the air increases . If there was a way to spread the hot exhaust down the side of the car without adding turbulence there should be a gain .

Offline Dr Goggles

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Re: Exhaust Flow and Aero
« Reply #16 on: June 28, 2011, 02:05:24 AM »
I have been pondering this alot too.  Running the exhaust out of a two 4in. collectors through the car will be tough in the Datsun we are building (slowly).  Shooting it straight out of the lower fender will look cool but will it hurt the aero? Could always turn them up through the hood like a tractor pull truck! Anyone that can comment on this with some knowledge will help MLSE and I out tremendously.

You can choose to imagine the air as flowing over the car or the car passing through the still air. Either way the closer the air follows the shape and the less "wake" you leave the better. (This part is easy to understand, it takes energy to stir something...if you leave a trail of eddys you are losing energy, the energy it takes to stir it up...)

Think about what exhaust travelling at 90 degrees to the direction of travel will do to the air that you are trying to leave , shall we say, as you found it.

Few understand what I'm trying to do but they vastly outnumber those who understand why...................

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Offline superford317

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Re: Exhaust Flow and Aero
« Reply #17 on: June 28, 2011, 11:11:32 AM »
Power equals thrust times velocity, so one extra pound of thrust at 400 mph is 1 lb * 400*1.5 ft/s = 600 lb*ft/s, a little over one horsepower for each pound of additional thrust. Bear in mind this is net, delivered horsepower - equivalent to at least five thermal horsepower of fuel consumption if it had been generated by putting in a bigger engine.
This is why engineers of the Thirties and Forties were so eager to derive extra thrust from engine exhaust - with or without augmentors - and from coolant heat. It also motivated efforts to use exhaust flow to pump cooling air.
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Offline superford317

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Re: Exhaust Flow and Aero
« Reply #18 on: June 28, 2011, 01:58:12 PM »
If the exhaust flow is being used to raise the temperature of the air hitting the rear tyres, it could have a significnat effect. Drag and lift from the tyres is directly proportional to air density. Raising the temperature 10 deg C will cut both drag and lift 3%. This would be a big boost as the wheels are a major lift inducer and create significant drag.
High hot gas volume in the leading edge of any part will reduce the drag as well as the aerodynamic force: lift or downforce.

 
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Offline superford317

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Re: Exhaust Flow and Aero
« Reply #19 on: June 28, 2011, 02:14:11 PM »
the effects of dumping the hot exhaust under the car would depend on several things, the shape and design of the underbody being the largest factor.
depending on the shape and design of the underbody and the exit location of the hot exhaust it could do anything from adding more down force and increasing traction, to lowering the overall drag of the vehicle and thus giving better aerodynamics and increasing speed.
  
« Last Edit: June 28, 2011, 02:31:41 PM by superford317 »
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Offline superford317

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Re: Exhaust Flow and Aero
« Reply #20 on: June 28, 2011, 03:27:08 PM »
aerodynamic force is directly related to air density flowing past the body.
half the air density and you half the drag.
half the air density and you half the lift.
aerodynamic force varies with velocity, double the velocity and you quadruple the drag.
even small well thought out plans for the exhaust can give BIG benefits in several ways.
it is well worth doing some research and doing more with the exhaust than dumping it without any second thought.
if it can drive turbochargers and make thousands of HP it can be used for many other things also.
« Last Edit: June 28, 2011, 03:46:54 PM by superford317 »
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Offline superford317

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Re: Exhaust Flow and Aero
« Reply #21 on: June 28, 2011, 04:18:11 PM »
without straying to much from the original post.
another thing to do some research on is the german heinkel HE100 fighter airplane first flown in 1938.
the HE100 used exhaust ejectors to boost air speed and had a unique evaporative or steam cooling system for the engine along the wings leading edge.
if rules permit engine cooling water could be circulated through double body panels at strategic locations and used to radiate heat to the surounding air flow without any additional drag and should reduce the density of the surounding air and make some gains in performance by reducing the air density along the skin.
Constructing a 1000hp turbocharged bellytank

Offline Dr Goggles

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Re: Exhaust Flow and Aero
« Reply #22 on: June 28, 2011, 06:52:22 PM »
Thanks Superford, seems you've had your nose down and tail up, good research.

That Heinkel was a complicated piece of kit, good looking shape......

Me thinks the allies were lucky this thing was pushed aside....check out the speeds and range under "Prototypes" here...

http://en.wikipedia.org/wiki/Heinkel_He_100


btw, show us some pics of your tank, we're all interested here.
« Last Edit: June 28, 2011, 07:01:39 PM by Dr Goggles »
Few understand what I'm trying to do but they vastly outnumber those who understand why...................

http://thespiritofsunshine.blogspot.com/

Current Australian E/GL record holder at 215.041mph

THE LUCKIEST MAN IN SLOW BUSINESS.

Offline manta22

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Re: Exhaust Flow and Aero
« Reply #23 on: June 28, 2011, 08:24:42 PM »
Dr;

I suspect that the He100 was too complicated for its own good. A case of a designer outsmarting himself-- TOO many pumps!  :?

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline TwinSpin

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Re: Exhaust Flow and Aero
« Reply #24 on: June 28, 2011, 09:50:44 PM »
Here is a different approach to exhaust flow that I have been thinking about, especially for a streamliner (if I ever get to build one).

Why not put the exhaust, single pipe, out the front center of the nose of the belly tank or streamliner?

