Aerodynamic Streamlining Template: Part-C

[aerohead]

Originally Posted by ChazInMT

All I have to say about this is HA! It illustrates what I've been saying all along that if the pressure on the front is the same as that on the back, then the air sliding down the backside is returning the energy to the car that it took to move it out of the way, this to me is the fundamental basis for aero design.

Yes,it's all about pressure recovery.If there's any separation,the wake base pressure will the same as at the point of separation.

[ERTW]

Originally Posted by KamperBob

Thanks, dude! Now I'm really looking forward to how this compares with your prismatic template model.

Your earlier point, ERTW, about modeling spheres as known problem solutions is actually a great idea!

So what was the trick for cutting computation time?

Actually Bob, the sphere example is in the software tutorial

It seems there is no trick I need a multicore cpu! Volvo quotes 500 to 1000 iterations for solid results, however, they did complex bodies and I'm doing simple shapes I find that the solver never quite converges - it just oscillates about a value. It 'stabilizes' within 10 iterations I should take the average of a bunch of data points.

My research yesterday totally blew me away! ...

[kah]

Saw a Citroen Xsara Picasso on the streets yesterday and this thread immediately came to mind.. I wonder how good the aero of this car is in reverse. It actually looks like PSA had the teardrop in mind when they designed it, but somehow got confused as to which way it should face.

[kah]

Photo didn't attach to my previous reply.. here's hoping it works.

[Fat Charlie]

Try it again. I think you need 10 posts before you can post a pic.

[euromodder]

Originally Posted by kah

Saw a Citroen Xsara Picasso on the streets yesterday and this thread immediately came to mind.. I wonder how good the aero of this car is in reverse. It actually looks like PSA had the teardrop in mind when they designed it, but somehow got confused as to which way it should face.

LOL.
There's actually a thread on reversed-aerodynamics as well

[ERTW]

Phil, do you have a front view and a plan view of the template? I don't think I'm getting the shape right. I set ground clearance at 10% of the body thickness with sharp edges and the Cd is coming in at 0.275 :/ Do you have a solid model?

I've also taken a 4:1 rear body with a 1:1 front end (same length as a 2.5:1 ellipse) which comes out to Cd 0.247. Using a 6:1 rear body (used to model dolphins) only dropped it to 0.243. Both had a 10% fillet and 10% ground clearance. What do you think?

edit: I rounded the bottom edge (R5" or 10%)...Cd 0.191, CL ~ 0.055. Sharp leading edges are a no no! I want to protrude the nose a little into the wake, and sharpen up the trailing edge. I believe it "wants" a 20% radius on the leading edge, and ~12.5° max angle. I think 25% ground clearance is closer to your template (and helps get it above the earth's boundary layer). I'll report it asap.

[drmiller100]

ertw, the air separates where the body turns down. What is the angle in relation to the floor there?

Also, are you modelling a fixed floor, or a moving floor like the real world is?

Also, can you model with ZERO ground clearance?

The "optimal" shape has always confused me - it violates the 20 degree slope, and I don't understand how the air is supposed to stay attached.

I bet if you change the slope of the roof to be the same all the way back, CD will improve.

I'd be curious if you just chopped the car at the slope change if the CD will improve.

[ERTW]

I'm already ahead of you drmiller...the solver finished this morning after 12 hrs and 373 iterations!

I learned a huge lesson this morning. The goal plot was pretty flat at 100 iterations. Even at 200...and then around 220 it became a step function It was flat out until 350 iterations - and I was ready to stop the solver there...and then another step. wtf! It goes to show that what I assumed was "substantially" accurate may not be. I'll just let the solver finish from now on!

A moving floor becomes useful below 5" of ground clearance. At 13", it's more free air, than ground proximity. If the under body is rough, then it may be of benefit to drop the car, and force more air over the body. If the under body is smooth, it's more beneficial to have higher ground clearance. It's a trade off that's best explored in a wind tunnel.

I have the tail at 15°, and it still appears to detach somewhat. I hypothesise that ~12.5° is max. I believe the largest loss is the vortices. I think we could stop streamlines from migrating from the bottom, around the sides by widening the tail...more like a duck tail than a point.

rho = 1.205 kg/m³ @ 273.15 K
v = 30 m/s (108 kph)
A = 2.138 m² (23.0 ft²)
Fx = 108.68 N

Cd = 0.094

Drag index = 0.20 m² (2.156 ft²)

did I do good papa?

[skyking]

Cool! Thanks so much for taking a hard look at it.

A moving floor becomes useful below 5" of ground clearance. At 13", it's more free air, than ground proximity. If the under body is rough, then it may be of benefit to drop the car, and force more air over the body. If the under body is smooth, it's more beneficial to have higher ground clearance. It's a trade off that's best explored in a wind tunnel.

1) Keep in mind that the real world still uses tires and wheels, and the farther you lift a body up the more the wheel frontal area and disturbance of the rotating disc come into play. There is no practical solution for fairing in the steering axle in particular, unless you have the budget of a race car company.
2) there is virtually always a crosswind component. A shape that kicks butt straight down the road will lose to one that is tested at a 1~3 degree crab angle, because that angle is reality.
3) #2 affects #3 as well. A disturbance from the wheel, projected across the underbody at an angle may really toss a wrench in the works, so to speak.