The Underbody (flat belly pan is suboptimal - explanation & solutions offered)

[He-man]

Ive been reading this thread all morning and am really confused as to what i should of for a belly pan. I see that the Chevy Ev1 has a perfectly flat belly pan and looking at aircraft the bodies are smooth as can be on planes. Many of the new high mileage cars have perfectly flat belly pans.

With all this being said, why wouldn't planes be dimpled on the body? I understand that dimpling works to reduce drag on spheres but where is the proof that it works on flat objects? And furthermore, what is the purpose of causing turbulent attached air on the bottom when the top and sides of the car don't necessarily have turbulent air but it is attached?

I am a newby to Aero stuff so if i said something dumb be nice

[Pooft Lee]

Originally Posted by He-man

Ive been reading this thread all morning and am really confused as to what i should of for a belly pan. I see that the Chevy Ev1 has a perfectly flat belly pan and looking at aircraft the bodies are smooth as can be on planes. Many of the new high mileage cars have perfectly flat belly pans.

With all this being said, why wouldn't planes be dimpled on the body? I understand that dimpling works to reduce drag on spheres but where is the proof that it works on flat objects? And furthermore, what is the purpose of causing turbulent attached air on the bottom when the top and sides of the car don't necessarily have turbulent air but it is attached?

I am a newby to Aero stuff so if i said something dumb be nice

As far as whats practical, a flat undertray will reduce drag. This thread is packed with hypotheticals and theories, so being confused is totally normal.

For practicality, a flat pan will cut drag even if it isn't perfect. An air dam is a much less time consuming place to start, with similar benefits

One reason dimpling isn't common is cost. Flat is way easier. The other big one, is that without the steep transitions that cause flow separation, you don't need it. The golf ball is fighting flow separation as best it can to shrink its wake.

Its been explored a little previously, but size is the biggest issue. A large object like a car will inevitably have turbulent flow anyways. You can use vortex generators to help, but those are adding drag upstream to hopefully ease it downstream, and not ideal

If you wanted more on why people are talking about rough surfaces to streamline, they're concerned about the boundary layer. Attached airflow has a greater influence on the vehicle. Metal intake manifolds leave the casting roughness to avoid attached airflow and keep velocity high, and boats use it as well. Ford tried it on the probe iv with difficult to measure results.

To be honest, I'm not quite sure when turbulent attached and laminar attached airflow are best. I don't think there is a "right" answer

[Pooft Lee]

Also, pressure and velocity are inversely proportional. Turbulent attached flow is faster, so you could potentially lower the pressure. If the airflow under the car wasn't already turbulent. Which it is after a certain distance.

There's a paneled front diffuser thread on corner carvers that has a lot of cool ideas
This thread is wayy in depth for the average project. You should look up wheel treatments for flat undertrays though. Wheel well and wheel drag is 25% of the drag on most cars

[freebeard]

There's a paneled front diffuser thread...

[citation needed]

I prepared a response yesterday, but it would appear I never hit Submit.

I believe you won't be getting laminar flow anywhere past the stagnation point, in ground effect. Turbulent flow, attached or not, is the domain ground vehicles operate in.

And furthermore, what is the purpose of causing turbulent attached air on the bottom when the top and sides of the car don't necessarily have turbulent air but it is attached?

The difference is that the underbody is moving relative to the ground plane. This causes shear forces that aren't experienced by the top, sides and ends. Your better wind tunnels have a moving ground plane for this reason.

[He-man]

So then i would be fine to do a flat belly pan? Can you point me to the paneled front diffuser thread? Also, is there a thread on doing the wheel well? Thanks guys, if the wheel well is 25% i think that will have to be incorporated into my flat belly pan.

[Pooft Lee]

Originally Posted by He-man

So then i would be fine to do a flat belly pan? Can you point me to the paneled front diffuser thread? Also, is there a thread on doing the wheel well? Thanks guys, if the wheel well is 25% i think that will have to be incorporated into my flat belly pan.

Apologies, I had it bookmarked
Paneled bottom diffuser thread - Corner-Carvers Forums

Lots of reading on underbody treatments, and how they've applied it.

As far as the wheels, they outline a solution on that thread that helps stop air from stagnating in the wheel well. The front undertray funnels air in inboard of the wheels.

Fairings for the rear wheels, and wind deflectors in front of the fronts are two options, as well as smooth hubcaps. Turbulence and pressure from the rotating wheel is the big issue. You can keep the air out, and you can help it leave easier, but the wheels have to turn and they'll always create some drag. If you look at endurance and other race cars, they typically include vents on top of the fender and behind the wheel arch. Whats tricky about referencing motorsport is they have very different goals than you do, although drag is very much on their radar they will trade it for a grip advantage

[freebeard]

Your search terms would be 'wheel spats' and 'air curtain'.

The shear forces in the underbody are more straightforward than what's happening in the wheelwells. Try this:

Numerical simulation of the flow around the wheel within wheelhouse - Sasan_Sarmast_Jan2010.pdf

See also:


A_New_Aerodynamic_Approach_to_advanced_automobile_ shapes.pdf

On page seven Dr. A. Morelli describes a Fiat with centrifugal fans built into the wheels to suck air out of the wheelwells and eject it through slots in the underbody to separate the external turbulence and complete his 'fluid tail'.

[aerohead]

Originally Posted by He-man

Ive been reading this thread all morning and am really confused as to what i should of for a belly pan. I see that the Chevy Ev1 has a perfectly flat belly pan and looking at aircraft the bodies are smooth as can be on planes. Many of the new high mileage cars have perfectly flat belly pans.

With all this being said, why wouldn't planes be dimpled on the body? I understand that dimpling works to reduce drag on spheres but where is the proof that it works on flat objects? And furthermore, what is the purpose of causing turbulent attached air on the bottom when the top and sides of the car don't necessarily have turbulent air but it is attached?

I am a newby to Aero stuff so if i said something dumb be nice

*The drag coefficient of a 3-D body cannot achieve it's lowest drag without a full turbulent boundary layer.
Here you can see the smaller wake and lower drag on the right,after artificial roughness has been introduced

*Airplanes are ruled by skin friction drag,they don't have profile drag like a car.Any thing they can do to reduce a turbulent boundary layer saves them fuel.So no dimpling!
The only exception would be for high-angle-of-attack takeoff or landing,when artificial roughness would make the difference between a crash.

[NoD~]

aerohead, so... with enough bugs on the front of my grill... same effect?

[Pooft Lee]

Originally Posted by aerohead

*The drag coefficient of a 3-D body cannot achieve it's lowest drag without a full turbulent boundary layer.

*Airplanes are ruled by skin friction drag,they don't have profile drag like a car.Any thing they can do to reduce a turbulent boundary layer saves them fuel.So no dimpling!
The only exception would be for high-angle-of-attack takeoff or landing,when artificial roughness would make the difference between a crash.

Here's where I get a little confused. Does turbulent attached flow produce more drag than laminar or vice versa? I understand the reduced wake is the end game, but is that the only plus to the dimpling?

If so, it makes sense a plane with a no-compromises shape wouldn't have the same separation issues and wouldn't want to run around chopping up air

I'm also curious what you mean by a car is mostly profile drag vs the plane