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 Post subject: Aero question
PostPosted: September 14, 2018, 9:50 am 
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Hey gang! Been a long long time! I use to build exos and other oddities and post here quite a bit. There were always some great minds on this board that certainly steered me in the correct direction over the years.
I've been building racing Porsches and Vettes since we last chatted and having a great time (mostly). Anyways I have a boss that bet me I couldn't build a fully tunneled Z06 out of a flood car he bought and didn't know what to do with. So I did (or almost) I've been reading everything from every book I can get my hands on and and the one question that never seems addressed is what is the load (so to speak) on the tunnels mounts? Is it being pushed down from the tunnel top area? The floor boards its attached to or the roof and top bodywork? Ive been assuming its the stagnant air on the other side of my tunnel that builds pressure and pushes but I can also see in my head the tunnel working with the bodywork and the roof and top bodywork being pushed down.
The owner has a Ferrari 550m that is slightly tunneled and its plastic is not mounted very robustly to the pan so maybe Im over building?
Not sure if I'm in the right area and apologize if I'm wrong or simply too far off topic.

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 Post subject: Re: Aero question
PostPosted: September 14, 2018, 12:54 pm 
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Google Bournouli's principal. The side of the panel with the accelerated airflow has a lower air pressure than the other, so the slower, higher pressure side pushes toward the accelerated lower pressure side.

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 Post subject: Re: Aero question
PostPosted: September 14, 2018, 1:40 pm 
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The tunnel mounts support the aerodynamic load due to the difference in pressure between the air on either side of the tunnel structure.

The tires and suspension support the aerodynamic load due to the difference in pressure between the air on either side of the car. The local pressure differences in between the top and bottom of the car may impart loads on the various internal structures, but all net out to 0 globally.

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 Post subject: Re: Aero question
PostPosted: September 14, 2018, 2:37 pm 
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So I guess where I get confused is with the air being seperated at the splitter like the leading edge of a wing. If the wing is solid core vs an open core...? Does the high pressure area change where it generates lift? Inside the wing opposite the wing skin accelerating the air? Or on the other side of the wing as a structure pushing up from underneath?
I'm familiar with Bournouli's principal and understanding the high and low pressures and why just confused on where. :roll:

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 Post subject: Re: Aero question
PostPosted: September 14, 2018, 5:00 pm 
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Does the air pressure inside of a ball cause it to move?

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 Post subject: Re: Aero question
PostPosted: September 14, 2018, 5:48 pm 
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I always found it counter-intuitive that moving air is low pressure, while stagnant air is at higher pressure, but it sure is. The splitter works because the air above it is stagnated against the nose of the car (high pressure), and pushing downward because the air below the splitter is moving fast (low pressure), hence, down force.

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 Post subject: Re: Aero question
PostPosted: September 14, 2018, 9:29 pm 
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No, I've got the splitter part. Im asking wrong perhaps. Let go to the ball. If I place that ball in a tube, remove the air from above the ball it will rise up the tube correct? Is the pressure to lift the ball coming from inside the ball or under the ball or both? Now lets view the car with tunnel/diffuser in profile. The air under is accelerated, lift is created (down force in our case) is that down force coming from the outer skin of the car (roof, hood, trunk) or the top side of the tunnel? If it's the top side of the tunnel then pretend I fill the interior and every cavity with foam? Does it still generate lift or stop? The original question is if a hypothetical tunnel/diffuser can create 500lbs of down force at X speed, does the support apparatus need to be able to hold the 500lbs or is some of that 500lb placed on the body of the car?
Sorry I'm confusing everyone.

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 Post subject: Re: Aero question
PostPosted: September 15, 2018, 1:01 am 
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pook wrote:
If I place that ball in a tube, remove the air from above the ball it will rise up the tube correct?
Correct.


pook wrote:
Is the pressure to lift the ball coming from inside the ball or under the ball or both?
Under. Consider that changing the pressure inside the ball will not change what happens to the ball in this scenario...Not counting mass/density/buoyancy effects.


pook wrote:
Now lets view the car with tunnel/diffuser in profile. The air under is accelerated, lift is created (down force in our case) is that down force coming from the outer skin of the car (roof, hood, trunk) or the top side of the tunnel?
Both, depending on your frame of reference. Globally it's coming from the outer skin of the car. Locally, at the tunnel mounts, it's coming from the top side of the tunnel.

Maybe it would help to put some numbers to this. All units are in gage pressure, which is relative to atmospheric pressure. So 0psi is atmospheric. Let's say you have two identical cars. The driver in Car A has the HVAC on low, creating a cockpit pressure of +1 psi. The driver in Car B has the HVAC on ludicrous, creating a cockpit pressure of +5 psi. Now lets say that the top surface of the car has an average pressure of +1 psi, while the lower surface of the car has an average pressure of -1 psi.

