Aero trunk lid project

[mnmarcus]

ChazinMt said"...In the back of a vehicle, the air is "Stupid" (No offense to any stupid people who may be reading this post)...

None taken, but here's a few stupid questions: How do I modify the template if it doesn't fit what I want to do with it? -and- Why isn't it a sphere with a cone for a tail?

I'm planning on a micro car project, half as wide and tall as a "normal" car but nearly as long. Let's say 2.5 feet wide and 4 feet tall by 10-12 feet long. I scale the template. Do I add flat sections to stretch it, or stretch the front only, or stretch the whole thing? I don't want to increase frontal area just to get a better cd and worse cdA.

Sorry for the off topic.

[California98Civic]

Originally Posted by mnmarcus

here's a few stupid questions: How do I modify the template if it doesn't fit what I want to do with it? -and- Why isn't it a sphere with a cone for a tail?

It is half of a somewhat cone-like form. That's why the flat bottom.

Originally Posted by mnmarcus

I'm planning on a micro car project, half as wide and tall as a "normal" car but nearly as long. Let's say 2.5 feet wide and 4 feet tall by 10-12 feet long. I scale the template. Do I add flat sections to stretch it, or stretch the front only, or stretch the whole thing? I don't want to increase frontal area just to get a better cd and worse cdA.

I think you could do more adjustments to the front end form than the back-end (there's that stupid air problem again). from the point of highest roof camber backward follow the template strictly. Forward you'll have a little more flexibility--how much I don't know. Others will say...

Good luck... sounds like a huge undertaking.

EDIT: and plan a sheer bottom belly-pan that is about 2" off the ground.

[ChazInMT]

I defer to the axiom, "WWVWD" (what would vw do) they have made 2 ground breaking cars, and the first sounds like a ringer for your deal The L1. The second is a more marketable side by side seating version XL1. Look to these for cutting edge inspiration.





Of course if they ever want this to sell in the US, they'll need a SUV version with 24" Rims and market it as the XXXXL1000. Anybody got a connection with VW here?? I think I have found out what I now must do.

[aerohead]

Originally Posted by ChazInMT

I think Aeroheads tryin to say this.

Thanks ChazInMT.
If the red outline is based on the height of the car,then this is the outline to use.
What I see,is that the 'max camber' for the side of the car lies about midway on the rear wheels.I would slide the 'Template' zero-point back to that position and look for a 'fit' in that area.
The roof begins its descent ahead of the sides of the car and it can be steeper than the sides.
The body side curvature will necessarily be 'retarded' with respect to the roof due to its 'late' start.
If you push the curve steeper,the shear will probably set up vorticity.
PS The body curvature ( camber ) will be different low on the car compared to midway up,and further up on the greenhouse.Technically,you'd be required to find the 'max camber' point for all locations on the body sides and adjust your curvature from each point.Is that hard enough?

[aerohead]

Originally Posted by CFECO

I was thinking, that "Follow the template always" may not be the best idea. If we had cars with the same front shape as the ideal, then yes, the rest should be as close to it as possible also. But since we have rounded off box shaped front ends, with setback windshields, the airflow will not match the ideal from the start, so I'm not sure the rest, always will either. Without a windtunnel, who knows? Back when I was doing cylinder head work, the flowbench showed me the air "wanted to go places" that I didn't always expect. A car with a blunt front end seems to like the waist area pinched in, looking from above, which is where the template shows it to be the widest. I imagine that is where the air is coming back to the body at an angle, and wants the body to match that angle smoothly. I think the speed matters also, higher the speed, the farther back the pinched in area should be. I realize this thread is all about the rear area, I'm thinking the air is coming back "down" from going up the windshield, and hitting the roof before the rear glass, and maybe needs the roof raised in the center in order for the spoiler to work. Just a thought!

As long as the onset flow from the front of the vehicle is attached to the point of maximum body cross-section,the vehicle should respond favorably to the 'Template'.Flow behavior will be constant from 20-to- 250 mph and the body need not vary as a function of velocity.It's different physics compared to a flow-bench.Exo-duct,not duct.

[aerohead]

Originally Posted by ChazInMT

Ironically, I posted this simplistic diagram the other day in another forum which illustrates your point, and shows an alternative point. It is a common misconception that the air is "Flung Out" by the windshield & front end, and to a small degree it may be. But the effect is much smaller that you imagine.

In my mind, I see the Template as being the ideal balance between having too aggressive a shape and too flat a shape. An aggressive shape creates a dynamically induced low pressure drag at best, and shreds attached flow at worst creating turbulent flow, both Cd killers. Too flat a curve which will make for a very long vehicle with no appreciable gain, or, more likely, a shortened version of a longer template which will have a larger rear area flat area leaving the air to come up with its own way of dealing with itself, which is never a good thing on the back end of a shape.

