Drag reduction with rough surfaces?? Anyone studied this?

[prfesser]

A post that boatgeek made indicated that surface roughness from glitter paint made little difference in altitude. A study of shark skin showed reduction in drag (though it was in turbulent flow). Has anyone in the rocketry community found a paint or other surface treatment that, although rough, actually appeared to decrease drag?

Best,
Terry

 

[boatgeek]

A post that boatgeek made indicated that surface roughness from glitter paint made little difference in altitude. A study of shark skin showed reduction in drag (though it was in turbulent flow). Has anyone in the rocketry community found a paint or other surface treatment that, although rough, actually appeared to decrease drag?

Best,
Terry

I think there's probably a few issues working together/against each other, which might make it hard to sort out the differences. And while sharks have been tuning their skin friction mods for a few hundred million years, we've only got a century or so of progress on the rocket end.

Larger grain sizes on the surface *should* always increase friction drag, unless you're monkeying with laminar-turbulent transitions and flow attachment the way that sharks (and golf balls) do. For example, this study says that textured shark skins only work while the shark is flexing. If the skin is held rigid, the flow resistance is 35%+ higher. Since we're not (usually ) wanting our rockets to flex in flight, that shouldn't be a factor. Also, we're pretty far into turbulent flow (Reynolds # ~ 1E5 to 1E6), so the transition from laminar to turbulent flow is far in the rearview mirror for most of the flight.

My guess is that the added turbulence does something to reduce base drag, which cancels out the increase in frictional resistance. I could be completely wrong, though. It would probably take a lot of study over nearly identical rockets with different surface textures. Someone would have to fly a lot...

 

[tsmith1315]

It would probably take a lot of study over nearly identical rockets with different surface textures. Someone would have to fly a lot.

And have consistent motors.

 

[Funkworks]

A simple but tedious test would be to build 100 identical rockets: 50 with smooth paint, and 50 with rough paint, and launch them all at the same time, to have identical wind conditions. With 100 rockets or so, all other parameters affecting altitude would be randomly distributed, but an altitude difference due to paint would stick out.

 

[BigMacDaddy]

How about smooth paint on nose and rough paint on tail? Finless rocket???

 

[OverTheTop]

A simple but tedious test would be to build 100 identical rockets: 50 with smooth paint, and 50 with rough paint, and launch them all at the same time, to have identical wind conditions.

I am not going to do this personally. Minimum 300 fins and 600 fillets is a real turnoff.

 

[G_T]

During transient conditions, flow will stay attached beyond the normal stall point. This is likely the case for the wiggling shark. A wing that might achieve a steady-state CL of, say, 1.2 right before stall, could readily achieve well over 2, though only very briefly. Note, this is when pitch rate is significant when compared to motion through the medium...

On a rocket, the nosecone shoulder is likely to trip the flow to turbulent within a couple inches past that shoulder. The various vent holes, shear pins, any other joints, are also going to have their effects. Once the flow is turbulent, the rough surface won't matter as much, within reason.

If you want to play, you can run some sets of numbers in RasAero to see how it predicts altitude change vs surface roughness. I have not run any of my own numbers to attempt to validate the results. My airfoil etc work has been at the low Reynolds numbers range.

Gerald

 

[dhbarr]

Rough boat tails would probably be both interesting and achievable for a little R&D. That and fluked fins I think stand a decent chance of being measurable.

 

[G_T]

Since the desired angle of attack for rocket fins is zero, what's being called fluked fins would accomplish nothing positive.

The nonlinear leading edge on the fins encourages vortex formation when the lift coefficient is not zero. It takes energy to produce these vortices, and that increasing drag vs laminar flow. However, the vortices allow flow to stay attached that would otherwise separate. So under high lift generation, the drag is acceptible, as otherwise the surface of that size and velocity would be stalled.

But if rocket fins are operating in a high lift state, something else has gone seriously wrong.

 

[Funkworks]

I am not going to do this personally. Minimum 300 fins and 600 fillets is a real turnoff.

Me neither! But it's the cheapest way I can think of. Compared to a wind tunnel or months of simulations on dedicated software. Like I dunno, less than $20,000? A lot for one hobbyist, but not that much for a paint company.

