CFD and models
 

OpenFoam is an opensource CFD package with apparently enough parts to do at least some analysis for us.
Others on this forum have shown some CFD work with systems they use at work.

I have been searching and the models found were not particularly good.
So does anyone have models they could contribute to such an effort?
Whilst I realise this isn't going to make our car into a hot bed of aero engineering it would be nice to visualise ideas we bounce off one another even if the results are not perfect.

The cost of creating a CFD model is way higher than you think.

Can you tell me why?
If it is just because of modelling time then I may have a way around that.
I own this so if i don't screw things up and get good weather it is the realms of possibility to fully digitise my car as an accurate point cloud based mesh.
I won't be able to do the bottom as the distance required would need a lift but if i can seal the rest of the mesh I can hopefully do a rough bottom model.

I know that models are not used in CFD they must be changed from a mesh to something else but there is a brief explanation of that here

It just seems worthwhile to at least try.

I work as a project engineer for T1 automotive heat exchanger supplier. There's a lot that goes on behind the scenes when an OEM customer comes to us and needs a CFD analysis done. Our virtual analysis team make things look easy as many times most of the variables are known right away as its always similar to work done in the past, but if you're starting from scratch I'd expect it to not be cheap to regress. Having models and proper mesh to use is just step one.

This link gives you a taste of things.
https://www.simscale.com/forum/t/cfd...-process/66560

Do not think I believe this will be straight forward. This is a huge field with many roles that take skill and wanting an end result would require at least some understanding of most of them which is not easy.
It just seems that we could benefit from something open source (ie free) and transportable to help each other and visualise our ideas more readily.

I can attempt to scan the car with the Occiptal sensor which will create a mesh but the underside will need some work realistically.
I did wonder about using the Tamiya Miata 1:24 model on some sort of gurney spinner to scan as it has exhaust pipes and A arms.

There are tutorials for OpenFoam doing analysis but i haven't found one on creating or adapting models yet.
Again this just seems sensible to try. If it too much to do then so be it but with all our chats about this stuff surely we should try to get something together. Especially as some do actually work in this field.

What's the goal of your cfd model? You've got to go into this with a specific problem in mind, such as the optimizing the drag vs downforce of a front splitter or optimum height of a gtc-300 wing. Many common design problems are either well understood, aren't worth modeling, or are better left to experimentation on track.

Finding the difference between stock and whatever the design is.
Optimisation can only be done so far in CFD this is more of a idea visualisation hope.

I agree with optimization through CAD. It surprises me that everyone is so against consolidating resources and working to improve aero with CFD. I have an nb model that I have no idea of the accuracy of the model. I play with it from time to time but my computer doesn't like the file size. PM me your email if you want it.

https://cimg3.ibsrv.net/gimg/www.mia...e82234f950.jpg

I have a na shell in solidworks that I can share. It's close to actual but wrong in a lot of places. I found it online somewhere

well upload models i dont mind playing with them

I'm building a cabinet to hide cables from my little one as there are reaching hands to climb currently so I won't be attempting this just yet but any and all uploads are welcome.
I will still try to photogrammetry my car as well as scan it with the occiptal in hopes of an accurate output but i am NA rather than NB or NC or ND.
There is an NB down the street and I'm yet to introduce myself so if the scan/photogrammetry works i'll ask if i can do it to his car.

I am comfortable doing basic modelling but I'm not digital sculpter so it will be as close as I can do.

If I may chime in here, as an aeronautical engineer... Automotive aerodynamics is incredibly complex - MUCH more so, than that for aircraft. This is because cars are not streamlined bodies, and they operate on a surface, and they have rotating tires, and a large part of the airflow is through cooling-components (ducts, radiator, ...). To be rigorous about aerodynamic modeling is a daunting task. Even at the highest levels, there is extensive empiricism and trial-and-error.

What we can however do, is incremental investigations, where the aim is to assess in rough terms, what would be the consequence of this or that mod. For example, any of the graphics already posted in this thread, could be run (with admittedly considerable effort) through a Reynolds Averaged Navier-Stokes solver... getting some (not necessarily all that good) answer. Then, some change would be introduced - say, blocking off the radiator, or raising or lowering the ride-height. The new computation, vs. the old, gives a comparison - a delta. It often happens, whether in computation or in wind tunnel testing, that the baseline result is contaminated, or otherwise unreliable... but the new result, with the aforementioned change, makes comparable error. So, the delta is still pretty accurate, because of cancellation of like errors. This is the philosophy in a lot of trial-and-error testing, where say lift and drag are measured in the wind tunnel for the baseline, and then with serious of components added or removed. The total result is inaccurate, but the deltas can be compared among themselves, with enough verisimilitude, to warrant more detailed testing.

