To Perforate or Not To Perforate

[winkosmosis]

Originally Posted by Christ

What about the boundary layer, frank? Are you thinking that because it's not a solid, the boundary layer "weeps through", and doesn't keep flow separated from the actual surface?

How can there be a boundary layer over a hole in the first place?

[XJguy]

In order for this concept to work, the panels would have to be in perfect alignment with the airflow, not possible.

There are many ideas and concepts that would and could work in controlled environments within strict parameters, however in the real world they do not.

[Christ]

Originally Posted by XJguy

In order for this concept to work, the panels would have to be in perfect alignment with the airflow, not possible.

There are many ideas and concepts that would and could work in controlled environments within strict parameters, however in the real world they do not.

air molecules like anything else will normally follow the path of least resistance which is normally not through perforation in a panel. At speed they'll be acting opposite of what you think.

it would however be nice if we could define the size of perforations were talking about here

my apologies for the lack of punctuation my phone doesn't support it via talk to text

[winkosmosis]

Originally Posted by Christ

air molecules like anything else will normally follow the path of least resistance which is normally not through perforation in a panel. At speed dial acting ackley opposite of what you think

it would however be nice if we could define the size of perforations were talking about here

my apologies for the lack of punctuation my phone doesn't support it via talk to text

Air would either go into the holes and bounce back out, causing drag.... Or try to flow straight and the bernoulli effect would suck air out of the holes interrupting smooth flow.

[Christ]

that specifically depends on the difference in pressure between area where the flow is versus the area being covered by the perforated panels, as well as on air speed and the size of the perforations.

I however am far from an expert on the matter.

[Frank Lee]

I'd speculate that a perforated floor would have flow into the cabin in some areas AND out of the cabin in some areas AND flow "along" the floor... too much going on. At any rate I think that the flow would be disrupted by the perforations in whatever axis... vs say a perforated grille block which is pretty much just oncoming (perpendicular) flow, and when the oncoming velocity gets high enough the mesh starts to behave as a solid with much of the air simply going around it because that is easier than going through it...

[Christ]

Originally Posted by winkosmosis

How can there be a boundary layer over a hole in the first place?

my thinking here is the high speed flow over a perforated surface creating movement in n out of the perforations at any rate would create a highly turbulent area thick enough to be considered a boundary layer without actually being textbook definition of one.

to answer the original posters question the weight reduction probably would not offset the difference in aerodynamics between a flat smoooth panel and a perforated panel of the same parameters.

[meanderingthemaze]

Wow, quite the interesting discussion generated by this question. I probably should have given more specifics, but some of those specifics are not decided yet (ie: what's above the belly pan, what is the cabin going to look like, etc). Also, I don't think the discussion would have been as informative.

I didn't think it was quite as straight forward as it might have seemed. It does depend on several factors and now I can design better with those in mind.

FWIW, it's a general purpose passenger commuter type car. Battery electric vehicle with an attempt to make super light and the more aero the better.

My goal is to get 100-120 range on lead acid battery technology and keep hard costs under $10k. I'm even considering a PVC frame, but need to think hard about the structural integrity in collisions on that one. Or maybe a hybrid frame.

Anyway, this leads me to the next question which I'll post in another thread to keep the discussions relevant to the post heading...

How much surface wobble is ok?

[euromodder]

Originally Posted by Christ

What about the boundary layer, frank? Are you thinking that because it's not a solid, the boundary layer "weeps through", and doesn't keep flow separated from the actual surface?

Air will weep through, even when it's perpendicular to the airstream, even at very high air speeds.

Examples ?
The rear F-4 splitterplates.
They are nearly perpendicular to the airstream.

F-15 movable intake ramp.
Here the holed intake ramp faces the airstream at high speeds.

[Christ]

That can only happen if the flow had greater pressure than the ambient on the opposite side of the membrane. If the pressure of the flow is equal, turbulence should create a boundary layer. If the difference is negative (highest purdue on the non-flow side), there should be an extraction of purdue as the flow passes each hole, which, by itself, will keep the flow from actually making contact or being able to permeate the surface.
Perpendicular, obviously the flow will have more force against the membrane than if it were parallel, so a higher ambient pressure behind the membrane would be necessary to prevent flow through.