It does make me wonder how much thought aerospace paint companies have put in their paints' drag characteristics. I bet simulating drying paint is much more interesting than watching paint dry.

 

[G_T]

It's not much of an issue for aerospace companies. Drag is way down on the list. Off the top of my head:

1) Bonding strength
2) Environmental protection
...a) Wide thermal range
...b) Abrasive resistance
...c) Chemical resistance
...d) UV and IR resistance
...e) Impact resistance
...f) - Most likely, insulating properties (assists in avoiding galvanic corrosion)
3) Minimal outgassing
4) Ablative rather than bubbling/peeling
5) Can do the above in a thin coat

In the small rockets we fly, drag dominates for energy suckage. In the large rockets professional companies and NASA tend to fly, weight dominates.

 

[OzHybrid]

Doesn't a golf ball with dimples travel further? I'll just chip that in....

 

[OverTheTop]

Doesn't a golf ball with dimples travel further? I'll just chip that in....

I think it can travel further because you put a backspin on it. The backspin and dimples cause a slight pressure increase under the front of the ball increasing distance. If the ball were hit flat I think the distance would be shorter than with a plain spherical ball.

 

[aerostadt]

Doesn't a golf ball with dimples travel further? I'll just chip that in....

Where have I heard that before?

 

[rharshberger]

I think it can travel further because you put a backspin on it. The backspin and dimples cause a slight pressure increase under the front of the ball increasing distance. If the ball were hit flat I think the distance would be shorter than with a plain spherical ball.

Maybe, but this article says a smooth golf ball would not fly as far spin or no spin.

https://www.scientificamerican.com/article/how-do-dimples-in-golf-ba/

 

[aerostadt]

Prof. Shapiro says that dimpled golf balls have profound improvement in range. See "Shape and Flow The Fluid Dynamics of Flow", Doubleday & Co., New York, 1961, page 170 (paperback).

 

[OzHybrid]

As we have access to 3D printers these days this should be simlpe..ish to test. Same rocket, 2 sets of fins. Diples on one, shark skin texture on the other, smooth on the last, 3 sets of fins.......... ( Think MontyPython Spanish inquisition.....)
But I'm not volunteering...

 

[Zeta]

A post that boatgeek made indicated that surface roughness from glitter paint made little difference in altitude. A study of shark skin showed reduction in drag (though it was in turbulent flow). Has anyone in the rocketry community found a paint or other surface treatment that, although rough, actually appeared to decrease drag?

Best,
Terry

3M Company did work on this under that same principle. They produced a film laminate that was used on boats and planes. I know other companies have jumped in it too : https://newatlas.com/aircraft/aeroshark-aircraft-skin/

 

[G_T]

Ok, lets go back to basics for a moment. As an overall ranking order, with suitable handwaiving to keep it simple, avoiding any technical rigour, talking ONLY about drag, not considering any form of boundary layer control, and going from leading edge of object towards the trailing edge:

Attached laminar flow has the lowest drag.

Attached laminar flow transitioning to attached turbulent flow is next lowest. The longer the laminar flow region, the lower the drag.

Attached laminar flow transitioning to detached laminar flow transitioning to turbulent flow transitioning to attached turbulent flow is next. Usually... Sort of...

Full turbulent flow might fit in here, if it stays attached...

Attached laminar flow transitioning to detached laminar flow transitioning to turbulent flow transitioning to attached turbulent flow transitioning to detached turbulent flow is next.... Usually... Depends on how detached.

Detached laminar flow is last.

Some of those in the middle of the list might not exist in practice depending on the Reynolds numbers range. The list is not exhaustive. And, I'm completely ignoring things like transonic, supersonic, and hypersonic flow. Each adds another level of complexity.

Golf ball without dimples -> detached laminar flow.
Golf ball with dimples keeps the flow attached farther around the ball.

And a sphere is a pretty draggy shape.

 

[OzHybrid]

Ok, lets go back to basics for a moment. As an overall ranking order, with suitable handwaiving to keep it simple, avoiding any technical rigour, talking ONLY about drag, not considering any form of boundary layer control, and going from leading edge of object towards the trailing edge:

Attached laminar flow has the lowest drag.

