Finding shape with lowest drag coefficient
 

Hi,

I have been thinking of a simple little project for which I would require your participation. The project is to find the most aerodynamic shape that can hold at least one person (laying down is ok).

Some background: I am a cross between software development and scientist (expertise in Computational Fluid Dynamics). I recently quit my job to focus on my own projects. One of which is Aerodynamic on Demand - add https in front of aerodynamicondemand.appspot.com

Further details: I would like interested people to use the above webportal (using the Discovery option for free) to find the most aerodynamic shape. Aerodynamic is measured by the drag coefficient Cd (i.e not the drag area), the lower the Cd the more aerodynamic the design is.

The project closing date will be 9th October 2013.

To participate it should be relatively simple:

1. Create your geometry in a CAD package (I personally use SketchUp):
   • Geometry does not need to be a car or bike (i.e do not worry with wheels, etc) - do not spend more than a couple of hours unless you have a very specific shape in mind
   • Geometry is to be able to accommodate at least one occupant
   • Geometry is not taller than 5m
   • The lowest point of the geometry is to be no more than 0.5m above the ground
   • See the geometry requirements on the website
2. Export the geometry in STL format (using meters units);
3. Go on the website to the "Start a calculation" page;
4. Fill in your details and use the promo code ECO1023 - if you click on "update" it should report a AU$0 cost for the Discovery option. Use a speed
   between 50km/h (13.889m/s) and 100km/h (27.77m/s) with a zero yaw angle;
5. Select the Discovery option.

At the moment, there are 50 free sims available (happy to increase if there is more interest) to be used by the closing date. If the website redirect you to Paypal, DO NOT PAY, send me an email instead!!!!

Note: the Discovery option is a coarse simulation.

Report your results: It is probably best to give your results by responding to this thread.

What am I going to do? I am planning to organise the results by decreasing drag coefficient to create 30 seconds YouTube video(s) of the geometries - I won't share or take credit for the geometries beyond the video(s). I would like to credit the participants so it would be great if when posting your results, you give out any twitter, facebook name that I can add to the credits (otherwise, I will just use your EcoModder username).

This is only running on the EcoModder forum, so please do not disclose the promo code outside. I will include a mention of EcoModder to the video (if acceptable).

Please feel free to ask if you have any questions.

Julien

Sounds cool. I don't have a cad program, but thanks for the idea.

Thanks Mike. You can try SketchUp - there is a free version caller SketchUp maker and is reasonably easy to use.

To get things started:

- Cd = 0.8908

- Results and images at: https://aerodynamicondemand.appspot....1-4ef147317882
- Twitter @HibouSoftware

Wonder what information is available for all the different bullet shapes? Obviously, they've done LOTs of aerodrag computations and analyses on the many, many, different military & civilian ogive 'shapes'?!?

parameters
 

*Since the ground is mentioned,this will be a road vehicle?
*If it is a road vehicle,must it contain a source of locomotion along with the occupant?
*It is to be operated only on a closed-course?
*Closed-course vehicles would have a chance to utilize laminar profiles if operating solo,on a dead calm track day.
*The 'laminar' profiles would never work on a real roadway.
*We might mention Reynolds number operating conditions as they affect boundary layer type and both pressure drag and surface friction drag.

started
 

There is a vast amount of empirical data regarding the aerodynamic drag of differing geometric forms in the public domain dating back to before the turn of the 20th Century.
Some minimums have already been established for forms operating in ground proximity.Typically,streamline bodies of revolution and ellipsoids and their half-bodies return the lowest drag on a frontal area basis.
There are delayed-transition 'laminar' forms which can work with torpedos,submarine hulls,airships,wing sections,and fuselage,artillery projectiles and bullets,but they don't perform in ground effect.
Unless your CFD has 'Full Navier-Stokes' capability with the ancillary turbulence models built in,it will be unable to anticipate 3-dimensional wake forms behind road vehicle bodies,something critical to accurate CFD drag prediction.

Yes. To keep it in the spirit of the forum

No. It is intended as a fun and quick experiment with shape and analysis, not a vehicle design.

Thanks for explanation as I was not familiar with closed-course. You can assume that it is operated on a closed-course. Since it is modelling based, you need to be aware that the model uses a turbulence model. I would be interested to see the difference in model prediction between a profile optimized for laminar flows and one optimized for transitioning/turbulent flows. I am assuming that the model will indicate that the latter is better - another limit of modelling...

Sounds good. Re = rho x U x L / mu where
rho = 1.205 kg/m3 and mu = 1.8e-5 kg/m.s
U is the travelling speed selected for the analysis
What is L ??? It could be a characteristic length taken as the longer of the height, width or length or based on the frontal area (L = sqrt(frontal area))?

I agree with you. My expertise is with software and CFD, not shape optimisation. I was thinking of trying a couple more shapes during the week, but I lack the background to go into it in a lot of details. Add I would love to have others testing my software.

