Had some free time and solidworks flow simulator

[Bigbadspoon]

So I recently got my hands on Solidworks 2010 with the Flow Simulator add-in and I mocked up a *really* rough model of my Civic (extruded a side profile, then extrude-cut the front profile out of it). I'm still learning surfacing, so that will be a future project, but I figured this was a good starting point.

So, I put up several configurations. There was just a baseline run, then with a spoiler somewhat similar to what came on the Si models, then with a grillblock and spoiler, then with a kammback and spoiler, a couple types of rear extensions, and a 3.5 inch drop. Since the model is not super accurate it has a flat underbody by default.

I had it test for a few things: drag force (Z-component), lift (Y-component), velocity (it's in in/s since mph is not a choice), and pressure. Since it's in IPS mode, I set the airflow to 1200 in/s, which comes out close to 68 mph, which is a bit under the speed limit here in Florida, but that's about how fast I drive.

Believe it or not, the spoiler didn't increase drag very much and it reduced lift quite a bit. Lowering was far and away the most effective at reducing drag force.

I put all my results (spreadsheets and pics) in a zip, but it seems to be a bit large for the forum, so I made a mediafire account where they can be accessed.

results.zip

results.zip

Supposedly, those are the same. I don't really know how it works, but I figured better safe than sorry.

[RobertSmalls]

Thanks for posting these. Flow simulations are always welcome.

As you said, the accuracy isn't perfect. I wouldn't draw any conclusions about ground clearance with a stationary road and no rotating wheels, for example, because those factors could well change the outcome.

How the heck did you get SolidWorks to tell you total lift and drag? Maybe that's a new feature to SW2010? I tried to estimate drag for this model, with and without tail extension: http://ecomodder.com/forum/showthrea...tml#post161595

But I couldn't get a CFD program simple enough for me to use, and complex enough to be confident in itself to spit out Fx and Fz.

Just as a sanity check, what does your Cd work out to for each case? That would be Fx/(.5*A*rho*V²), with V being free-stream airspeed and rho being air density.

[Bigbadspoon]

I'm on my way to class right now, so I'll have to calculate the Cd when I get home, but when you run the simulation, there is an icon where you can pick "goals". In the goals menu there are about 20 or so different things to have the simulation check for, including component force and velocity.

[gone-ot]

...visually, only looks subtle, but probably very quantitative differences?!?!

[Bigbadspoon]

Not HUGE differences. I have spreadsheets in that zip file that give the specifics. The only one that I noticed was really big was when I put the spoiler on, the lift component went down by about 40 pounds (almost 50%). Collectively, the decrease in drag was like 15 pounds. But, these are all very rough estimates since, like Robert said, rotating wheels are not accounted for and the road is technically moving the same speed as the car (stationary). Also, the shape is very rough.

Robert, I'm going to sit down and calculate that Cd right now. I think I saw somewhere that solidworks can calculate sectional areas, so I'm gonna look for that function.

[Bigbadspoon]

So, using the data from the spreadsheets and my model, I calculated the Cd for the baseline and then the final run, which had lowering, lower extended, spoiler, and grille block.

The baseline Cd was 0.52 and the second one was 0.46. The rest fell somewhere in between, but in the interest of time and my sanity (I do monotonous calculations all day at school), I didn't calculate them.

Needless to say, that is substantially higher than the actual number, which I found on wikipedia to be 0.36 for the generation of civic right after mine. They have a very similar shape, though, so it should be close. So, I guess we could adjust these drag numbers down by multiplying them all by (.36/.52).

[RobertSmalls]

My first thought:

Sanity check fail.

But look at the biggest ways your model deviates from the car. The road moving at the speed of the car impedes airflow underneath, increasing drag. You lack tires, so your reference area was smaller, increasing Cd. And I assume you used hard chines instead of large radii on the left and right edges of the car, which would contribute substantially to drag.

I would advise against a scaling factor. Instead, look at it as "Feature X provides an improvement of x%" or "-0.0x Cd".

What's the chance you can get valid, usable results from SW2010, and use them as a basis for engineering decisions? And what effect would my Insight model's tail extension have on drag?

[Bigbadspoon]

Well, the model has tires, but the plane that intersected the model at that point went through the center of the car and not through the tires, so you couldn't see them. Either way, without rotation they're not too much good.

SW2010 would more than likely provide reasonable results, but I'm pretty new to it. It is a fairly popular program in the engineering community, after all, though I'm sure dedicated aerodynamics companies would probably export models to a more specific program.

As for your Insight, I'm not 100% sure. Do you have a link to your model? Also, you mentioned something about Pro/E. If it was modeled in there, how did you get the picture in the background? I fooled around with trying to find that for like 3 hours one day and couldn't find it.

I'll slap a cone on the end of my Civic for now and see what happens.

[Bigbadspoon]

It brought the baseline Cd down from .52 to .40.

Also, I attached a view of the model I've been using.

[sendler]

Can you help with a sim?
.
http://ecomodder.com/forum/showthrea...tml#post382462
.