Amazing what you can do with tire data

I mentioned earlier that some race tire manufacturers do have empirical tire data available if you ask nicely. Unfortunately it also tends to be confidential so I can't really show much detail, but you'll get the idea.

Believe it or not this is fairly rudimentary and only took about a full day to throw together, in between getting assorted other crap done. It does allow you to do a lot, though.

The most important thing is that I can do engineering as opposed to tinkering (see aside*) and do high-level design work rapidly. I don't want to spend weeks bullshitting around with suspension points to see what they do. I can pick my kinematic curves, roll stiffness, lateral load transfer distribution and all those basic macro parameters... and once a lot of these are defined they really lock in where your points have to be to achieve them. There's fine tuning to be done with the realities of what rates are possible given constraints of chassis and wheel dimensions, etc... but this gets you pretty close, pretty quickly.

Also makes it much easier to anticipate things like, "If I buy a couple sets of tires to test, when I take off Set A and bolt on Set B, what if anything do I need to change?"


As an *aside... and I'm not sure if I mentioned this earlier... when it comes to design like this there are two ways you can go about it. The way we did at CU for many years was not suspension engineering. It was tinkering. You play with suspension points, move one up, another one over, and see what it does. Maybe you have an idea of where you're trying to go with the design, maybe not. This is a ground-up design approach. Can take days or weeks to get where you want to be.

The other method is to engineer a solution from the top down. You pick the performance parameters you want. Start at a high level. Once you start defining what you need to get the car to handle how you want, the small stuff falls into place. If I want a caster angle of 'X' and a mechanical trail of 'Y' there's one unique solution for how that works in the side view. It does the work for you!

Revised brake duct and inner wheel fairing

Hadn't really been a fan of that previous brake duct inlet. Looked like a leaf blower. Revised that, and added the start of a simple inner wheel fairing. Easy drag reduction, why not.

At the moment the materials are defined as CRFP just for looks, but there are some rules I'll have to revisit on what can be carbon fiber as opposed to glass. S-glass still gives a fairly rigid structure between typical E-glass and carbon. It looks complex but if I use one-off molds with dissolvable foam, it shouldn't be too bad.

Alternatively what might be the best bet would be to direct rapid prototype those complex curvature duct pieces. SLA gets a bit expensive, but 3d-printing out of black ABS is relatively cheap. The machine we had at school was something like $5 per cubic inch of material, and there's not an awful lot of material there.

CFD - Revisited

Keeping boundary conditions and air properties the same, but this time going with a much less aggressive airfoil pattern. NACA3409.


Pressure field looks a lot better...


As does velocity. Flow does start to separate toward the end there, which we can fix... by adding a Gurney flap. If you're not familiar, Google it.

Does pretty much exactly what you'd expect. Extra low pressure zone directly aft of the flap helps suck the air flow back to the foil and moves the separation point further back.


Also, as one would suppose, builds up some stagnation pressure ahead of the flap on the top side of the foil. All in all you still have drag, but should be more downforce.



Pretty bad ass.

Next up will be to try to do some validation work on something simple like a NACA0009 profile to see if it at least captures trends for cD and cL at varying AoA. From there I can look through some airfoil catalogs, pick a handful that look promising and start playing around with increasing their operating envelope and performance level through slats, flaps, and all that good stuff.

CFD - What in the hell...

I forgot to mention the last time, another integral part of making good use of FEA/CFD is being able to interpret the results properly. Easier said than done for someone whose background is in big billet aluminum pieces, tires, and suspension.

So I figured I'd play with some CFD with a stock airfoil profile. Not knowing what kind of camber, thickness, and AoA is appropriate, I took a random guess. Used an inverted NACA7412, a crapload of AoA, and a 25 m/s (~55mph) inlet velocity boundary condition. Boundary conditions for the top and bottom of the mesh are dU/dn = 0, and V = 0, with outlet being some extrapolated pressure.

Kinematic viscosity 2.59e-5 (a bit high for air I later remembered)

Since I'm ultimately interested in downforce and drag, I decided to look at pressure after my model ran for a bit.

Results sucked!

What in the hell is that shit? Pressure balls floating off my airfoil? Is that real? Wtf? Even as a tire engineer I have some sense to know that's probably not good.

Upon closer examination of the velocity field...


Kinda looks like flow separation. Or at least that's my guess. Major league flow separation. Like whoa. Edit - That looks like a typical von Karman vortex sheet now that I think about it... at least in the velocity view. Had never looked at pressure before!

I'm just playin' around for now, have to figure out how to grab cL and cD from these, but after some experimentation maybe this airfoil is a bit aggressive for the application (at least unmodified with no slats, flaps, Gurneys, etc)! I think this one below I even ran at at more proper viscosity of 1.59e-5


To be continued... later tonight.

CFD - General

I'll write this up while I wait for the drier to finish.

I'm sure as soon as I get into the meat of any CFD or FEA I'll get comments of criticism. Some out of genuine concern and interest (cool), others just so they sound like experts while really just being a dick (not cool).

The accuracy of such analysis always comes into question, and rightfully so. In my limited experience doing designer-level structural FEA in college, and just from watching some stuff at work, good results inevitably come down to the following:

1. Assumptions made. Could be assuming your upright material is isotropic and homogenous (more or less true on billet, not at all on a weldment!) or that flow around an airfoil is incompressible.
2. Boundary conditions / loads / constraints. Maybe it was just CU, but FSAE students seemed notoriously off in this regard, just from inexperience.
3. Refinement of mesh. Pretty self explanitory. Capturing fine details (be it a Gurney flap or a non-radiused corner for a stress riser) requires localized mesh refinement.
4. What you expect to get out of it.
   

As an example of 2 & 3, on one of our FSAE cars we had a highly-loaded structural part (wheel hub). Billet aluminum, with material properties well-defined. Design criteria was based on von Mises stress, with FOS=2. The part had a thin tube section of about 0.06" wall thickness. I believe the mesh used globally in the model was 0.10 - 0.12". Additionally when I looked at it years after, the constraints were off. As a result, the first time driving the car this part sheared as soon as the driver got on the brakes. FOS=2, not so much.

On the other hand, we designed the same part years later with a different method. Billet steel. I think it actually wound up being lighter than the aluminum one (steel rocks). Design criteria again based on von Mises stress, with FOS=1.2. May sound low, but 20% is a lot of overshoot! Very carefully looked at the constraints and conditions with a highly refined mesh. Part never failed.

Good examples of how changing a few small things has a big impact on results. For this 2-d CFD work I'm not looking for absolute numbers. I'm interested in directionality in how to increase negative lift, reduce drag, look at effects of Gurney's, etc. Hopefully I won't get in too much trouble.

Unsteady 2-d flow simulation

A preview of things to come in the blog?



Very possible.

Darren Engwirda put together Matlab code for a 2-d mesh generator, and 2-d Navier-Stokes solver. Pretty slick and easy to use. We shall see how well it works.