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. |
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. |
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. |
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. |
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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ducting
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The lowest drag bodies are always 'streamlined' bodies,with the most minimum of internal flow. |
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. |
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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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:
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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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I like the way you think. Instead of Aerodynamics of Road Vehicles, try Viktor Schauberger: https://ecomodder.com/forum/member-f...logicalsub.gif https://duckduckgo.com/?q=viktor+sch...sine&ia=videos The air curtains and your decklid mod are external ducts. The trick is short and fat to move a lot of air with least skin friction. The first example that caught my eye was this Ferrari: https://i.imgur.com/Z8q8D.jpg All it does is bleed air away from the A-pillar. A favorite would be the GTI W12-650 https://i.imgur.com/564oWm0.jpg Quote:
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Einstein’s General Theory of Relativity is Questioned But Still Stands ‘For Now’ – W. M. Keck Observatory
Einstein’s General Theory of Relativity is Questioned But Still Stands ‘For Now’ Posted July 25, 2019 Detailed UCLA-led analysis of the star’s orbit near supermassive black hole gives a look into how gravity behaves The analysis suggests the stars light is red-shifted by climbing out of the [invisible] black hole's gravity well. At the perigee of it's highly eccentric orbit, it's traveling at 2.5% the speed of light. |
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Yeah but wouldn't installing a black hole in the bed of my truck cause a significant loss in gas mileage?
Not sure if it's tailgate up or down works with the black hole mod. |
squeeze it forward
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heat the air
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math
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Shauberger
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I wasn't so much thinking it would generate "thrust". I guess I was seeing X amount of pressure applied to each side of a watermelon seed at the widest part somewhat cancelling each other out (180 degrees apart resulting in a 0). But the same X amount of pressure applied to each side of the tapered part being more than 0 because the side pressures being less than 180 degrees apart. Not that it would result in more than X, but rather a greater percentage of X being "usable". Does that make sense?
I can see the concept in my mind but just can't put it into words. Especially in tech speak. |
I think the difference is that the air doesn't apply force at right angles to it's flow. It's trying to get from fore to aft with the least disturbance.
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cancelling
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Due to entropy, derived by kinetic energy converted to heat via viscosity and surface friction drag,it's impossible for there to be as much energy at the back of the body ,as at the front. We're stuck with losing perhaps 12% of overall energy to skin friction.The good news is,that we can reduce pressure drag by basically 88%.Unless we're driving on the Moon,we're stuck with the 12% loss. |
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Thank you everyone.
I'll have to read up on that D'Alembert paradox. Between that and freebeard's comments about how pressure would be more in the direction of air flow, I can see where my ideas were skewed. This is one of the things I truly appreciate about this site. You folks are always willing to try and educate and share your knowledge, to help someone like me understand. |
Looking for a thread to drop this in. Close enough? https://theansweris27.com/naca-submerged-duct/
theansweris27.com is an interesting site. They show the wind tunnel they replicate in CFD and the mesh representing the atmosphere. https://theansweris27.com/wp-content...ind-tunnel.png https://theansweris27.com/wp-content.../mesh_clip.png It ends with an example of a Ferrari that moves away from a submerged duct to a more scoop-like form. |
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