Fluids , Mech and Physics please look at this.
Discussion
I know that there are lots of people with really good ideas on here, but if you don't mind 
off if you don't have a proper engineering answer this time I'd really appreciate it, in the nicest possible way ofcourse.
A horizontal fin( the blade is on a horizontal axis( I.E . it flaps left to right) working in a confined space of 170 mm radius pipe with the water filled 70 mm( and cannot be changed) deep and is not working. The fin is 40 cm deep at the bottom (draft ?) ( the top is at the surface) and is 120 mm long. We haven't been able to measure the speed of the fin due to equipment and brief constraints. The fin is a piece of plastic 40 mm by 120 mm. Is there a shape ( or "flappyness" of plastic) that works in these confines or a speed of motor that will produce a force over 2-3 newtons ?
Thank you very, very much.
A technically inept product design student.
Alex
Bump it please for beer tokens.
off if you don't have a proper engineering answer this time I'd really appreciate it, in the nicest possible way ofcourse. A horizontal fin( the blade is on a horizontal axis( I.E . it flaps left to right) working in a confined space of 170 mm radius pipe with the water filled 70 mm( and cannot be changed) deep and is not working. The fin is 40 cm deep at the bottom (draft ?) ( the top is at the surface) and is 120 mm long. We haven't been able to measure the speed of the fin due to equipment and brief constraints. The fin is a piece of plastic 40 mm by 120 mm. Is there a shape ( or "flappyness" of plastic) that works in these confines or a speed of motor that will produce a force over 2-3 newtons ?
Thank you very, very much.
A technically inept product design student.
Alex
Bump it please for beer tokens.
http://putfile.com/pic.php?pic=12/34222495535.jpg&am
The water isn't moving. The fin needs to produce a force of several newtons. More is not a problem infact it's perfect. Less than 1 won't move it so is no use. Thanks. We don't know what speed the fin is moving is at but that is adjustable so what ever you want I suppose. Well it can move from 90 and less and however many times per second changing from -90 to 90 is possible aslong as it isn't something silly( i.e 0.00something or 10000000something.)
BTW I know it's short but if it's possible the answer needs to be available about 14.00 on monday.
Edited to add : What we think is happening is that there is a rebounding wave off the sides that is canceling out the "movement" from the blade because it works fine in a bath. This is sort of verified by people running a whale or dolphin flappy up and down tail that works fine but were not sure why this runs because it surely has the same constraints of the surface and the bottom of the tube ?
>> Edited by thebluemonkey on Saturday 10th December 05:22
The water isn't moving. The fin needs to produce a force of several newtons. More is not a problem infact it's perfect. Less than 1 won't move it so is no use. Thanks. We don't know what speed the fin is moving is at but that is adjustable so what ever you want I suppose. Well it can move from 90 and less and however many times per second changing from -90 to 90 is possible aslong as it isn't something silly( i.e 0.00something or 10000000something.)
BTW I know it's short but if it's possible the answer needs to be available about 14.00 on monday.
Edited to add : What we think is happening is that there is a rebounding wave off the sides that is canceling out the "movement" from the blade because it works fine in a bath. This is sort of verified by people running a whale or dolphin flappy up and down tail that works fine but were not sure why this runs because it surely has the same constraints of the surface and the bottom of the tube ?
>> Edited by thebluemonkey on Saturday 10th December 05:22
Looking at the picture i assume the fin is pivoting from the point closest to the end of the pipe? and you want the 'fin' to move back and forth to produce a flow of water down the rest of the pipe?
Assuming this i would suggest maybe an amount of flex in the 'fin' would be beneficial to give a smooth flow, if you were using a rigid fin you are likely to see a lot of disturbance on the end of the fin loosing a lot of potential flow. Picture the way the dolphins tail moves as it pushes it through the water, it is not rigidly straight, thats the sort of flex you are looking to achieve.
Assuming this i would suggest maybe an amount of flex in the 'fin' would be beneficial to give a smooth flow, if you were using a rigid fin you are likely to see a lot of disturbance on the end of the fin loosing a lot of potential flow. Picture the way the dolphins tail moves as it pushes it through the water, it is not rigidly straight, thats the sort of flex you are looking to achieve.
It's pivoting at the point furthest away and the fin has already been tested with quite a few "flapynesses" ratings ranging from very very flappy to totally stiff,it doesn't seem to make a difference ( that's what is really confusing us, it has pretty much the same acceleration in a bath despite the "flappyness" well within 5%.but stick it in the trough and it does all.) The fin is on a vehicle that is moving through the tube. I'm sure my explanations are making it harder for you , sorry !
all.) The fin is on a vehicle that is moving through the tube. I'm sure my explanations are making it harder for you , sorry ! 
