Cross Sectional Area of Car (for drag)

haroon_zabair

Hi, I hope I am not being too much of a noob but I am following a tutorial for a CAD system to make a vehicle aerodynamics wind tunnel. In order to do so, I must input the cross sectional area (of the front of the car) for only half the car - I am only using one side of the car due to lack of necessity. The image I have provided shows how they did it using On Shape in the tutorial. Please could someone highlight how to do this with proper steps. Thanks!

alnis

Welcome to the forums @haroon_zabair! No bad questions here!

Here's how I would find the cross-sectional area of something for aerodynamics (plus the center of pressure as a bonus for flying stuff ):
https://cad.onshape.com/documents/2250dac4fbab018f4cc3678f/v/36b9eb1e182daa3b198c6f5f/e/b795927e08cf29a1716fbf2e

Using the measure custom feature absolutely isn't necessary, I just like to use it so that I don't have to manually select the face and measure it each time. You can add it here:
https://cad.onshape.com/documents/77baa8153589a7fc5f289829/w/cffd0f2a7077380d5378a885/e/181cb871f3008e6b885df46a

I've used the Silhouette Outline custom feature for this:
https://cad.onshape.com/documents/447c6fe48283e84c4c382e67/v/ac0bdf1d607b2f9b985a2a1f/e/1aca5fb8241964de0b77dd68

@MBartlett21's Project Body custom feature looks like a more advanced version of this, I've just used the outline feature since it's what I'm used to.
https://cad.onshape.com/documents/201b33ee5450cefbca3b34b8/w/95004b1f32bddc1e032970e5/e/03f0c71d4856a8408c7766a0

Hope this helps!

tony_459

Oh wow, those are super custom features. I'll have uses for them. Thanks for listing them here, Alnis!

tony_459

Now I'm curious how others find the drag and lift coefficients for their models. CFD simulation? Wind tunnel experiments? Empirical equations published, e.g., by Hoerner? If simulation, what software?

alnis

@tony_459 all credit goes to other people for those custom features! I'm still very much a beginner at FeatureScript. Now that it's break, I have no excuses for not learning it haha!

As for drag and lift coefficients, throwing the model into CFD software like SimScale usually lets you get the output forces based on a given wind speed, upward force = lift, backward force = drag, etc. I actually ran this airplane model through a SimScale CFD simulation to try to figure these things out, but I have lots to learn on that front! Here's a video of what it looked like (note the wing stalling out from the turbulent flow behind it):
https://youtu.be/NzYPIAJ3qtQ

haroon_zabair

Oh wow, @alnis_smidchens. I have been trying to make a simple vehicle aerodynamics wind tunnel following a tutorial on Simscale (where I got my attached image from). I am learning a lot. I am still not sure how they managed to calculate the area with the method shown in my image. But, I will try your first method you mentioned. Thanks!

tony_459

Ah, SimScale... Yeah, they're pretty awesome... though last time I tried to get drag coefficients they didn't have a simple, clear solution... This was years ago, and they've had requests of this nature for a while now, so things must have improved? I'll have to check them out again. Thanks for pitching in, Alnis!

tony_459

It would be pretty wild to make your own wind tunnel. I don't imagine it's trivial or cheap? Then again, I've seen people do far more surprising things for almost nothing, so I have to wonder...

It would have to be a pretty small wind tunnel and all subsonic, I'm sure, so no sonic booms, ha ha... This is probably the dumbest thing I've ever wondered out loud

alnis

@tony_459 yeah, I can't remember how I ended up getting the final, useful values out of the simulation. I'm also not an aerodynamics expert by any means!

For making a wind tunnel, I think it isn't really an unreasonable thing to do at home! Not a dumb idea at all! It looks like there are even some kits out there:
https://www.wardsci.com/store/product/8867453/basic-wind-tunnel


Sure, it won't be as scientific as a proper wind tunnel, but it could absolutely be a fun at-home project.

haroon_zabair

Oops, I didn't quite mean a real wind tunnel - just a virtual one using the really helpful tutorial on sim scale. But, @tony_459 , I think you would be surprised how easy it is to make a home made wind tunnel. Similar to the one @alnis_smidchens showed above , I have found one (see link) that seems simpler whilst still showing the effects of aerodynamics  via the wing moving on a vertical axis. It's actually pretty intuitive and not too hard to create/understand.

https://youtu.be/qDQncRSlL8c

@tony_459 , If you were wondering if Sim Scale has improved, it sounds like they have. I will also leave a link to their vehicle aerodynamics wind tunnel tutorial if you want to check it out - Its great to see a free system have such easy yet in depth features and tutorials to follow.

https://www.simscale.com/docs/tutorials/aerodynamic-simulation-vehicle/

tony_459

Oh wow, big thumbs up to Bartlett's Project Body custom feature. The Outline feature is nice but it handles only one body at a time, so to get a cross section on a whole articulated assembly would take some boolean welding---if it could be done at all. Bartlett's feature lets you do many separate bodies in one go, which is way more practical. Very cool

Fda

I leave some links in case it is useful to you. It is a project that was made in the Fab lab in my city.
http://cesaretopia.com/cierzscience-tunnel/
https://hackaday.io/project/12080-cierzscience-tunnel
https://youtu.be/g2Y6GBYWttk

tony_459

Oh wow... Very cool, Fda. Thanks for sharing!