Accellerator Funnel

If the water can be forced into the funnel at 3 mph regardless of friction or back pressure from the reduction in opening size, the answer is 12 mph.

This will likely not be accurate in the real world.

Would that indicate that at 3 mph, the water would NOT be able to exit fast enough and there would be a pressure wave or 'bulge' spilling back out of the funnel?

That is correct.

Although you will have an acceleration at the discharge end, it will not likely be 12 mph (I'd give it a s.w.a.g. of half that).

You will have back pressure spill from the entrance to the funnel. The amount of the back spill is problematical.

But your post stipulates that it does go in at 3 mph. In that case it comes out at 12 mph.

The device is attached to the inside of a net. The net is towed at 3 mph .

That's makes it a different, and more difficult question, IMHO. The velocity at the inlet is likely to be lower than the 3 mph boat speed. It might be possible to estimate inlet velocity by considering the headloss in a taper (in a pipe system) but I haven't got the data to hand.

12mph (due to ratio of areas). NB water is an incompressible fluid.

Good point on the 'incompressible fluid' note...water doesn't work like air because of that trait so a 'velocity stack' effect won't be created without gravity or pump induced pressure.

I'd expect that the factor of four velocity increase will be a maximum value possible. In reality if you were pulling this funnel through water at 3 mph I would expect that a pressure wave at the opening of the funnel will make the water that enters the funnel will be less than 3 mph.

Agree on 12mph based on areas.

Beyond the requested solution, there are some interesting happenings. Since the velocity is quadrupled, the Kinetic Energy is sixteen times as much at the small end. That means that the Potential Energy, the pressure, is reduced by that amount.

A similar thing happens on the outside, the large end of the funnel leaves a reduced pressure on the outside and may extend to the the small end.

These are qualitative, I have no idea how to calculate the numbers.

If you tow this funnel at 3 mph, much of the water ahead of the funnel will spill off around the rim, and the rest may actually enter the funnel at about 1 mph, and exit 4x as fast.

Ok. Then that is what is happening right now. The device is slowing down the gear and producing a larger than normal bollard pull. Its acting like a sea anchor of sorts.

Next questions:

1. How much speed could I conservatively expect THIS arrangement to handle?
2. What would be the Exit Velocity at that conservative speed ?

Tricky question, I know. BUT YOU All are the geniuses....NOT me!

A sea anchor "of sorts"? That's exactly what a sea anchor is!

I would modify the shape to make it more hydrodynamic....you might get 1.5-2.0 mph as is...

I am limited to the placement of it inside of an already designed sampling trawl. I can only operate within 30">24" dia. by 30" deep constraints.

Thank you all for the help so far.

Lot of designs here....or is this you?

http://www.aquaticresearch.com/default.htm

Plankton nets. Mostly engineered mechanical parts with some Nitex or PEcap for filtering . My stuff is more broader ranged and not so standardized. These are some high end gadgets for plankton sampling though. Nice quality, at least what I have seen from time to time.

Water is not compressible! If it came out the back of the funnel faster than it went in, the funnel would pass you by!

To make it work you need to put a small 1/4" metal tube around the inside of the bigger end of the funnel with a lot of holes in it and pump air into the tube to create an air cushion around the inside of the funnel that would be compressible and flare the smaller end of the funnel out about 4" long and an inch bigger in diameter so the air would have a place to expand into.

If you are using a 3 point harness, each of the 3 tow lines could have flexible tubing run down to the funnel mouth and connected to the air induction tube at 3 points to get an even air cushion on the inside surface of the funnel.

Then this chart might partially apply:

http://en.wikipedia.org/wiki/File:Nozzle_de_Laval_diagram.svg

Wait! If you are just sampling fish and not water, put slits in the funnel sides equal in area to the difference between the areas of the outlet and inlet...gives you neutral drag but no loss of fish?shrimp? samples.

Well,

It depends what's coming in the front, too.

The funnel is "held in place" by hydraulic pressure on the large frontal area as compared to the outlet size. There's also hydraulic pressure on the outside of the funnel to consider.

