Boundary Layer Airflow

So what is the current word on riblets? I have not read anything about them for several years. Why are they no longer available?

Is that golf ball like?

I think what you are referring to are essentially boat tails that are too short for laminar flow to stay attached. When the flow separates, you have more turbulence and effectively more base drag. A turbulent boundary layer is less likely to separate than a laminar boundary layer. In some cases intentionally tripping the boundary layer to turbulent is enough to maintain attached flow and reduce drag. In some more severe cases, you can add vortex generators to help energize the boundary layer and keep it attached. This is usually just a quick fix, and you would have been much better off to use a longer boat tail instead.

Alan

Yes, sucking down the boundary layer can help maintain a laminar boundary layer. but sucking requires power.

I don't think you would get enough suction that way. You would be better off using a boat tail to eliminate or minimize the base area. OTOH, if you are adding ballast to get up to optimum mass, you could replace the ballast with a small battery driven vacuum.

What I would suggest is exploiting natural laminar flow. Shape the body to have favorable pressure gradient over the fore body and make the laminar region as long as possible. Follow that with a ramp or trip strip to transition the flow to turbulent. Finish off with a short low drag boat tail. This approach would work best for some of the FAI events, where the rules force you to use fat bodies, and where you can have motors custom made to provide optimal thrust. The big gain will likely come from optimal thrust, but this also means that you will spend more time at low Reynolds number constant speed, that provides more opportunity for natural laminar flow design.

There is not a great deal of literature on natural laminar flow design of bodies of revolution, and most of what is available is under hydrodynamics. However, you can also glean useful design ideas from

Alan

A passive system that uses the base drag as the "power source" would be self-functioning.

Pardon the tech post.

Oh, BTW, waxing the rocket reduces drag far more than these methods.

Pardon the tech post.

Jerry

Alan,

Thanks to Geoff Elder's challenge I entered the I-Lite Bowling Ball contest at LDRS this year (I placed next to last). His excellent site is

The riblets I mentioned was a film manufactured by 3M years ago to reduce airplane drag and helped win the America's Cup Challenge. The film worked so well that it was outlawed and no longer available.

Although I ran hundreds of Rocksim simulations, there is nothing like testing the real thing. I love the smell of APCP in the morning :-)

As a fellow competitor Drake "Doc" Damerau says: Science rules!!

Regards, Rich Kroboth TRA#4148 METRA Northern New Jersey

Thanbs. I was aware of the 3M film riblets, but not the outlawing. I'll check out your links when I have time.

Sometimes the effect of the thing you want to investigate is just too amall to measure in a reasonable number of tests, and you have to resort to analysis and simulation. I love the mell of APCP in the afternoon, evening... ;)

If only that were true!

Alan

Pardon denied. If science ruled, you'd be having a necktie party. ;)

Alan

Thank god for zealot trolls, eh?

:)

Ablation, relaminarizing and maximizing frontal presentation along as much of the length as possible. That's what dolphins do. They exhude gel-like "hairs" that relaminarizxe the flow. These flake off constantly, replacing themselves every 2 hours, the capturing of the flake increasing the mass of the turbulence and slowing it. And they get bigger and bigger towards the rear and transition to smaller comparatively quickly.

A spike with loose fur should be optimal.

Of course there's no such thing as truly relaminarizing, but there is such a thing as controlling chaos. You just have to figure out how to minimize the increase in complexity, and can actually induce low dimensional chaos of greater amplitude than some higher dimensional chaos (like broken up boundry flow)and overwhelm the higher, preventing it from bifurcating (getting worse) for a longer time than it naturally would.

I don't know of any rocket stuff on this. I do know some guys from Santa Fe Institute and Los Alamos that study this stuff with respect to explosions.

The Brown effect used in electrostatic "lifters" may laminarize airflow via ionic wind. The bad news is you need high voltage. The good news is you can use a gas jet to generate said voltage; a rocket has one. Again, no rocket stuff I know of, but I have some stuff on the B-2 as well as Brown's patents that looks promising. The other bad news is that a breakdown of the resistance against the high voltage could, in a rocket, result in a discharge shooting through the bird. Bad news indeed.

With a low enough Rn and a smooth enough gap, that gap might not trip it, but yes, the bottom line is it's going to trip anyway--you don't have to rough up the paint to accomplish it.

Scott Orr

Oh my! Did I say that out loud?

Alan "Ah stands all ah kin stands, till ah can't stands no more!"