The CFD is based on a steady-state RANS simulation package. The simulation uses a k-omega SST turbulence model - with is quite common, but lacks a number of capabilities (which most 2-equations turbulence models lacks): namely laminar to turbulent transition and anisotropy of the Reynolds stress tensors closure.

transition and L
 

For road vehicles,the source flow is considered already fully turbulent and the vehicle totally immersed in a turbulent boundary layer.Which is good,as the TBL can feed kinetic energy in,in a hostile region of adverse pressure gradient over the aft-body,helping to prevent reverse-circulation and ensuing separation in this 3-D domain.
In a protected,closed-course environment,with a lone vehicle on the course at a time on a calm day,it might be able to get a bit of laminar boundary layer up to the first minimum pressure position,then transition over.But vehicles have such little surface area compared to say an aircraft,the skin friction is insignificant compared to pressure drag.
------------------------------------------------------------------------
With respect to the Rn,yes L=body length as we treat it with vehicles.

anisotrophy/Rstclosure
 

I peeked at some online CFD stuff,Imperial College,etc..
' looks like they recognize some contextual limitations with the models,but boy are they advancing the state-of-the-art!
When Wolf H.Hucho was with Volkswagen,they would put a car model in the wind tunnel,use visualization techniques to define the wake,along with velocity and pressure probe survey,then import the car-wake coordinates into the software as an aerodynamic singularity,panelizing the wake right along with the car within the wireframe virtual domain.
This gave them a priori knowledge of the actual 3-D wake with which to include in the numerical analysis for comparison to the empirical measurements found in the tunnel.
The CFD just gets better and better.:)

I totally agree with you. It is quite an evolving field, and it is difficult to keep up to date with research and best practice.

From a methodology point of view, there has been some interested development with the so-called meshless models (I think that Autodesk Falcon is one of them) - but I have not used any and find it difficult to make the difference between claims and reality.

This being considered, simple models using combination of empirical/physical test/models can be far more accurate than very advanced models... It is often about validation... validation... validation... and experience...

I just tried a sphere:

- Cd: 0.0846
- Frontal area: 2.2 m2
- Reynolds number: 1.6e6

Results: https://aerodynamicondemand.appspot....5-74a3a2e8fc3a

One of the very interesting feature is that the flow seems attached at the downwind side apart from the lower part (close to the moving ground surface) of the downwind side.

Shape Effects on Drag

Just saying, but really fluid dynamics of an object in a jet stream is simple. Mostly the object that yields the lowest cd is a tear drop or double ended bullet. Relative to the above mentioned site and any lecture about it begins with talking about a rain drop. Another relavent way to conceive its design is fire, either waving a stick around or more interesting ways of seeing how the shape ultimately forms a tear drop shape.

empirical
 

Even in wind tunnel tests of 3-D models things get weird and contextual.
Professor Alberto Morelli developed a CFD tool from a 1947 NACA (NASA) numerical algorithm.
In 1976 a scale model of Morelli's sent an earthquake and aftershocks through the aero community,registering Cd 0.161 with mild tail truncation.
The thing that wasn't mentioned was that the body was not at a practical ground clearance.
In 1978 the Italian govt. funded a full-scale demonstrator and with wheels,and realistic ground clearance,the car registered Cd 0.35,much higher than what was anticipated.
Aptera attempted to capitalize on Morelli's form and to my knowledge attained no remarkable Cd when compared to more old school,retro,vintage,antique,or Pleistocenic forms.
PS If my memory serves me,a NOVA broadcast showed a computer graphic simulation of cosmic radiation deflection by Earth's magnetosphere,creating a classical teardrop form.

I'll save you (but mostly myself) the time, the old Lamborghini Countach had the world's lowest drag-coefficient, you can take that car's shape and put it to the test anyday, wheels and all...

Here's another:
http://cr4.globalspec.com/PostImages...FE10BC3519.png

It's a simple thing, starts with a point, ends in a point.
No getting around it, that is the world's most aerodynamic shape.

Actually the newest Lamborghini Veneno is super close, too:
http://en.wikipedia.org/wiki/File:La...profile%29.jpg

Now my work is done.

Needs a boattail! :P

What's the number and where did you find it? I'm finding a 0.42 Cd on Wikipedia for the 1974 Lamborghini Countach.

http://en.wikipedia.org/wiki/Automob...ag_coefficient

My Forester is only .45!

Thanks for the idea. I refine the concept a little bit as follows:
- Design is to include a box of 1100mm W x 1200mm H x 1900mm L
- Drag is measured by drag area (Cd.A) in place of Cd.

1) Simulation a - Cd.A = 0.6501 m2
Results
https://cfdgallery.s3.amazonaws.com/...ometryonly.jpg

2) Simulation b - Cd.A = 0.5737 m2
Results
https://cfdgallery.s3.amazonaws.com/...ometryonly.jpg

3) Simulation c - Cd.A = 0.5318 m2
Results
https://cfdgallery.s3.amazonaws.com/...ometryonly.jpg

4) Simulation d - Cd.A = 0.4159 m2
Results
https://cfdgallery.s3.amazonaws.com/...ometryonly.jpg

5) Simulation e - Cd.A = 0.1661 m2
Results
https://cfdgallery.s3.amazonaws.com/...ometryonly.jpg

6) Simuation f - Cd.A = 0.1431 m2
Results
https://cfdgallery.s3.amazonaws.com/...ometryonly.jpg

7) Simulation g - Cd.A = 0.1176 m2
Results
https://cfdgallery.s3.amazonaws.com/...ometryonly.jpg