Mesh kammback

[bogurt]

Anyone used mesh such as screen door mesh etc for aero parts? I know its not as slick as something solid, but im pretty sure the surface friction would be minimal in comparisson to the advantage gained by redirecting the airflow. Anyways I want to try mesh since its somewhat see through to and decently durable. Any negative input before I build a kammback for my new beetle using mesh stretched over a light weight frame?

[ChazInMT]

In my mind, the solid surface gives the air something to push on as it falls back behind the vehicle, this push, in addition to the lack of vortex generation derived by a properly shaped form is what gives us a lower drag coefficient. Not sure how well the mesh will allow the air to push. Not saying this will give you worse mileage, I'm pretty certain a mesh form will not give you anything close to optimal for a given shape. If in your mind you see 95% of the air "Skipping" across the mesh, I see 50-60% pushing straight through it. One of those pressure gradient things where the air wants to go into the low pressure area and does not care how it does it.

I've driven behind a new beetle in a rainstorm before and seen the huge counter rotating vortex's generated by the serious down wash created by the air moving over the top of the rear window and being slammed down. It is quite impressive. I'm pretty sure this is where a large amount of drag is created and why a large "Whale Tail" style spoiler is effective in reducing drag by disrupting the formation of these large vortices.

[Cd]

There is/was a guy that did this on his bellypan and reported that it seemed to work.
With all the discussion on crosswinds on motorcycle aero fairings, screen seems to be a good idea to try since it would allow the wind to flow through the fairing, yet retain most of the tapered wake.

As far as using it for a kammback, i would love to see it tested, but i doubt it would be effective unless the mesh was super tight, since unlike the sides of a motorcycle or the underside of a car ( relatively flat in shape ) there is a huge open area that has a massive amount of force from the vortexes back there pulling the air downward.

The screen would allow the air to be pulled downward, thereby making itself useless.
If you have some extra screen just laying around i think a lot of us here would like to see it tested - just for curiosity !

[justme1969]

It is a wonderfull idea in that the perforations will create more air cushion at any transition area. Now window screen may not be a good choice but screen with perhaps 20% - 35% perforated surface on Kamm back should do wonders in dispersing the high pressure stream and low pressure pockett that results from transition.
Most rear spoilers are raised for similar effect allowing low pressure pocket to be blended into high pressure over flowing stream.
Exception would be if compressing rear end for down force sticking surface into high pressure stream is key and even more turbulence is created.
This is why most OEM trunk spoilers are just for looks and hurt performance in both categories.

I ment that most spoilers are raised allowing low pressure air to be vaccumed into overflowing stream from under them reducing low pressure pockett signature.

[mikeyjd]

Try it with the expectation that it would be less than optimal. Just my 2 cents.

[bogurt]

Thanks for chiming in. I'll still give it a try. if it doesn't work, I'm not exactly out too much money. I got the idea from oem convertibles that use a mesh back screen to successfully redirect air for comfort reasons. Any recommendations for alternate materials? Again, thanks for all the input.

[aerohead]

Originally Posted by bogurt

Anyone used mesh such as screen door mesh etc for aero parts? I know its not as slick as something solid, but im pretty sure the surface friction would be minimal in comparisson to the advantage gained by redirecting the airflow. Anyways I want to try mesh since its somewhat see through to and decently durable. Any negative input before I build a kammback for my new beetle using mesh stretched over a light weight frame?

If the screen is over an air-tight region it can help,but if there's any place air can flow to underneath,it won't do any good at all.

[freebeard]

Any recommendations for alternate materials?

Buy a used trampoline at Goodwill, cut up the black nylon mesh and recycle the frame. To allay aerohead's concern, maybe have a forward facing air scoop to provide a positive pressure beneath it?

How would you deal with the hatch?

[Grant-53]

I use 1 inch hex wire mesh between two layers of tarp plastic (0.005" thick) bonded with contact cement.

[freebeard]

Bogurt -- Here's an example of mesh used in an active aero device. If you want to find the video they discuss how it works.