Ponderings on under body aero

[jakobnev]

A video in the "I was wrong!" thread reminded me of something I was thinking a couple of years ago but never made a post about.

The idea is that if the flow is turbulent, the air traveling with the car will be mixed with the air nearest to the road, and that this will lead to momentum transfer from the car to the road.

This could be minimized by delaying the onset of turbulence, but without testing it's hard to estimate the magnitude of the effect.



Thoughts?

[aerohead]

Originally Posted by jakobnev

A video in the "I was wrong!" thread reminded me of something I was thinking a couple of years ago but never made a post about.

The idea is that if the flow is turbulent, the air traveling with the car will be mixed with the air nearest to the road, and that this will lead to momentum transfer from the car to the road.

This could be minimized by delaying the onset of turbulence, but without testing it's hard to estimate the magnitude of the effect.



Thoughts?

* It must be considered on a case-specific basis.
* A JEEP Wrangler will be different of a Citroen DS 19.
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*If a vehicle has an airdam, side skirts, and rear valance of equal ground clearance, no lower than the lowest underbody component, it will essentially carry a pool of dead air with it, as it travels down the road. ( ignoring cooling and wheel openings).
* There would exist some shear-related drag between the dead air and the active flow beneath it.
* A full belly pan insures that the air in this region IS dead, sequestered inside the pan.
* If the entry to the pan is well conceived, the flow will flow down it, with only surface friction drag and no appreciable turbulence-related losses.
* The belly pan does add 'skin' which was formerly turbulence, and a turbulent boundary layer will form on it's bottom, which does, to some degree 'choke' the flow as it grows larger and larger the further downstream.
*This accelerated flow can be beneficial with respect to lower underside pressure and its implication to high-speed stability.
* If you throw in a proper diffuser, total drag and lift can be reduced even more.
The convention since the mid-1970s, is to limit the amount of air flowing under the vehicle, optimizing for drag. For typical driving velocities,the lift kinda sorts itself out automatically.

[freebeard]

Thoughts?

I can't discern the difference between 'unrealistic ideal' and 'doable?'. Is it suggesting that the former is not achievable?

I suspect there is untapped potential in the underbody. Consider the M-hull boat. It has a needle nose and barge boards that capture the bow wave, provide lift and reduce the wake dramatically.

Lower the stagnation point, widen the barge boards into wheel pontoons* and add a diverging diffuser; and it turns into an untried and unproven ground vehicle concept. Yer welcome.

edit:
For clarity, the wheel pontoon would have a flat outer face, and an inner simple curve into the throat. The needle nose would have a compound curve to induce the vortexes the M-hull employs. The belled exit could be simple or compound curved.

2nd edit:

* The belly pan does add 'skin' which was formerly turbulence, and a turbulent boundary layer will form on it's bottom, which does, to some degree 'choke' the flow as it grows larger and larger the further downstream.

This is why Cal-look VWs have a rake. The underbody diverges from the front torsion bar.

It's possible that a perforated pan could turn the underbody (or parts of it) into a semi-permeable membrane that would make the underbody a resonant cavity. There's a CFD thesis in there.

[j-c-c]

Originally Posted by freebeard

It's possible that a perforated pan could turn the underbody (or parts of it) into a semi-permeable membrane that would make the underbody a resonant cavity. There's a CFD thesis in there.

To what potential effect?

[freebeard]

Good question. No idea what I was thinking. Maybe quieter?

Back in the real world you have louvered panels for to ventilated the engine compartment and &.

[Piotrsko]

I thought the Cal look was to stop the annoying tendency of a bug trying to lift the front off the ground above 80 mph. 75 in my super started getting twitchy on a calm day, 55 was scary in a headwind.

[freebeard]

I thought the Cal look was to stop the annoying tendency of a bug trying to lift the front off the ground above 80 mph. 75 in my super started getting twitchy on a calm day, 55 was scary in a headwind.

I just revisited the thread and saw your comment adjacent to my Profile picture.


The Beetle body would lend itself to modern racecar practice

The Aston Martin Valkyrie for instance gets a lot of downforce from the underbody. I guess the OP was about skin drag coupling in the underbody plenum.

[aerohead]

On the worst-case-scenario vehicle (circa 1987, deep ladder-type chassis, body on frame pickup/ VAN/ SUV ), the addition of a full belly pan, with 2.8-degree upswept diffuser would reduce drag by Cd 0.07.
The 'cleaner' the vehicle to begin with, the less improvement. Diminishing returns.
With today's Tesla Model-Y, there'd be very little left one could do to lower drag, except to add wheel fairings, like on the Volkswagen XL1.