Is there any point in wind tunnel testing a 1:43 scale model

[TMcRally]

Hi

As the heading suggests.

It is easy to get a reasonably accurate model of my car in 1:43 and make a bench wind tunnel. I was even thinking of making a rolling road set up.

I can get 1:10 but it's not as accurate a model.

Is this a waste of time or are there some benefits to be had. I'd like to test different shapes under the car, the rules are *sort of free.

*Sort of = not really, but I'm allowed any under tray for body protection, it can't be obviously for aero or for structural stiffening otherwise it's free.

I read elsewhere in the forum that air speeds will need to be ridiculously high to give results at small scale and that they use water tanks in small scale tests.

Are there CFD programs that work reasonably well and if so how can I get a model of my car, is there a library ?

Thanks
Dave


Thanks
Dave

 

[MikeHalloran]

Try the live DVD of CAELinux. OpenFOAM is the CFD unit.

http://www.caelinux.com/CMS/

If the live DVD doesn't work on your system, don't bother installing it to your hard drive; it will just put you into Ubuntu upgrade hell, and stop working almost immediately.

Some of the documents are in French. Some translations are available. The demos and tutorials are based on older versions, which are also available, and will put you into PCLinuxOS upgrade hell if installed.

I'm not recommending it; you asked for free, and that's what there is. For me, it's worked just well enough to keep me hoping that if I learn a little more about it, or hold my tongue differently, it will actually do something useful. So far, no joy.





Mike Halloran
Pembroke Pines, FL, USA

 

[evelrod]

TMcRally---

Is this a waste of time or are there some benefits to be had. I'd like to test different shapes under the car, the rules are *sort of free.

I just posted a reply to the slot car aero ? and happened on this thread. I won't repeat what I said, you can look it up. However, in building the wind tunnel and all the attendant 'engineering' required to do the aero testing on our 1/24th scale cars...the fun we had...cannot be without value. Did we learn anything? Sure. Mostly not what we were looking for. Go for it!

Rod

 

[GregLocock]

OpenFOAM is used by at least one OEM as a full strength CFD, so there is little doubt that it could do the job.

One use of dimensional analysis is to identify a liquid that will perform correctly at an appropriate speed for your scale model. I know some racing teams used water.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[TMcRally]

Thanks all

I should point out that this for a full scale rally car and that the test is to hopefully find some benifits that I can transfer from the small model to the full size car.

Dave

 

[TMcRally]

Hi all

Can someone have a look at my numbers for running the model in a water tank, I'm out of my depth here by a long shot.

If my target speed for measuring the effects in air for the full size car is 140kph.

If at 20deg C water is 998.2071kgm^3 and air is 1.204kgm^3 then water is 829 times denser than air.

If my model is 1:43 then air flow for the test should be 140x43=6020kph

To run the model in water the model speed in the water should be 6020/829 = 7.26 kph.

If that is correct then a 1:10 scale model is much more manageable at 1.7 kph.

Thanks
Dave

 

[TMcRally]

It doesn't sound right that would mean a full size test in water would be 0.2 kph to simulate 140 kph.

 

[GregLocock]

We ran a 1/5 scale model of our solar car in air at 100 kph and measured the pressure distribution along the centreline and that seemed to work OK.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[ufe777]

Those calculations seem reasonable to me and are similar to what I came up with about a year ago.

The problem I found was by the time you have a channel big enough to reduce wall effects then you water flow rate starts to become quite large. There was a sweet spot in the trade off between model size and required flow rate.

 

[TMcRally]

Ha Ha

The 1:43 scale model arrived - I didn't even think to work out the size of it, the add said it was 198mm long so I just assumed that was right. When it arrived it was about twice the length of a match box. It also said it was an accurate reproduction - maybe after Chris Atkinson drove it for a week !

The box it came in was about 198mm. I can't see this model being of much use - although if I was part of Mythbusters it would be fun to see it in a 6,020kph wind tunnel. (that can't be right).

Thanks all
I'll start again with a bigger one and let you know how I get on.

I did find this site for 3D models -

http://www.turbosquid.com/3d

-not sure how accurite they are.

Can you buy reasnobly priced CFD software ?

 

[evelrod]

Just a educated guess on my part...based on past experiences/memory...go with at least a 1/8th scale. Should give you a model of ~10" to work with.

Rod

 

[Peter7307]

If it is of any help here...Citroen used scale models for their cars in wind tunnels and the smallest scale was 1:10.

I recall reading a comment from them along the lines that anything less was not sufficiently accurate to base decisions on.

This was back in the 1970's so it may well have changed since.

Pete.

 

[Murec]

If the 1:43 scale model is 198mm long, the actual car would be 8514 mm long?