Does it not need to move quickly and sharply initially from maximum deflection at the edge, and then reach the opposite edge slowing down, then snap quickly back to centre, and then slow down towards the opposing edge again.
Ie, a non-linear movement. I'm assuming now that it is a sinusoidal output (cicular cam driving the motion), which just won't generate any more force in one direction than the other in a confined space.
I'm not 100% sure, but I have done degree level fluid dynamics and have many friends doing CFD and finite analysis stuff (bit beyond me), and I believe the problem is the motion. The fin is moving water backwards for 50% of the time, and 50% forwards as it passes the perpendicular to the desired direction of travel.
So if the moves to straight from the side, it generates thrust backwards, but then if it moves round to the front, it is pushing water forwards down between the edge of the channel and the fin, negating the initial thrust where it is pushing water backwards.
Just use some eccentric motion for it and imho it will work fine. For now all your doing is swishing a panel left and right over the same arc generating equal forces both front and back in a confined space. Aim for more force back while the fin tip is moving backwards, and less while it is moving forwards.
A dolphin has the advantage of a stiff or floppy fin, so it can succumb to the water at the ends of it's stroke to reduce trubulence and then become stiff on the power stroke, which probably allows it more luxury with a sinusoidal output fin tip deflection over time.
You don't have that luxury so imagine how a diver uses their flippers. Hard down for the thrust, but then more gently up.
Not sure if it's technically right but the water will develop more reactive force to an object trying to move more quickly through it, so use that to your advantage and move quickly when the force will work in your favour and slowly and gently when it won't.
If your still stuck I'll ask my friend who works in aeronautical (ground based F1 car) engineering.
Dave
Ie, a non-linear movement. I'm assuming now that it is a sinusoidal output (cicular cam driving the motion), which just won't generate any more force in one direction than the other in a confined space.
I'm not 100% sure, but I have done degree level fluid dynamics and have many friends doing CFD and finite analysis stuff (bit beyond me), and I believe the problem is the motion. The fin is moving water backwards for 50% of the time, and 50% forwards as it passes the perpendicular to the desired direction of travel.
So if the moves to straight from the side, it generates thrust backwards, but then if it moves round to the front, it is pushing water forwards down between the edge of the channel and the fin, negating the initial thrust where it is pushing water backwards.
Just use some eccentric motion for it and imho it will work fine. For now all your doing is swishing a panel left and right over the same arc generating equal forces both front and back in a confined space. Aim for more force back while the fin tip is moving backwards, and less while it is moving forwards.
A dolphin has the advantage of a stiff or floppy fin, so it can succumb to the water at the ends of it's stroke to reduce trubulence and then become stiff on the power stroke, which probably allows it more luxury with a sinusoidal output fin tip deflection over time.
You don't have that luxury so imagine how a diver uses their flippers. Hard down for the thrust, but then more gently up.
Not sure if it's technically right but the water will develop more reactive force to an object trying to move more quickly through it, so use that to your advantage and move quickly when the force will work in your favour and slowly and gently when it won't.
If your still stuck I'll ask my friend who works in aeronautical (ground based F1 car) engineering.
Dave
I reckon your system is too simplified, and you're trying to implement it in a space that is too confined.
If you look at a fish, when it swims it's whole body moves. Whilst the fin is the part that pushes the water aside and develops thrust, there is an enevitable sideways reaction that the fish counteracts with motions in it's body. Some of the fastest fish even have dorsal fins, that I'm sure, help them to move more forwards than sideways.
I think your fin is too big in relation to the size of the trough (or the water in it) to see the forward force that you are seeking. Presumably your power system is inherently absorbing the unwanted sideways thrust. I still think you need deeper wider water, and a stationary vane that sits in front of the mobile one. This will help to ensure that the turbulence generated by the mobile fin can only reach equilibrium by moving behind the system.
At the moment, I'm guessing that there is as much water movement in front of your "fish" as behind it. I don't think that's good for locomotion.
>> Edited by dilbert on Saturday 10th December 11:33
If you look at a fish, when it swims it's whole body moves. Whilst the fin is the part that pushes the water aside and develops thrust, there is an enevitable sideways reaction that the fish counteracts with motions in it's body. Some of the fastest fish even have dorsal fins, that I'm sure, help them to move more forwards than sideways.
I think your fin is too big in relation to the size of the trough (or the water in it) to see the forward force that you are seeking. Presumably your power system is inherently absorbing the unwanted sideways thrust. I still think you need deeper wider water, and a stationary vane that sits in front of the mobile one. This will help to ensure that the turbulence generated by the mobile fin can only reach equilibrium by moving behind the system.
At the moment, I'm guessing that there is as much water movement in front of your "fish" as behind it. I don't think that's good for locomotion.
