Turbulent Boundary Layer to Prevent Flow Separation

Hi all,

Back in 2012 my team won the Innovation award in the UKAYRoC competition (British TARC) for a simple and effective way to limit the altitude of a rocket, to zone in on the target 800ft. It consisted of wrapping black electrical tape around the upper section of the rocket so that fine adjustments could be made by adding or removing single layers of tape. The idea being this adds only a small amount of weight (so we can stay within the 650gram limit) but increases the drag by quite a lot. It was quite effective and we placed 5th overall with with a tongue-in-cheek naming of the system as the 'Esconian Ring'. At the time we weren't able to get a hold of any F22 motors which the design had been based around, so instead we had to use the more powerful F52 and this resulted in a very large ring.

In 2012 we used the same idea for fine adjustment, but being able to use the motor the rocket was designed for the diameter of this ring was much smaller and we also tried to taper it. While we were happy to use drag to alter the altitude, we were aware that it was one of our biggest variables between flights so wanted to keep it as low as reasonably achievable.



One of our concerns was that the drag ring could be messing up the air flow over our fins making them less effective.

Could the drag ring be causing flow separation over the rocket body, and could it be preventing clean air from reaching the fins?

We believe that this could be the case and so to combat this problem, last year we toyed with suction techniques listed here https://www.rocketryforum.com/showthread.php?57811-Ducted-Rocket-and-Pressure-Gradient

This year to overcome the same original problem we are intending to attach a wrap of sandpaper in front of the drag ring. The aim here is to force a transition in the boundary layer from laminar to turbulent over the drag ring as it is my understanding that a turbulent boundary layer is thicker and less prone to separation. The result being that more of each fin will be in cleaner air.

This is something we intend to experiment with over the course of the year, but what are peoples initial ideas? Does anyone think there is merit to this ides or are we creating turbulence and drag unnecessarily?

Cheers,
Esconian

Esconian said:

Could the drag ring be causing flow separation over the rocket body, and could it be preventing clean air from reaching the fins?

Click to expand...

In subsonic flow, a turbulent boundary layer is in a sense already separated flow. However, in the size scale of a model rocket this turbulent boundary layer is not going to be very thick (only on the order of a 1/4 inch) and most of the fin area will see clean freestream airflow outside the thickness of the boundary layer.

I can't see from your illustration what the gory details are of your trip-strip; if the outer edge was sharp (turned up edge of tape?) or rounded (due to a "wrinkle" rolled into the tape) or how much it might have been rounded. This is purely a SOTP observation, but I find it difficult to believe that your drag tuning technique would accomplish much more altitude change than maybe 20 or 40 feet for typical low-power model rocket altitudes. Is that about the same order of magnitude that your calculations showed?

We were using ALOT of tape

Our first flight in 2012 without the drag ring (at the time my first taste of F class) took us to 1020-something feet, and just by adding wraps and wraps of tape we took that down to 800ft. We were effectively making a BT-80 body tube 24mm wider in diameter but very abruptly and only for about 15mm in length.

It was a similar story for 2013, but we managed to get away with only making it 14mm wider in diameter, and stepped it down more gradually behind. The below pictures show the two rings in question.



Because of the very sharp square like transition from 66mm to 90mm it seemed hard to believe that this couldn't be causing disruption to air further down the rocket (although none of us have any real aeronautics knowledge beyond the basics).

The plan for this year was to try and limit this supposed detrimental effect to the air flow by forcing a turbulent boundary layer just before the ring. If such a layer is approximately a 1/4inch thick then this sounds ideal as the ring sticks out about this much (5/16th ish) all the way round. The purpose of forcing this turbulent layer is not to tune the altitude, we do this by altering the thickness of the ring, forcing the turbulent layer is simply to improve airflow over the fins so that they can be as effective as possible. We would in fact counter any change in drag (and therefore altitude) arising from this forced transition by altering the diameter of the ring. We are aiming for the same exact altitude every flight.

I guess really the question is which is the least bad scenario for the fins? Flights at the mercy of the drag ring, or flights where a turbulent layer is forced?

Seems like a pretty simple solution: put your ring below the fins.

That's how a saucer flies and you will also be increasing something called base drag. Not only is it another kind of drag to add, but it Increases stability.

The strip isn't going to separate the flow from the entire rocket - that's pretty impossible to do, given how rockets are normally shaped. What it is going to do is create a wake zone where the boundary layer is well and truly separated, a short distance aft of the ring, much like a ball does. The separation zone will eventually collapse and the boundary layer will reattach.

The strip will have two effects:
1: tremendous drag, as you've seen. Wake zones are extremely draggy in and of themselves, much less for what they do to airflow over the rest of the rocket.
2: If the boundary layer isn't turbulent aft of where you put the ring, it is once the ring is there.

As for the boundary layer thickness, I agree with Powderburner - it's not going to be extremely thick. A couple feet downstream of that ring you'll have a bit thicker boundary than you would have had if it wasn't there, but the majority of your fin area will still be in clean air.

...of course, all this has me wondering what would happen if you crammed a BIG motor in there and busted Mach. You'd get an interesting little double-shock (one at each joint between BT and the ring), which could cause all kinds of aerodynamic fun. Or rip the ring off (and take the rocket with it).

I had sort of imagined a couple wraps of tape, not a couple rolls of tape.

You're right, you should have achieved a significant altitude change with that approach.