Stepped shape for flow attachment.

[Frank Lee]

The Chrysler PNGV car had a step for the backlight and it still had very low Cd.

[Big Dave]

I've been thinking about this to smooth out the air flow over my pickup tonneau yet retaining visibility out the rear window.

[JasonG]

I seem to recall the navy was experimenting with stepped hull design for high speed craft.
They got the idea from some shark or snake, memory not working.......scales/sawtooth shape.
It worked but was impractical to build and keep clean.
Seems it had a side effect of pulling air in from the sides due to the low pressure in the channels. That reduced drag which was the design intent.

[Frank Lee]

Originally Posted by Big Dave

I've been thinking about this to smooth out the air flow over my pickup tonneau yet retaining visibility out the rear window.

Exactly! If/when I ever dig out the aerocap I started building years ago to finish, I planned on doing the rear window like the Chrysler.

[euromodder]

Originally Posted by JasonG

I seem to recall the navy was experimenting with stepped hull design for high speed craft.

Stepped hull designs are nothing new.
It's been used on seaplanes and speedboats for about 100 years.

They got the idea from some shark or snake, memory not working.......scales/sawtooth shape.

Probably reinventing the theory and scaling the steps down while adding many more of them ?
It's been done with overlapping plates, on boats and aircraft alike.


If you do the same with a stepped rear deck, you end up approaching the aerodynamic template

[beer]

As promised I tested the theory today, mainly becouse I had hopes that the vortexes would reattach the flow even at a greater angle.

I have acces to a flowbank, mainly used for engine tuning (flowing channels).
So today I made a setup through which I could visiualize the flow attachmend with different angles and with smooth and stepped surfaces.

I made a basic druplet shape buth with squared top and sides.
For the modelling of the exact shape I used a kind of clay.

@cfg83 CFD simulations were pretty acurate, but his models were shown at a relative low angel of attack. (which I could make out from the vids)


Brief Summary:
Test with a smooth "druplet kind of shape" shows deattached flow at angles above +/-17degrees.

Test with relative big steps.
Showed attachemend also till 17 degrees, you could clearly see that the flow was turbulant right after the step and reattached half way a step.
See the picture below to give you an idea how the flow unfolded. Red=turbulant
Yellow=laminair


Test with smaller steps.
The flow was often turbulant but reattached with the shallow angle of about 11 degrees.
See picture


Conclusion
Stepped shapes have the same flow-attachment angle as a smooth surface, but the flow must have a chance to reattach to a surface.

[beer]

Originally Posted by California98Civic

That flow indicator is kinda cool. BTW, the last set of steps seem the best.

My (real world) test showed that the 2 second vid shown by @cfg83
is the best for flow attachemend with stepped slopes

[aerohead]

Originally Posted by beer

My (real world) test showed that the 2 second vid shown by @cfg83
is the best for flow attachemend with stepped slopes

beer,you'll want to keep an eye out for flutter.The steps will have a natural frequency and harmonics of that frequency in which they'll vibrate like a tuning fork.If not sufficiently stiff enough for dampening they can shake themselves apart in the vortex.

[cfg83]

aerohead -

Originally Posted by aerohead

beer,you'll want to keep an eye out for flutter.The steps will have a natural frequency and harmonics of that frequency in which they'll vibrate like a tuning fork.If not sufficiently stiff enough for dampening they can shake themselves apart in the vortex.

So at air speed X it could shake apart. Does that also mean that at air speed Y it could perform at it's best? Is there an optimum step shape for every MPH?

CarloSW2

[ChazInMT]

I still think this is one of the coolest pictures ever taken. I marvel at the complexity of the design. It is a sharks skin close up using a scanning electron microscope. I think these look like little alien spacecraft all lined up.