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

[aerohead]

Originally Posted by NoD~

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

*you don't even need those.
*above 20-mph,you'll have about an inch of laminar boundary layer (according to Feysal Ahmed's Master's Thesis),and the rest of the car will be full-blown turbulent boundary layer,with fully-attached flow if the body is shaped correctly.

[aerohead]

Originally Posted by Pooft Lee

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

*you always want attached laminar outer flow (sometimes referred to as inviscid flow).
*you cannot have attached laminar outer flow on the vehicle's afterbody unless you have a full turbulent boundary layer.
*and unless the afterbody is very much like the template,you lose both.

[aerohead]

Originally Posted by Pooft Lee

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

*Automotive streamlining is about reducing,or eliminating flow separation,almost exclusively in the afterbody.
*If you have separation,the kinetic energy of the turbulence of the separated flow can never be reclaimed.
*If you don't have separation,the afterbody flow will decelerate,lose it's velocity,gain pressure,and when it leaves the rear of the car,will be at it's highest possible pressure,which reduces pressure drag,the primary component of profile drag.
*A turbulent boundary layer will allow fully attached laminar flow "IF" the body is shaped in the back like the 'template.'
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*Dimpling was a way to increase the Reynolds number of little golf balls to a supercritical value,to force the transition from a high-drag laminar boundary layer,to a low-drag turbulent boundary layer.
*Without the dimpling,even at 110-mph off the club head,the wake would be enormous and they wouldn't make it far down the fairway.
*Vortex generators(real ones !)would improve the range of all golf balls,compared to dimpled balls,but as soon as you struck the ball with the club they'd be destroyed.
*The dimple is a practical compromise,providing artificial roughness to force the LBL-toTBL transition.
*No matter what direction the ball is placed on the tee,the air effectively 'see's' the same 'face' of the ball as it flies toward the green.
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*Aircraft are very long in relation to their width.
*These large fineness ratios make pressure drag virtually impossible.
*Their enormous exposed surface area is responsible for the lion's share of their drag.
*TBL surface friction is higher than LBL surface friction.
*The lowest drag "laminar" aircraft hold the widest portion of their wing and fuselage very far back to postpone the inevitable 1st-minmum-pressure which forces the LBL-to TBL transition.
*"Laminar" aircraft is kind of an oxymoron.All aircraft have turbulent boundary layers.It's just that they're minimized as much as possible.
*Aircraft are also designed for 'flight conditions',at a given altitude and velocity,trimmed for lowest drag,in the thinnest air that they can operate in.
*Aircraft and automobiles serve completely different masters and their design paths diverged a long time ago.

[Xist]

If dimples do not work on anything larger than a golfball, why not attach golfballs to your car?!

[freebeard]

Thank you for your contribution.