Here is my thinking; torpedoes and Johnny Weissmuller. First, Weissmuller won 5 Olympic gold medals in the 1920's. When interviewed during his movie years as Tarzan on how he swam so fast, he said: the fastest way to swim in water is to swim out of the water. He explained how he arched his back to bring most of his torso out of the water, except for his arms and legs.

I remembered Weissmuller when I heard about the U.S. Navy having 300 mph torpedoes. I don't mean that the 300 mph torpedoes actually move "out of the water," but that some way the water is moved out of the way, I guess? Therefore, IF...IF...the Navy must create some form of cavitation of the water at the torpedo's nose and along the sides, then I could believe there are 300 mph torpedoes.

Alright, apply this theory to a streamliner to echo what John Burk said in Reply #15: "... spread the hot exhaust down the side of the car ... for a gain." Those New Jersey guys always have good ideas.

If someone tries my theory and gets a record, I'll be the first to buy him or her a steak dinner. :cheers:

Now, that's my food for thought on exhaust flow.

Offline superford317

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Re: Exhaust Flow and Aero
« Reply #25 on: June 28, 2011, 11:50:17 PM »
i have been in racing for several years, i am currently in iraq working as a civillan for the US government for the last 5 years.
thanks Dr Goggles, your tank is one i have ben lookng at for sometime and the way the exhaust exits the rear. the meredith effect could be implemented on the tank exhaust at the rear by using an exhaust augmenter where the exhaust exits the tank, smooth the body panels on the sides of the engine where the air louvers are and replace them with a NACA duct on each side. the large suction action of the exhaust augmenter would be like a vacume cleaner running and pulling cool air in the NACA ducts around the hot engine compartment and out the back, contributing more heat and expanding gasses to the air stream behind the car, filling the void behind the vehicle and increasing aerodynamics.
an earlier post beat me to the punch, it got to late for me last night so today i was going to elaborate about the american and the russian 300MPH torpedos, they use compressed gasses released along the front and sides of the torpedo to lower the drag and increase the speed, the gas bubbles create a boundry layer of gas around the torpdo body, pushing the water out of the way, water being many times more viscous than air and subsequently creating more drag.
i think the best way to release the exhaust gasses would be to imagine 2 long, flat header collectors 1/8 to 1/4 inch tall but 36inch in width surounding the body about its circumferance, releasing the hot exhaust gases into the air strem smoothly around the entire body diameter.
the ideal placement would be as close to the nose as possiable so the hot gasses could cover as much of the body over the longest distance, if you have a bullet shaped nose, which a LOT of the tanks and streemliners fall under this, you can imagine a bell shape being placed over the nose and it having a small air gap between it and the body for the exhaust to escape and the outlet device would have minimal effect on the vehicles aero. the bell shape with a circuler exhaust inside would distribute the exhaust evenly, if you only had a straight pipe sticking out the front, the gas distribution would be all over the place, the aero impeded, plus the on coming air would be working against the exhaust flow, trying to push back down the pipe.
« Last Edit: June 29, 2011, 12:13:21 AM by superford317 »
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Offline superford317

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Re: Exhaust Flow and Aero
« Reply #26 on: June 29, 2011, 12:16:37 AM »
imagine the exhaust augmenter working in reverse capped over the nose of the vehicle.
the exhaust augmenter works a lot like the bernoulli principal in design and function.
the vehicle moving at high speeds would actually help to pull the exhaust out of the system and improve exhaust flow.
« Last Edit: June 29, 2011, 12:29:17 AM by superford317 »
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Offline Dr Goggles

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Re: Exhaust Flow and Aero
« Reply #27 on: June 29, 2011, 12:58:29 AM »
....that distant crunching and grinding noise is the cogs in the Reverend's head starting to turn...

I think I see an exhaust augmenter in the Spirit of Sunshine's very near future......

Have a biscuit Superford , that is the single best peiece of info I have seen posted here for some time......
Few understand what I'm trying to do but they vastly outnumber those who understand why...................

http://thespiritofsunshine.blogspot.com/

Current Australian E/GL record holder at 215.041mph

THE LUCKIEST MAN IN SLOW BUSINESS.

Offline superford317

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Re: Exhaust Flow and Aero
« Reply #28 on: June 29, 2011, 01:54:58 AM »
the russians & americans have used the idea of ejecting compresed gasses to lower hydrodynamic drag and there by increasing the speed of torpedos and entire submarines, they use the nuclear reactors to recover air from the sea water, store it in tanks and during a battle if they need a sudden burst of speed to evade an enemy torpedo they release the air along the hull of the submarine and reducing the hydrodynamic drag enormously. plus all of the air bubbles in the water help to throw off the tracking system of the torpedo.
Constructing a 1000hp turbocharged bellytank

Offline superford317

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Re: Exhaust Flow and Aero
« Reply #29 on: June 29, 2011, 07:14:35 AM »
non streamlined shapes have to worry more about pressure drag and streamlined shapes have to worry more about skin friction drag. so how to get the most benefit from the exhaust would depend on the shape that you are pushing through the air.
the worse your vehicles aerodynamics the greater the effect of dumping all of the exhaust and what ever hot air you can extract, into the wake behind the vehicle to help fill the void behind the vehilce and to decrease the pressure drag.
for the more streemlined vehicles the largest benefit would be gained from letting the exhaust flow over the body to decrease the skin friction drag.
never mind the name of the picture, it is was a side note for some research  i was doing.
« Last Edit: June 29, 2011, 10:45:29 AM by superford317 »
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