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Old 07-22-2019, 06:47 PM   #1 (permalink)
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High pressure to low pressure air movement

Hello again everyone.

Yep, it's the guy with the silly questions and crazy ideas again. I feel like the underachieving student with the brain trust here at EM, but asking silly questions is just a part of my learning process. This again is something I would never do to a vehicle, but I sometimes wonder how changes affect the aero penalty of a vehicle. Most of the changes we make to our vehicles don't completely negate an aero penalty but lessen it so that the net result is a smaller penalty (positive gain?). This is just another one of those silly ideas that popped into my head while leafing through Aerodynamics of Road Vehicles again.

I know there is a high pressure area in the front of a vehicle as it moves down the road, and a lower pressure area in the rear. I believe the idea of "air curtains" is to help remove some of that high pressure in the front and move it to the sides of the vehicle so it can flow with the attached air on the sides. To me this makes a positive gain because it lowers the difference (delta ▲?) between the two pressure areas.

So the first question I had was..... what if you could move some of the high pressure from the front of the vehicle and use it to fill the low pressure void in the rear? Imagine a 6" tube from the grille running through the vehicle that exits above the bumper into the low pressure void. I'm assuming that this would create a smaller difference between the front and rear and be a theoretical benefit?

Then, while watching a NASCAR race I saw the NACA (?) ducts that move air to help keep the drivers somewhat cool. They openings must not have much aero penalty or they wouldn't use them like that. So the next crazy question was.....what if you cut two NACA ducts into a trunk (top or sides I'm not sure) or the outsides of a truck bed and tubed that air out the back above the bumper or out the tailgate (with some sort of bed cover not an open bed)?

Neither idea is practical because of having to open a trunk repeatedly for storage and losing the carrying capacity of a truck bed and I get that. But in my mind, it's a "what if....." or "wouldn't it be neat if....." kind of idea. Definitely not practical, but a way to try and better learn a concept.

Thanks for the thoughts on this and for always being willing to help me work out my crazy way of learning.

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Old 07-22-2019, 07:12 PM   #2 (permalink)
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We've been down that road before.

IMHO if you have the space to spare for a duct you are better off chopping the top and reducing frontal area. Ducts have drag too. Think surface area. Ducts actually add surface area. I'm suspicious of air curtains for that reason but they've made it into production so I guess the delta made it worthwhile.
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Old 07-22-2019, 07:18 PM   #3 (permalink)
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Are there threads on this? Neat. What should I search under? I'd like to read about it.

I never thought of the drag from the surface area of a tube as being very high. I guess I thought the amount of pressure (flow?) of air pretty much negating this? But then again this is an area I have absolutely no expertise in.
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Old 07-22-2019, 08:16 PM   #4 (permalink)
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Search the internet, you can find pressure drop calculators and should be able to specify psia for vacuum instead of psig.

https://www.tlv.com/global/US/calcul...sure-loss.html

I would assume blocking the front of the vehicle and installing under body panels would be a better solution.
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Old 07-24-2019, 12:01 AM   #5 (permalink)
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You need to think a bit differently. If you have a fan, and it moves air with a motor, it's pretty intuitive that the fan uses energy to make the air move, right? When a vehicle moves through the air, it also is going to make the air move, the air you must think of as wanting to be stationary, but the car moves it out of the way, and drags some air along with it, this making air move requires energy, the amount of energy needed is considered drag. So, the real name of the game is making the air move as little as possible as you drive through it thus keeping the energy required to make it move as low as possible.
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Old 07-24-2019, 10:58 AM   #6 (permalink)
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ducting

Quote:
Originally Posted by hat_man View Post
Are there threads on this? Neat. What should I search under? I'd like to read about it.

I never thought of the drag from the surface area of a tube as being very high. I guess I thought the amount of pressure (flow?) of air pretty much negating this? But then again this is an area I have absolutely no expertise in.
It's impossible to recover more usable kinetic energy from internal,ducted flow,than from streamlined exterior flow. It violates the second law of thermodynamics,via skin-friction heating that would not be present in exterior flow.Also,you're robbing flow from the outside of the body,which always needs all it can get.
The lowest drag bodies are always 'streamlined' bodies,with the most minimum of internal flow.
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Old 07-24-2019, 06:56 PM   #7 (permalink)
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I think I'm understanding it better now.