The floor pan of Car A sees a pressure differential of (-1 - +1 = -1-1 = -2) -2 psi, which means the floor is experiencing 2 psi in the downward direction. The roof of Car A sees a pressure differential of (+1 - +1 = 1-1 = 0) 0 psi, which means the roof is experiencing no directional pressure. Now notice that if you add the floor and roof effects you get (-2 + 0 = -2) -2 psi, or 2 psi in the downward direction...Which just so happens to be exactly the same as if you only look at the pressure below and above the car, as if it were filled with foam, which works out as (-1 - +1 = -1-1 = -2). Now is this a coincidence because of everything being only +1 or -1? Let's take a look at car 2 to find out.

If we look at the air above and below the car only for Car B, it is exactly the same math as when we did so for car A. But breaking it down into upper and lower surfaces, rather than the foam filled car, here is how it works out. The floor pan of Car B sees a pressure differential of (-1 - +5 = -6) -6 psi, which means the floor is experiencing 6 psi in the downward direction. The roof of Car B sees a pressure differential of (+5 - +1 = 5-1 = 4) 4 psi, which means the roof is experiencing 4 psi in the upward direction. Adding the floor and roof effects, you get (-6 + +4 = -6+4 = -2)

So no, it was not a coincidence. The overall (global) effect on the car as a whole is the same no matter what the pressure is inside, because the pressure inside is exerting an equal an opposite force to the roof as it is the floor. However, also note that the (local) floor pan in Car A only needs to support a 2 psi of pressure. Meanwhile, even though Car B has the same total downforce, the floor pan must support 6 psi of pressure, and therefore must be able to support 3x the load as that of Car A.

Your tunnel mounts are analogous to the floor pan in my example.


pook wrote:
If it's the top side of the tunnel then pretend I fill the interior and every cavity with foam? Does it still generate lift or stop?
As noted above, yes.


pook wrote:
The original question is if a hypothetical tunnel/diffuser can create 500lbs of down force at X speed, does the support apparatus need to be able to hold the 500lbs or is some of that 500lb placed on the body of the car?
If you mean that the tunnel itself creates 500 lbs of downforce relative to the air on the other side of it, yes that is what the mounts need to support...But if you mean the car as a whole simply makes 500 pounds of downforce, that does not provide enough information to determine how much load the tunnel mounts alone are supporting.

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Last edited by Driven5 on September 15, 2018, 5:16 pm, edited 1 time in total.

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 Post subject: Re: Aero question
PostPosted: September 15, 2018, 9:42 am 
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Thats what I was looking for. I guess the airfoil picture I've included had me asking is the high pressure pushing up from inside the wing as it appears or just the bottom of the wing as a whole or some combination of the two? That wouldn't make sence though since a wing could have a solid core and still generate lift. I'll assume the skin is being tugged on by the velocity of the air/fluid acting on the boundary layer OVER the wing? .
Attachment:
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 Post subject: Re: Aero question
PostPosted: September 15, 2018, 11:49 am 
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pook wrote:
Thats what I was looking for. I guess the airfoil picture I've included had me asking is the high pressure pushing up from inside the wing as it appears or just the bottom of the wing as a whole or some combination of the two? That wouldn't make sence though since a wing could have a solid core and still generate lift. I'll assume the skin is being tugged on by the velocity of the air/fluid acting on the boundary layer OVER the wing? .
Attachment:
0566090-v40-6.jpg


Hey guys! First post here, but been lurking for awhile. I feel like I can shed some light on the picture of the wing, I’m an instructor pilot to add a frame of reference. What you are seeing in the picture is the air inside the wing expanding due to the lowers pressure that it sees at altitude. Most aircraft hold fuel in their wings and are thus pressurized in order to avoid fumes being vented.

Thus at ground level/during fueling the pressure is normalized with the field elevation pressure, but then during flight you have a lower pressure on the outside of the wing versus a higher pressure on the inside. Very similar to the car analogy above where the one car had the blower on high thus increasing the internal pressuree of the car. The amount of increase in lift on the wing from the higher pressure inside the wing would be non-existant, however the change in shape of the wing due to the slightly bulging panels might increase lift at a very small rate. This would be due to the airflow at the surface of the wing being relatively slow compared to the speed of airline just an inch or 2 above the wings surface.

I could keep going on in detail about the change in lift characteristics due to airflow and wing shape, but what I wrote above should clarify what was occurring the wing.


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 Post subject: Re: Aero question
PostPosted: September 16, 2018, 9:06 am 
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Very cool! Never thought about the pressure inside the wing to explain it. Thank you.
The "ball" would expand at altitude... duh :roll:
Im trying to make it more complicated than it needs to be. I watched an hour long lecture on wings and lift the other night discussing Bournouli and Newton and I'm actually more lost than when I started. But fascinating stuff!!!!

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 Post subject: Re: Aero question
PostPosted: September 16, 2018, 11:58 am 
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pook wrote:
Very cool! Never thought about the pressure inside the wing to explain it. Thank you.
The "ball" would expand at altitude... duh :roll:
Im trying to make it more complicated than it needs to be. I watched an hour long lecture on wings and lift the other night discussing Bournouli and Newton and I'm actually more lost than when I started. But fascinating stuff!!!!