But I do have to ask - what, other than piddling around and enlightening ourselves - would be the objective of such a venture? Normally, computations are used, when actual experiment is intractable, say for example when a large number of test-cases are to be run, and this would be prohibitively costly. But in the case of amateurs, working alone, trying to improve their own cars - well, what would computations enable, that is superior to that possible by just running the car down the road? I ask not from cynical or condescending perspective, but from genuine curiosity.

I get this strange feeling that Holmes and Joe Perez would get along really well.

I believe the delta you're referring to is what members want the CFD model for. Other than that, its kind of like when someone buys a 3D printer. They're not buying it to start a prototyping business, they're buying it to play around.

I think the answer is both, to play around with and for people to try and take it forward.....the hive mentality can and does work. Megasquirt is a good example.

It would only take one or two "aliens" to make a dramatic step forward.....plus, you never know till you try......building the platform of repeatable and multiple users testing and comparing the data, looking for errors and fine tuning has a lot more chance of moving forward than each person doing real world tests that aren't repeatable or even comparable.

CFD files shared across the internet would/could be...

Its not anywhere near where I am capable of playing but to me there is no reason to not encourage it.

IMO.

As curly and kiwi have said it is to have the freedom to play but also take ideas forward.
Spending 30 minutes knocking up a model change for an idea then some more time to run a standard type of test (what that is we don't know yet) would give us a delta.
Couple to that an accurate model and that delta becomes less theoretical.

I'm a software engineer with a biology degree who built a synthetic aperture radar in his shed.
It wasn't necessarily helpful but it was interesting and functional. I was able to do that however through shared knowledge and help.
The gent I was talking to has now published a . This covers the lectures he gives on the subject as well as construction of what I miniaturised for my drone.

If we can get some models, get some basic use case knowledge there may a chance that as a community we could get some explanation for our ideas.
No one here expects this to be straight forward. No one here expects the model to match real world. We simply want the possibility of testing our knowledge and improving our cars.
With some experience I could pass this on so others can model ideas for their far more impressive endeavors than I will do.

Look into ONshape cloud-based modeling, the free version can take you pretty far. There are a ton of tutorials.

It does surprise me that this hasn't been done yet.

After talking with a few consultants in the past the difficulties lies in meshing surfaces appropriately and getting the computing power to support a moving road, rolling tires, and iterations at each and every ride height and yaw/slip angle.

Considerations for later. No one thinks this is a walk in the park. First of all we need some decent models. Then we can attempt to get some sort of analysis. Once we have this then we can attempt realistic conditions.
Remember this is an entire industry in its self taking it up from scratch won't be easy, pretty or quick anyway.

My only caution, is that anyone who has actually done an analysis this complex, knows how easy it is to make bad decisions off a model that you think is correct. Especially with this, it's tough to know whether you have done the right thing without taking the car to a wind tunnel. If it's just to play around, cool, but if you are hoping to make a competition beating time attack car by designing it in CFD, I would want to temper your expectations now. I'm not against this at all, but I just know first hand how hard it is to make good aero changes to a vehicle with an untested model.

No. Literally everyone interested has stated this is to provided comparisons at best. You only have to read the few sentences in this thread to see that isn't what this is about.

Having done my own share of CFD simulations (some Miatas and other fluid dynamic systems) and CAD modelling, I can say that if your model is not accurate or if you don't know the errors of your model, it is really difficult to get any relevant data. Small surface imperfections have a big difference on your simulation results as for example they play a big role in flow attachment. (Good example is that OEM manufacturers use small sharp edges near the rear corners of cars to disattach the flow for better Drag Coefficiency) Also running these simulations is another story itself, if you want to get results with good resolution, expect to leave your home computer screaming at 100% load for couple of days to get just one simulation done. At work we use powerful multi-computer render-farms any time there are 3D fluid dynamics involved (or any other complex structural simulations for that matter, like fiber-orientation dependent calculations). Even generating the mesh for the simulation can take hours even if done really well.