Attached laminar flow transitioning to attached turbulent flow is next lowest. The longer the laminar flow region, the lower the drag.

Attached laminar flow transitioning to detached laminar flow transitioning to turbulent flow transitioning to attached turbulent flow is next. Usually... Sort of...

Full turbulent flow might fit in here, if it stays attached...

Attached laminar flow transitioning to detached laminar flow transitioning to turbulent flow transitioning to attached turbulent flow transitioning to detached turbulent flow is next.... Usually... Depends on how detached.

Detached laminar flow is last.

Some of those in the middle of the list might not exist in practice depending on the Reynolds numbers range. The list is not exhaustive. And, I'm completely ignoring things like transonic, supersonic, and hypersonic flow. Each adds another level of complexity.

Golf ball without dimples -> detached laminar flow.
Golf ball with dimples keeps the flow attached farther around the ball.

And a sphere is a pretty draggy shape.

Dammit Spock, Why do you have to be so logical......

 

[Greg Furtman]

3M Company did work on this under that same principle. They produced a film laminate that was used on boats and planes. I know other companies have jumped in it too : https://newatlas.com/aircraft/aeroshark-aircraft-skin/

 

[lakeroadster]

A 1% decrease in fuel usage with the AEROshark skin. With all the varying conditions during flight, this claim makes one wonder just how they did the testing?

Did they fly two planes, side by side, under identical conditions to verify identical fuel usage? Then applied the film to one plane and then conducted the test again?

Are individual jet engines similar enough such that their own fuel mileage, from one engine to another, are within 1%?

 

[Zeta]

A post that boatgeek made indicated that surface roughness from glitter paint made little difference in altitude. A study of shark skin showed reduction in drag (though it was in turbulent flow). Has anyone in the rocketry community found a paint or other surface treatment that, although rough, actually appeared to decrease drag?

Best,
Terry

perhaps velocity matters..... boats are slow, rockets are fast

 

[cwbullet]

One of the teams at Spaceport America Cup did a golfball dimpled nosecone.

 

[BEC]

A 1% decrease in fuel usage with the AEROshark skin. With all the varying conditions during flight, this claim makes one wonder just how they did the testing?

Did they fly two planes, side by side, under identical conditions to verify identical fuel usage? Then applied the film to one plane and then conducted the test again?

Are individual jet engines similar enough such that their own fuel mileage, from one engine to another, are within 1%?

Turbines that power airliners probably are that consistent….at least if they’ve similar time on them.

But every time we (I‘m retired from Boeing Commercial) looked at this the thing was the benefit only lasts if you can keep this sharkskin treatment clean and dry. So it helps for awhile and then the real world kind of compromises it. Or at least that’s what I remember — aerodynamic tweaks were not my area of expertise but were studied by others in my group.

That Lufthansa Technik video suggests that they are going to implement it anyway… it will be interesting to see if they really get the drag reductions long term. It will also be interesting to see how well those films stay stuck on the airplanes….

 

[KenECoyote]

Check out the Estes Rock-It. The nose cone has a chipped rock look and it would be easy to test it for altitude, swap nose cones for a smooth one and compare ad nauseum.
Here's a pic of mine with matching rough texture paint.

 

[KenECoyote]

BTW Dimples do decrease drag... Mythbusters did a test with a dimpled car.

 

[OzHybrid]

So my takeaway from this would be:-
For a lumpy object or lumpy part of my rocket, dimpling would probably reduce drag in the subsonic speed range.
For a fully streamlined rocket that would operate substantially with laminar flow, there is no advantage
An advantage from dimpling would only be useful if I were aiming for some competitive advantage. Implementing the dimpling could result in a weight penalty which would negate the advantage.
A golf ball is a lumpy object and has a benefit, but the golfing body rules prevent any competitive advantage as all golf balls used competitively are regulated for performance.
And that takes us back to the start of the hampster dance. And Dosado now Promenade......

 

[OzHybrid]

BTW Dimples do decrease drag... Mythbusters did a test with a dimpled car.

29mpg for dimpled 24 for no dimples.

 

[shockie]

Wrap a body tube with sandpaper outer wraps. Use different grits. Compare altimeter data from ricket launch with no sandpaper wraps versus various grit wraps.