If you have any questions on it, I can elaborate a good amount. It is literally what I do for a living.


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 Post subject: Re: Aero question
PostPosted: September 16, 2018, 1:10 pm 
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pook wrote:
The original question is if a hypothetical tunnel/diffuser can create 500lbs of down force at X speed, does the support apparatus need to be able to hold the 500lbs or is some of that 500lb placed on the body of the car?
To quote our newest member Castrol,
Castrol wrote:
It is literally what I do for a living.
Okay, I haven't done much of it lately, but before I got on this homebuilt sports car kick, I was active in oddball aircraft design. Many thousands of examples of my designs are flying, and if you're a James Bond fan, you've seen a couple of them (Parahawk, The World is Not Enough, Switchblade, Die Another Day). Thus endeth my qualifications. And now, on to my pedantic lecture.

All you need to know is that the lift/downforce is created by pressure differential -- it doesn't matter if that differential comes from one side having its pressure reduced or the other side having its pressure increased, if there is a differential between them, there will be lift. There's really no such thing as a "solid" core in aviation, and when designing with foam cores, one must (or at least should) consider the shape changes that come with pressure changes. This question (where, exactly, is the 'lift' being generated?) can't be ignored in coreless designs. In a conventional aluminum-skinned aircraft, is the lift generated by the upper surface...which means the aircraft is being supported by the rivets that hold the skin on top of the wing...or the lower surface?

It's even more significant when the wing surface is flexible, such as hang gliders and ultralights with sailcloth wings--significant enough that considerable effort is put into optimum placement of vents. If the wing's internal vents are in a (relatively) low pressure area of the wing, the bottom surface carries the load, and may be pushed up into the wing structure. If the vents are in a (relatively) high pressure area of the wing, the top surface carries the load, and may bulge between the ribs and disrupt the airflow.

And for wings with pneumatic structures, such as ram air (rectangular or elliptical) parachutes, if you ignore where on the wing the lift is generated, you don't have a structure at all. Those wings have to be vented at the stagnation point at the lower leading edge or thet never get wing-shaped.

All that preface is to show that the topic (where on an aerodynamic structure is the lift generated?) is an important and well explored science, and pook's question can be answered, and the answer is: on most cars, and all open cars, the lift is physically attached to the car by the floor. In theory, one could seal the car and vent the cabin/cockpit at a low pressure area, and if one did, the car would be pushed down at the roof, not sucked down at the floor, but I doubt that is done much in practice. So yes, pook, attach your tunnel components securely to the floor, they are likely where the downforce is generated.

That said, if the space between the tunnel components and the floor are vented to the tunnel, then there's no pressure differential between the tunnel components and the floor, and thus little aerodynamic load between them. I'd say this is better taken advantage of when determining how flex-resistant the tunnel parts need to be, than determining what strength is needed to keep them from detaching themselves to the car during operation.

PS
pook wrote:
Sorry I'm confusing everyone.
Don't worry about it. Most of us were born confused. :)

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 Post subject: Re: Aero question
PostPosted: September 18, 2018, 7:32 am 
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Thank you, thats why I love this site. Such a wide variety of talent and knowledge on this forum all because of our common passion for the Automobile!

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 Post subject: Re: Aero question
PostPosted: October 7, 2018, 9:12 am 
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Have a question. Probably going to mostly display my ignorance of this topic, but I'm here to learn...

I understand that faster moving air has lower pressure than slower moving air. Seems counterintuitive, but obviously airplanes fly.

My understanding of this as applied to a vehicle is that, typically, the path of air below the vehicle is straight or nearly so and the path above is nearly always longer, due to the passenger compartment and getting over the front of the hood. The distance is greater so the air taking that path moves faster in relation to the vehicle skin as it passes through. This causes lift overall.

Where I get some confusion is that most vehicles have air dams up front to keep air from getting under the vehicle. Again, if I understand correctly, this is to prevent air from getting beneath the vehicle and causing high pressure there. That would seem to slow the air even further, causing more overall lift - but isn't the point of not having the air under there to prevent lift? Are there two separate phenomena occurring underneath? Is the ground playing a role that isn't a factor up top? If you could prevent any air from getting under, then wouldn't give the ultimate amount of overall lift, which is what you're trying to prevent by high pressure air underneath.

In my case, I'm planning a much higher clearance vehicle - maybe 8-10 inches. An air dam is impractical. I've been thinking about what I should do to the underside (or if I should even bother given I'll likely never drive more than 80mph unless I went to a track, which I have no plans to do). For example, would it be better to make the bottom as flat as possible to try to get any air that gets under to exit? Better to make it a shallow inverse arch to give the air a longer path and thus lower the pressure? Maybe better to make it concave so an air that does get past the boundary has space to expand?


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