So the most efficient "model" would be the AST viewed in 3-D. Like a teardrop. Not practical for a vehicle so a near half teardrop with the road being the mid-plane (?) of the teardrop.

So another question.....in my mind I'm seeing a half teardrop with lines representing the (laminar?) flow along it's surface. As the pressure travels rearwards along the teardrop (and comes back together) does it's cumulative effect "squeeze" the tail of the teardrop causing theoretical forward movement? Like pinching a watermelon seed between your thumb and forefinger and shooting it at your wife? So if you rob some of the pressure through the middle (6" tube) you decrease the amount of pressure along the sides of the teardrop and lessen the "squeeze"? Not only would it decrease the pressure moving down the sides but could create drag along the insides of the tube because of the friction?

That would be why a boat tail works so well? It helps direct the flow along the sides into the low pressure void at the rear of the vehicle? I can see now how an aero cap and a boat tail would be beneficial for a pickup truck.
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Old 07-24-2019, 08:12 PM   #8 (permalink)
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its an idea that is almost good
the thing is its incomplete
you have to overcome the penalties you incur by taking some of the air away from the carefully curved and angled areas in the back that helps squeeze the car forward like a watermelon pip and by adding surface area

there are however ways that you can do that
primarily I think would be to heat the air so that it supplies -more- volume to the low pressure zone than it robs from the air foils of the body work
if you took that heat from the engine cooling system you could shrink or even eliminate
the radiator doubling up as it were on your benefit

If your tube was tapered like a long rocket cone ????
maybe even a little thrust
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Old 07-24-2019, 08:20 PM   #9 (permalink)
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Quote:
Originally Posted by hat_man View Post
As the pressure travels rearwards along the teardrop (and comes back together) does it's cumulative effect "squeeze" the tail of the teardrop causing theoretical forward movement? Like pinching a watermelon seed between your thumb and forefinger and shooting it at your wife?
No--that would be impossible, since you would be getting more energy out of the system than you put into it. But yes, after the flow goes over the top of the car, it accelerates over the shoulder (the high point of the roof or the windshield/roof junction, depending how the car was designed) thus lowering its pressure. If the body behind that point curves away gently, the pressure is gradually recovered as the flow decelerates back to the free stream velocity; if it drops too fast, the flow in the boundary layer actually starts to stall or reverse and it detaches from the surface. However, you can't recover more pressure in the rear than the body's movement generated at the stagnation point in the front. If you taper the tail all the way to a point, with attached flow all the way back and pressure drag = 0, you will still have drag from the increased friction drag. There's no free lunch.

You mentioned you have a copy of Aerodynamics of Road Vehicles; you might find a lot of useful information reading Chapter 2 ("Some Fundamentals of Fluid Mechanics") all the way through.

Quote:
So another question.....in my mind I'm seeing a half teardrop with lines representing the (laminar?) flow along it's surface.
ETA: Also, no--not laminar, and that's a good thing. The transition from laminar to turbulent flow is determined by the Reynolds number, which = (free stream velocity * length) / (kinematic viscosity of the fluid). At critical Re = 5 x 10^5 or thereabouts, the flow transitions from laminar to turbulent--and the drag decreases. Turbulent flow also will stay attached to surfaces that are too steep for laminar flow to stay attached. Turbulent flow has higher friction drag, but the potential lower pressure drag far outweighs that.
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Old 07-25-2019, 12:54 AM   #10 (permalink)
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why not
just try it ?

many times in the world experts will tell you the math says it can not work

but
in reality the math can lie , the math is wrong on many things .
because much of the math is broken deliberately ....

the math says you can not see Sardinia from Genoa but in reality
you can .
the math says gravity bends light , reality and observation says it does not .

just give it a try , what do you have to loose ?

even if you do not see the results you wanted to see , you will learn ,


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