>> Edited by dilbert on Saturday 10th December 11:33
I've tried to visualize the system you're describing. I'm thinking of something rather like a diver's fin, mounted on edge and flapping sideways, in a horizontal tube which is part filled.
Having the fin close to the surface limits the pressure difference you can achieve across it. If you try to generate too much pressure difference it will entrain air and stall. The shallower the fin is the more of a limitation this will be, so if you have any opportunity to make it deeper then take it.
I think the trick to get fins like that to work is to get the correct amount of flex in the fin. It is this flex which makes the 'outward' (away from center) and 'return' (towards center) strokes assymetrical, and you need them to be assymetrical in order to generate any net thrust. (The earlier suggestion to change the speed throughout the stroke might be another way to achieve the same thing.)
With a simple mechanical fin part of it will be creating thrust forwards, and part will be creating thrust backwards. The further forwards the 'pivoting axis' that you are flapping the fin around, the more net thrust you will get. If you can move the base of the fan sideways without twisting (equivalent to a pivot point infinitely far forwards) you will get more thrust. If you can add an 'ankle' that lets the fin pivot through a small angle with no resistance it might work even better, although you will sacrifice some travel. You need some experimentation to find the best compromise.
Gut feeling is that you want enough flex to have the trailing edge of the fin at about 45 degrees to the centerline in order to achieve slow forward motion.
If you want to go faster you'd increase the frequency and use a smaller deflection, but with the fin being surface piercing in an enclosed space I think you'd very quickly stall the fin (I suspect this is why it isn't working at the moment). So slow it down until it stops splashing, adjust the flexibility until you get about 45 deg worth of deflection in the fin, space the fin as far away from the pivot as you can and try again.
Having the fin close to the surface limits the pressure difference you can achieve across it. If you try to generate too much pressure difference it will entrain air and stall. The shallower the fin is the more of a limitation this will be, so if you have any opportunity to make it deeper then take it.
I think the trick to get fins like that to work is to get the correct amount of flex in the fin. It is this flex which makes the 'outward' (away from center) and 'return' (towards center) strokes assymetrical, and you need them to be assymetrical in order to generate any net thrust. (The earlier suggestion to change the speed throughout the stroke might be another way to achieve the same thing.)
With a simple mechanical fin part of it will be creating thrust forwards, and part will be creating thrust backwards. The further forwards the 'pivoting axis' that you are flapping the fin around, the more net thrust you will get. If you can move the base of the fan sideways without twisting (equivalent to a pivot point infinitely far forwards) you will get more thrust. If you can add an 'ankle' that lets the fin pivot through a small angle with no resistance it might work even better, although you will sacrifice some travel. You need some experimentation to find the best compromise.
Gut feeling is that you want enough flex to have the trailing edge of the fin at about 45 degrees to the centerline in order to achieve slow forward motion.
If you want to go faster you'd increase the frequency and use a smaller deflection, but with the fin being surface piercing in an enclosed space I think you'd very quickly stall the fin (I suspect this is why it isn't working at the moment). So slow it down until it stops splashing, adjust the flexibility until you get about 45 deg worth of deflection in the fin, space the fin as far away from the pivot as you can and try again.
I haven't read everyone elses answer so apologies if this has been said.
I think you problem is this: A flapping fin cause water flow backward (thrust) and water flow laterally. In open water the lateral flow is not a problem it just causes the vehicle to wobble back and forth as it goes along. However, in a pipe the lateral flow is stagnating against the side walls, this is causing a pressure increase next to the flipper. This pressure increase is working on the angled flipper surface in such a way that it is causing a backwards thrust, canceling your forward effort.
The only chance this has to work is with a much smaller flipper. A small flipper going fast should easily be able to give you 3-4N.
Have you thought about using a tail rather than a flipper?
I think you problem is this: A flapping fin cause water flow backward (thrust) and water flow laterally. In open water the lateral flow is not a problem it just causes the vehicle to wobble back and forth as it goes along. However, in a pipe the lateral flow is stagnating against the side walls, this is causing a pressure increase next to the flipper. This pressure increase is working on the angled flipper surface in such a way that it is causing a backwards thrust, canceling your forward effort.
The only chance this has to work is with a much smaller flipper. A small flipper going fast should easily be able to give you 3-4N.
Have you thought about using a tail rather than a flipper?
Try two fins with either the same axis or axis very close.
Set them to move forwards closed (together - minimum resistance) and backwards open (like open pages in a book - maximum resistance). To do this you will need stiff fins and a stop to prevent them from opening too much on the power stroke.
Set them to move forwards closed (together - minimum resistance) and backwards open (like open pages in a book - maximum resistance). To do this you will need stiff fins and a stop to prevent them from opening too much on the power stroke.
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