So, in effect, air in front of a vehicle is "Smart" and can overcome problems by sort of creating its own shape, going from the ideal crappy front end, to an ideal aero shape front may gain you 20-30% overall gains in Cd.

In the back of a vehicle, the air is "Stupid" (No offense to any stupid people who may be reading this post) and does not manage itself well in the absence of a shape to follow, we must, as a people, as true eco-modders, devise ways to guide the stupid air. One of the easiest most effective ways is to follow the template. It is through proper use and adherence to this template, that 70-80% of the achievable gains may be realized. As always, YMMV (which is really just another way of saying your probably doing it wrong).

Finally, let us ponder the mystery of the shape, and give it our praise.

In the fore-body of the 'Template' the ram pressure against it provides a 'favorable' pressure gradient which holds the turbulent boundary layer against the 'Template',up to the point of max. camber,where the velocity at the boundary of the 'body' is zero.
Beyond the point of max. camber,there no longer exists a 'favorable' pressure gradient ( it's in the lee of the 'Template' ),the flow is decelerating,and according to Bernouli's Theorem,the velocity would have to slow even more,but it's already at zero!
If the body does not taper at a very gentle rate,the boundary layer will detach ( flow separation ),the flow will reverse directions,beginning eddy flow which will soon grow into full-blown turbulence.
The point of separation determines the base pressure of the wake,which along with the forward stagnation pressure creates the 'pressure drag' we're trying to eliminate.
The 'Template' will not allow separation.It has no wake.It has no pressure drag,only surface friction,which we can't do anything about.
No real mystery,just good empirical science.

[aerohead]

Originally Posted by mnmarcus

ChazinMt said"...In the back of a vehicle, the air is "Stupid" (No offense to any stupid people who may be reading this post)...

None taken, but here's a few stupid questions: How do I modify the template if it doesn't fit what I want to do with it? -and- Why isn't it a sphere with a cone for a tail?

I'm planning on a micro car project, half as wide and tall as a "normal" car but nearly as long. Let's say 2.5 feet wide and 4 feet tall by 10-12 feet long. I scale the template. Do I add flat sections to stretch it, or stretch the front only, or stretch the whole thing? I don't want to increase frontal area just to get a better cd and worse cdA.

Sorry for the off topic.

There are no stupid questions,only the one that was never asked.
With respect to modifying the 'Template',if you must do something,go 'hyper-'Template' and make things longer with a little extra surface friction.
If you go hypo-'Template' you'll get flow separation and really high drag.

[CFECO]

Here are some pictures to ponder. On the flat shape, the Corvette headlight, the air does not create it's own "Streamlined path" and follow the hood, as in ChazInMT's drawing, "What the air is doing". The air is shown to be coming back down to the hood at a fairly large distance back of the headlight. The other page shows the "template" does not perform to it's potential, without the front streamlining also. Just my humble opinion.

[ChazInMT]

It looks to me like the air on the vette is arcs back down to the hood 14 inches or so behind the light. Keep in mind, I don't mean to say a flat front is the same as an aero one, I'm just saying it isn't as big of a hit as people want to make it. Obviously an aero front helps.

As far as the illustration goes, it's just that, a drawing, someones opinion of what is happening. Interesting though.

[aerohead]

Originally Posted by CFECO

Here are some pictures to ponder. On the flat shape, the Corvette headlight, the air does not create it's own "Streamlined path" and follow the hood, as in ChazInMT's drawing, "What the air is doing". The air is shown to be coming back down to the hood at a fairly large distance back of the headlight. The other page shows the "template" does not perform to it's potential, without the front streamlining also. Just my humble opinion.

I'm not going to speak for Chaz,but if you'll look at the smoke,you'll notice that the air stream is deforming quite a bit ahead of the headlight housing.Dr.Prandtl called this a 'line of discontinuity.'
Air will avoid extreme accelerations and instead,warp its path around an object.
Certainly,any leading edge will require a minimum radius to enable attached flow,however,once attached flow is accomplished,very little drag reduction is realized from increasing the radius.I think this is inferred in Chaz'z illustration.
With respect to the second thumbnail.I don't think this is 3-dimensional flow being illustrated.I think the word 'sphere' should be replaced with the word 'cylinder.'
The flows are not correct for 3-D flow.
The 'pointed' nose of the 4th image is never used for sub-sonic design.In non-zero yaw it would actually have higher drag.Instability.
Not even the X-15 uses such a nose.
If you haven't seen the thread 'Flow-Images' you might want to check it out.
As to the 'Template',it's based on science.Below 250 mph,the semi-hemispherical nose is recommended for lowest drag.