I spent close to 1-2 months to make and detail my own NA and NB models, and still I would not consider them to be that good (these were used for my friends thesis and for my own car). I was planning to borrow a laser scanner from work to do a cross-comparison of the models, but so far motivation to do that has not stroke me. As an advice for workflow, this is probably a good approach for this anyways. Only use your point cloud data for cross comparing. Normally point cloud data can have a inaccuracy of anything between 1-30mm (depending on orientation and angle of surface being captured) which is a bit too much for any kind of simulation. Also point cloud data is not usable in a simulation anyways even if you convert it into a mesh, because it will have really bad mesh geometry.

For modelling you should model your stuff in quad-based mesh that is following any hard edges to avoid issues when you subdivide the mesh to have higher resolution.

It is fun to play around for example with the Autodesk FlowDesign, but it is really hard to get any real results as there are many different details that play a big role in the simulation. Which resolution, which solver, which equations, which model, rolling road, rotating wheels, what speed, etc etc. and most importantly, how do you compare your data to real life?

From my perspective it would be an opportunity to figure out IF a certain mod has potential to just try.

from there the benefit might show up in improvement in laptime, better feel or even just looks......

i am really keen to just see the results of the standard 949 style air dam with different length splitters. of particular interest to me is the thickness of the splitter. V8 supercars are 3-4" deep versus 1/2 inch ply or ACM etc...

Hi, what my friend found out in the simulations he did, is that the area of the splitter indeed does affect the down-force (duh), shape not that much. But what had a lot bigger impact on the created down-force was the angle of the splitter. With a 2 degree angle (nose down) it was possible to double the total downforce of the whole car. (this was without spoiler, just a OEM lip at the back) (also the center of downforce was further back) I know he reads these forums and I know he would kill me if I posted his results here... It was a lot of work for him to do these studies so posting them here would feel like stealing his work anyways.

Thats fine with me. I have a server rack in a custom cabinet venting through my roof with G8 HP hardware in the house as it is so this wouldn't change much from the day to day noise of that.

Interesting that you found models from mesh were more "noisy" I hadn't though of this ramification from doing the IR scan plotting the Occiptal (basically a pumped up Kinect) can do. Accuracy was stated as 3mm so this would provide some wild variations although smoothing of the model through various packages should reduce that sort of noise somewhat.

The idea is to visualise ideas not get data. Seeing a beneficial difference between standard and test would simply provide an idea for actual testing. This will likely never be actually helpful moreover a fiddling location to see how stupid an idea may be by comparison not in absolute units.

I wanted to provide a photo of what I mean by the quad-based mesh that follows hard edges. This is my own model so by no means perfect, but decently optimized:


https://cimg8.ibsrv.net/gimg/www.mia...cc870ec5d5.png

This is more than decent. I am unfamiliar with any open-source/free tools that allow mesh modification and model preparation, however we use ANSA at work and I simply love it.

I am not sure if you mean modelling software or meshing software. Its a bit confusing as with the modelling software you do a mesh, but a "mesher" converts your model into a mesh usable by the simulation software.

I would highly recommend Blender as a free modelling tool. Its mainly meant for doing 3D animations and rendering such things, but it has really nice set of tools for learning to do mesh by hand. You can simply start by adding on some reference starting points to space, connecting them, and slowly starting to extrude faces and build the model.

I think BlueCFD had some opensource tools for meshing. I am not 100% sure, but it is worth looking into if you are curious.

Meshing/re-meshing, not modelling ;)

BTW you got a PM

I found Openfoam to be a very difficult peice of software to use and draw any meanful conclusions from. The workflow can be quite cumbersome also - exporting model to stls, importing to my linux computer, some tweaks which are needed in linux to the stls, time running the mesh (hope the mesh works and any changes in the model havent crashed my mesher) and then running the solver. Also setting up monitoring of the simulation to make sure it isnt diverging :/ (This can be streamlined with a virtual box and some scripts, but I never got around to it.)

Finding a good meshing setup can also be quite painful - I'd recommend SnappyHexMesh if you are set on using Openfoam. The setup of this will be dependent on what your computer is and how you have setup your model/stls - which is usually driven by what force information you want to output - ie: if you need individual component forces and moments. (resultant moments are incredible useful and often not well understood and missed.) Post processing this information and comparing it can also be quite difficult to understand.

You can get some pretty pictures/videos from paraview though I guess :likecat: