Not much of an update, the virtual car next to the real one…
After much stuffing about I’m getting somewhere with the CFD, some of the results of this are shown below:
According to the CFD this shape is allegedly making 476lb of down force with 571lb of drag at 150mph. This gives a CD of about 0.54.
If the numbers turn out to be even vaguely correct then I’ll be happy.
I’m still struggling with openFOAM.
The graph below shows what I’d expect to see on a typical run, notice the data starts off wild but within the space of a few hundred iterations its basically become stable for the rest of the simulation. The three lines shown here are X, Y and Z forces over 2500 iterations, the values here aren’t really the point just the shape of the graphs:
Then there’s what happens when things go wrong, as shown below, the graphs here are from 4 seperate runs showing X forces for the first 1000 iterations.
The purple line is the sensible data, the green and blue lines show a phenomena where the results instead of converging yo yo exponentially out of control. The red line shows a different situation where a value will just keep climbing.
Looking at these numbers its not hard to spot something has gone horribly wrong, whats harder to work out is what it was and how to stop it from happening. At the moment I think most of these of issues are caused by mesh problems, so currently that’s the direction I’m looking. Hopefully I’ll find a way to stop this sort of thing from happening too often, which it is at the moment.
I’ve started working on the design of the body more seriously than before. My previous attempts at using CFD software had essentially been a failure in terms of the body design but I had learnt a few things about the process.
I’m now starting the process again having switched software to OpenFOAM this is open source (free) software most commonly run on Linux, however I’m not familiar with Linux and didn’t want deal with learning too much of that so I bought a setup ported to run on Windows from blueCAPE. Which you can just install and have work out of the box. You can recompile the Linux version on to Windows yourself for free if you wish but for 50 odd Euro I couldn’t be bothered with the effort.
From my previous efforts I’ve number of different body shapes to run through before I need to get into any real new design and learning how to get what I think are reasonable results from the simulation has taken a fair bit of time to date.
Looking at the models I am running… The volume I am using is 20m long by 8m wide by 4m tall the basic mesh breaks this down into 100mm cubes, so that’s 640,000 cells right there, then when the model is introduced there’s a process of refinement to through a program call snappyhexMesh (part of openFOAM) which refines the mesh to fit the shape introduced into it. By the time this is done there’s generally about 7.5 million cells in the simulation.
After quite a bit of experimentation and not being too happy with the results I came across this paper on the web, finding good advice for this type of problem seems quite hard, you have to be able convince yourself you are or are not looking at rubbish results coming out of the software and it’s easy to get rubbish results I know.
So following the example in the paper I run the simulations now for 2500 iterations and take the last average of the last 500 iterations as the result.
Thankfully I have constructed a fairly decent PC to runs the sims on (i7 2600k running at ~4.5GHz, 16Gb of ram). Each run typically takes about 13 hours to mesh and complete, which sounds slow but in practice is ok. Thankfully I have figured out how to run the problem in parallel so it makes full use of the cores available in the CPU otherwise this time would be about 5 times longer.
Below is a video made using ParaView of one of the early runs, I believe visualization of whats going on in the simulation is a very important part of the process of trying to work out what to try next.
I’ll start posting some results and images as I get further into this.
Below are some pictures of the car in the state it’ll go into storage, this is the most assembled in one go I think it has been so far.
The KBS paint looks better in the pictures than in the flesh.
This’ll be the last post of the build for some time as we’ll be moving to Japan in a few weeks for a few years. Hopefully during that time I’ll be able to work on the body design as well as a revised improved design for the chassis.
Here is the chassis frame back from being sand blasted and primed. Since these were taken it’s had a coat of KBS Rust Seal and I’m now in the process of building the whole thing back up so it can go in to storage, more pics soon.
In an attempt to reduce the dozen or so spam bots that register on the site each day I’ve implemented a Captcha plug-in, which should slow them down (stop them I wish!).
I’ve been through the registered users and deleted the ones that I think were spam, if I got it wrong and deleted your real registration, sorry please register again.
Hopefully if this works I can open up the security on the comments a bit more.
As usual its been too long since I updated the blog…
The news is essentially the build of the car is going to be put on hold for approximately 3 years.
This is due to me changing jobs and in a couple of months myself and family will be moving to Japan until about the end of 2014. Which as you’ll understand will make working on the car a bit difficult.
So in the mean time efforts are being dedicated to getting the chassis to a point where I can get it blasted and painted ready for storage. I’m nearly there with that and pictures will follow in time.
I intend to carry on the design process for the body of the car and other outstanding items during the time away. Also I’m thinking of revising the chassis design to come up with the MkII version and fix all the bits I’m not happy with on the current version. I’m not sure who’ll build the MkII, it certainly wont be me as the MkI chassis is 95% there so I am stuffed if I’ll start again. But it seems somehow a shame to keep all the design work bottled up in one car, I’ll need to think some more on what that process might look like though. I had been determined to keep the car design to myself until it was constructed and running and proven to be worth the effort, if I do that now though it’ll be at least 5 years away before it see the light of day (i.e. a long time), on the other hand I don’t want to waste peoples time building something that turns out to be horrible to drive. I guess I’ll need to think some more on this. Thoughts?
I”ve not been home much for the last couple of months, hence not much has been done on the car. In actual fact I’m away on business again now, so still no work on the car.
However I did get some time at home since the last post and here’s where its at…
Firstly I sourced from a local wreckers a handbrake from a Toyota Land Cruiser of all things, below shows it installed, due to limited floor space in the car it needed to be installed vertically, the cable retention bracket for the floor is yet to be done.
This was pretty much the last thing that I could think of – could be bothered to source before stripping the car for final welding and painting. I’m in no doubt I’ve forgotten to weld something on I should, but I needed to draw a line somewhere and this was it.
So here it is partially dismantled…
And here it is back on the rotisserie for welding…
It’s now of of the rotisserie and the lifting brackets have been cut off, so now I’m using the engine crane to pick it up and rotating the thing.
On one of my trips this year I had time enough to get to Barber Motorsports park in Birmingham, Alabama. Where there was a legends of motorsport event happening, one of the many cars there for the weekend was the one below an Argo JM16 which I really like looking at so post it for no reason other than that.
No progress to report as such, but used a Gigapan Epic 100 to take the image below.
This is a link to the image on the Gigapan.org website and the full screen version.
This image is hundreds of regular pictures stitched together to form a 720 mega pixel image. So you can zoom in to a tiny level of detail.
By the way the welding on the sheet metal is just tacking it there so it looks a bit tacky at the moment.