We have used Accelerator funnels for decades now. They were made of netting. The water flow coming out of the AF was always faster then what came in. We put a diver in the water and tested it repeatedly. However, the funnel was made of 62mm stretched mesh x 48" dia. x 56" long with a 36" exit. NOT the same we are experimenting with now.

Now I am attempting to step up the velocity with a 'tarp" accelerator (ATF).

The device is sewn into a trawl net so it can not move on its own. I want as much velocity as I can, given that I am limited to a funnel under 30" in diameter and approximately 30"> 36" ( max) long. By reading everyone's post on the subject I see that the EXIT aperture is the key to making it happen or causing the IPW ( Internal Pressure Wave).

Thank you again for any help. Sorry to say but the air bubble thing is not going to work with a trawl set up like this. However the idea might have possibilities in another static net system (Passive Gear)

It is a sea anchor.

I spoke with the crew tonight on their way back from Illinois.

Earlier today , after reading your posts, I instructed them to cut off 5" around the Exit of the funnel. this opened the Exit to almost 19" dia. I then had them cut 6" slits in back of the 8 pleats I had sewn into the funnel to reduce the dia.

At 2 knots , they took 300-700 juvenile fishes in the 30 to 100mm size range. All in excellent shape.

At 3 knots the catch rate dropped to a dozen fish.... as you all predicted...too much water trying to enter the funnel mouth.

I think we solved this issue. they will be using the set up again in 3weeks on 100-300mm fish...faster swimmers...

Thank all of you for your help in this project.

Volume flow in equals volume flow out. Volume flow equals velocity times area. Output area is 1/4 input area so output velocity is 4 times input velocity, 12mph.

Why do you need the funnel shape?Are the fish being recovered into a smaller frontal area?

If not,then a straight opening would be better,possible with flexible pleating or tassels inside front edge to reduce damage to fish.Perhaps a layer of soft(seaweed-like) material on the inside of the opening?

Fish respond to pressure waves,but there is a limit to their response time.It may sound counter-intuitive,but perhaps 2 mph may be one of many "sweet spots" to catch the fish:not enough pressure wave to alarm them.

3 mph may be enough to alarm them,yet slow enough to allow them to get out of the way.

Perhaps there is another(faster) speed that would exceed their response time,yet not inflict excessive damage on the catch,and overall,may yield more quantity of viable fish.

Just thinkin'.........hmmm.....

The gear that I designed for this project works on the very surface of the water . Due to the way this net lays out and the doors used to spread it open, it has limited speed. On most species we have no issues. It collects samples anywhere from the Pacific Gyre to the Penobscot river.

Now we are concentrating on juvenile Asian carp. These little buggers feel IPW (Internal Pressure wave) very quickly and their response is instant. The ATF is designed to help draft them into the cod end where the 12mm mesh holds them.

This was the first ATF I have made this short (30").... the longer and wider ones never had an issue up to 3-4 knots. But now we looking at smaller gear and hence smaller ATF.

Without the ATF, the net collects 30-40 fish in a 7 minute run. WITH the ATF the net takes upwards of 100 , (30mm-100mm) plus per minute.

As of last night, the best speed is indeed 2 knots on a 8 meter trawl.

I just want to be able to expand the size and speed IF need be. Right now we can do 8 meter trawls with a short, more open ATF.

Thank you for the input.

Obviously, I am out of my depth here(pardon the pun) but it seems that the bow of the ship would also trigger a response from the target fish.Therefor, it seems to me,that outrigger-type nets,placed behind the bow,at say,a 45 degree angle from the centerline of the ship,and extending forward of the bow, would capture those that flee the pressure wave of the ship.The ones that sense the nets would flee in all directions, perhaps some toward the center.

As I said, this is not my bailiwick,but perhaps studying and imitating the greatest of all small fishers,the Baleen Whale could help you in designing a similar structure.The whale has no trouble filtering tons of fish thru his mouth, and from what little I know, they never feel him coming.

Also, from what I have seen, a small electric current will make them jump out of the water.Perhaps in combination with the out rigger or other net, they could be temporarily stunned to disable their flight instinct long enough to be netted.

Again,I'm just an old bull, ruminating.........