Help me evaluate CFD results

[aerohead]

Originally Posted by M_a_t_t

I can't find the diagram I remember seeing, but I was hoping to try and get the airflow to be re-directed away from the roof at the windshield and have it come back down and reattach following the roofline. Similar to what seemed to happen with the hood to windshield model (post #3).

Are you eyeballing the slopes or calculating? I have been eyeballing. When I overlay the template (I understand has limitations) the tail pretty closely matches the upper shape. I would like to do some tuft testing, but don't really have the stuff required to record it.






Thanks for finding that. They don't have the correct year escort though




Do you think a gurney flap at the tip of the tail would help flow? I remembered I have 2 wide angle dash cams I might be able to use to record from a chase vehicle. I will try to get something this weekend.

In Wolf Hucho's 'Aerodynamics of Road Vehicles,he presents empirical results of aerodynamic testing,dating back to 1922.
He dedicates an entire section to forebody design,drawing on Jaray,Fachsenfeld,Kamm,Waters,Pawlowski,Carr,Ahmed ,Hoerner,Emmelmann,Janssen,and Hucho.
From the visual schematics,I can just look at your rendering,and know that you're already at saturation.There'll be no problem with attached flow over the roof peak and onto the rear slope.
The important part is the slope/contour back there.That's where your streamlining potential is.The 'template' contour is derived from the lowest-drag half-streamline-body of revolution.My model which used it,measured Cd 0.12 just as predicted.You'll never find a shape with lower drag potential.That's why I shared it.

[M_a_t_t]

I appreciate the advice from you guys.

Can you expand on what you mean by saturation?

Are you saying the profile is as good as its going to get and I won't see much improvement modifying the boat tail anymore? Therefore, I need to focus on the rest of the car.

Also, I finished the bottom section of the boat tail I just hadn't when I took that picture.



Edit: Assuming I am correct in what you mean by saturation I think I have some useful information. On a particular road in town that was recently repaved and has a 55 mph speed limit the tail makes absolutely no noise. If there was a lot of turbulence the unsupported sections would flap around right? The road is decently protected from crosswinds on both sides by trees and buildings.

[freebeard]

Thanks for posting that. Just yesterday in another thread I was lamenting the fact we haven't seen a full boat tail (on a Metro) lately.

Do you have construction pics? That show the internal structure (if any?). What do you do about tail light and the license plate?

I'd be curious what it is like living with it. Parking and & etc.

Are you saying the profile is as good as its going to get and I won't see much improvement modifying the boat tail anymore? Therefore, I need to focus on the rest of the car.

Get some test results (coast-down or whatever) before other mods. To isolate the gain from that one modification.

As for 'as good as its going to get', consider truncating it less than a foot. See the Peter Brock Cobra Coupe or Dave Cloud's Dolphin.

https://ecomodder.com/forum/showthre...tml#post559726

[aerohead]

Originally Posted by M_a_t_t

I appreciate the advice from you guys.

Can you expand on what you mean by saturation?

Are you saying the profile is as good as its going to get and I won't see much improvement modifying the boat tail anymore? Therefore, I need to focus on the rest of the car.

Also, I finished the bottom section of the boat tail I just hadn't when I took that picture.



Edit: Assuming I am correct in what you mean by saturation I think I have some useful information. On a particular road in town that was recently repaved and has a 55 mph speed limit the tail makes absolutely no noise. If there was a lot of turbulence the unsupported sections would flap around right? The road is decently protected from crosswinds on both sides by trees and buildings.

At a certain point of increased radius or angle,no additional rounding,or steepening will produce any further drag reduction.You've hit 'saturation.'
The tail looks delicious! Thanks for doing that!

[M_a_t_t]

Originally Posted by freebeard

Do you have construction pics? That show the internal structure (if any?).
It's pretty simply put together. Framed it with 1" by 1/8 flat stock and covered using a roll of roof flashing. Sheet metal screws for attachment and tape to seal it up. I overlayed the template on the car and imported it into CAD. From there I plotted points along the curve and used the back of the trunk as a scale in CAD. I started with putting a loop horizontally on the car. I leveled it using cardboard and jackstands. Once that was together I started on the profile shape. There are 2 pieces that I attached to the bumper. The bottom shape I just kind of eyeballed. To get the metal above to bend I just welded the vertical parts to the cross piece and while it was hot I bent it around so that It would hit the roof. I did have to weld an extra 3 ft or so to actually reach the roof. After that I skinned it in the aluminum flashing.You can kind of see it in this picture. I can get a picture from inside later.


