Soft sided aero instead of hard forms?

[stillsearching]

Has anyone experimented with the use of aerodynamic treatments which are less than completely hard like a body panel? I mean could a metal pole frame with a tarp held tightly over it for a whale tail work as well as a physically hard one, giving you the option to have a collapseable one that you only deploy for highway driving?

Is there any possibility a soft sided vehicle (not whale tail, the whole vehicle, roof, trunk, hood...) would actually work BETTER than a hard one? I ask this having seen some documentary about I think it was sharks or dolphins, about how the turbulence from the water flows along their skin, which is less than perfectly hard along their sides... the softness allows eddies or something to 'stick' to the sides, the skin moves up and down a bit with the particular forces preventing turbulence or cavitation of any sort. They were trying to adapt the nature of that to submarines or somesuch in the docu I saw.

I'm aware air is way less dense, and maybe it's highly dependant on the right amount of give vs firmness (sort of like a shock absorber - too loose and there's no control, too tight and it's jarring) and perhaps there's no easy way to really make it work but if anyone had further ideas I figured this crew might.

[kach22i]

Originally Posted by stillsearching

........... the skin moves up and down a bit with the particular forces preventing turbulence or cavitation of any sort.........

The shark skin also has a micro texture similar to tiny scales which also makes a contribution as I understand it.

You are right in that there are many similarities between air and water as both fall under fluid dynamics. However air has boundary layer flow, where as water has parasitic drag.

Over at the boatdesign.net forum, people (experts) are always hesitant to draw too many similarities between air and water. I suppose there are a lot of reasons for this.

However to a layman like myself, golf ball dimples are like fish scales in some respect, so why cannot a flickering skin make a difference?

In short, I cannot answer your question, but think it's a good one.

I think that I've read before that a pulsating skin on air vehicles is a very bad thing as it disrupts the boundary layer of air too much and causes more drag. However I'd like to know a little more about the "why" of this if true.

[Frank Lee]

You'd want to get that skin taut; flapping is bad aero and saps lots of energy.

[freebeard]

I was at an Electrathon race this weekend, and one of the racers implemented this very idea. The fabric-covered nose was a loose fit and it had lateral waves on it from the air pressure. They run 20-60 mph and vary from Bonneville-style streamliners to bare cages.

[aerohead]

Inflated structures have already demonstrated benefits.A small blower can provide enough tension to the envelope such that it will not deform during 'normal' driving.
It's critical that the membrane maintain it's shape,and it may require some internal structure to restrict oscillatory motion as well.

[freebeard]

I like to point out to people that an inflatable structure is the only thing that will stand up to the abuse between your rims and the pavement.

/read Frei Otto back in the day

[stillsearching]

thanks to aerohead, the notion of inflatable something gave me an idea for tow vehicle/trailer coupling as well then

[Otto]

NASA (Bushnell?) did studies of compliant wall surfaces about 30+ years ago. Google for technical papers.

The answer is yes: Compliant wall surfaces such as feathers and fur dampen flow oscillations at the source, so have less drag.