What do you do about tail light and the license plate?
The tail light is a trailer light, one of the narrow ones. It's like 1" by maybe 8 or 10 inches. The turn signals I chose initially are terrible. I bought some 4" flush mount trailer lights I have to put them in yet. I don't have a hole saw big enough. For the license plate I put 2 bolts in the sheet metal and then I put the license plate on and sandwiched it between the 2 nuts.

I'd be curious what it is like living with it. Parking and & etc.
It's not bad. Rear visibility is terrible without the side view mirrors. Right now I have the passenger side off and I don't like it. Parking isn't really a problem other than not being able to see behind you very well. Its just about the same length as my mom's 01 Pontiac Grand Prix. I usually park where there is a curb with grass overhang (I'm not sure what you call them). This way I just back over the curb and its not an issue.


Get some test results (coast-down or whatever) before other mods. To isolate the gain from that one modification.
I did a run using the instructables coast down method and excel sheet and got a Cd of .30 (vs stock .34, not verified just googled. I also had the passenger mirror off and didn't try to adjust the frontal area.) and Crr of .01285 I think one of my tires was a little low when testing though so it might be a little better than the data I got

As for 'as good as its going to get', consider truncating it less than a foot. See the Peter Brock Cobra Coupe or Dave Cloud's Dolphin.
Are you suggesting trying to follow template better and eith run a clear panel or just leave it truncated?

Originally Posted by aerohead

You've hit 'saturation.'
The tail looks delicious! Thanks for doing that!

Thank you, pretty much everyone has said positive things about it, including "normies".

[aerohead]

Originally Posted by M_a_t_t

Thank you, pretty much everyone has said positive things about it, including "normies".

I believe that at 70-mph,a 10% reduction in Cd produces a 6% increase in mpg.I apologize if I screwed that up.

[teoman]

Originally Posted by M_a_t_t

I appreciate the advice from you guys.

Can you expand on what you mean by saturation?

Are you saying the profile is as good as its going to get and I won't see much improvement modifying the boat tail anymore? Therefore, I need to focus on the rest of the car.

Also, I finished the bottom section of the boat tail I just hadn't when I took that picture.



Edit: Assuming I am correct in what you mean by saturation I think I have some useful information. On a particular road in town that was recently repaved and has a 55 mph speed limit the tail makes absolutely no noise. If there was a lot of turbulence the unsupported sections would flap around right? The road is decently protected from crosswinds on both sides by trees and buildings.

Holy moly!

They let you ride around in that? Gotta love the USA.

[freebeard]

Thank you, pretty much everyone has said positive things about it, including "normies".

Don't underestimate the significance of aerhead's approval.

0.34=>0.30 sounds about right. Now go after the wheelwells.

They let you ride around in that? Gotta love the USA.

"Hells yeah — 'Murica!"

The result may vary by jurisdiction.

[teoman]

Check out the software that freebeard linked a couple of months back. Appears to be slightly more capable that the Autodesk stuff.

[freebeard]

Permalink #4 was just for purposes of illustration.

The key to low-compute-power CFD IMHO is OpenVDB:

Originally Posted by DDG

OpenVDB
[Search domain www.openvdb.org] https://www.openvdb.org
OpenVDB is an Academy Award-winning open-source C++ library comprising a novel hierarchical data structure and a suite of tools for the efficient storage and manipulation of sparse volumetric data discretized on three-dimensional grids.
OpenVDB - Download

[Search domain www.openvdb.org/download/] https://www.openvdb.org/download/
OpenVDB includes a small number of Maya nodes, primarily for conversion of geometry to and from OpenVDB volumes and for visualization of volumes. This archive contains several example scene files that demonstrate how to use the nodes. Maya Examples zip - Aug 8 2016

OpenVDB - Wikipedia
[Search domain en.wikipedia.org/wiki/OpenVDB] https://en.wikipedia.org/wiki/OpenVDB
OpenVDB is an open source C++ software library comprising a novel hierarchical data structure and a large suite of tools for the efficient storage and manipulation of sparse volumetric data discretized on three-dimensional grids.

It's all about the sparse inverted B-trees, if ya know what I mean. And being able to attach arbitrary attributes to each voxel. If you need it, the odor of hickory smoke.

https://i.ytimg.com/vi/R8Y9VqU_YTI/maxresdefault.jpg
Instead of computing every voxel in the virtual wind tunnel, it looks more diligently closer to the aeroform.

It's above my pay grade.