# Hydrodynamic form

Discussion in 'Hydrodynamics and Aerodynamics' started by FredrikA, Mar 24, 2014.

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### FredrikAJunior Member

Hi,

I´m a designstudent workin on my degree doing a future vision of how we can collect algea in the Baltic Sea.

I´m working on my concept now and i need some help or guidance regarding good forms to minimize waves from the machine.

Think of the machine as a lawnmower or a vacuum cleaner. It takes one section each run and then turns and takes the next one. The front of the machine will suck algea and water into the machine then filter befor letting the water out again. Therefore i dont want the algeas on the edges of the machine to move in the water so its still there for the next section of harvesting.

Algea can disolve or drift away when the waves from the boat hits them. I want to make as low wave impact as possible and direct the flow straight backwards. is complicated but hopefully this illustrations will help.

What hull design have least waves when moving in 4 knots?

How can the end of the "wings" keep the flow straight back instead of pushing water towards the machine or away from it?

Kind Regards
/Fredrik

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### Eric SponbergSenior Member

Hi FrederikA--Welcome to the forum. What an interesting concept! I think that you are correct in looking at a submersible, and I think you are correct to be wary of unwanted eddies off the tips of the collectors. Here are a couple of thoughts:

First, what is the purpose of the two dihedral wings below the main body of the craft. If they are control surfaces (wings with ailerons) they may not need to be as big as you show them. The thrusters at the back look to be on a wing of sorts, and these could have an aileron to control pitch. Then instead of two dihedral wings below the main body, put one small inverted rudder at the back of he craft between the thrusters. That eliminates one wing and reduces the size of the remaining one, plus puts it in the center of the body back aft. I hope I am interpreting your drawing correctly.

Second, the collectors--you may have to do some testing in a pool or towing tank to come up with a suitable design or configuration to change the tip shape. And for this, an idea comes from jet airliner design. You may be aware that some jet liners like the Boeing 737 and the Airbuses have wingtips at the ends of their wings. These are to control the lift coming off the wing and to reduce the size of he vortex that swirls off the wingtips. Your collectors will not necessarily be like wings creating lift, but there may be a tip vortex nonetheless, as you suggest. Therefore, a wing tip blade, perhaps like the rudder that I mentioned above, would be good for here. It would not be acting like a rudder, however, more like a spoiler.

Say the water flow off the tip wants to diverge outward from the track--then the spoiler would be set with a slight angle inward (toe-in) to turn the flow back in line with the track. Wings always have downwash which goes in the opposite direction to lift, and that is why a toe-in of a wing tip would help flow divert back parallel to the track.

On the other hand, if the flow off the tip of the collector is converging toward the track centerline, then the tip spoiler would be set to an angle of attack outward (toe-out) so that the downwash diverts the flow away from track centerline back to parallel with the track.

We don't know which way flow is going to divert on your current design, and that is why you may have to do some pool tests. Trial and error will determine which way the flow naturally wants to go without wingtips, and differenct designs and settings of wingtips will determine which is the most effective.

I have a question--why is the nose of the main body shaped the way it is? Is this to direct flow and volume filled with algae into the collectors?

The above spoiler idea is just one of many that may be offered by others, but this is the most obvious one that comes to mind.

Interesting project! Good luck!

Eric

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### daiquiriEngineering and Design

I think we actually can make a fairly accurate guess about the flow direction by looking at the shape of the scoop. The way it is designed at this stage, it is imo obvious that the water cannot go outwards. It will hence tend to go inwards, towards the centreline of the vessel.
There are two reasons which make me believe so:
1. The highly non-streamlined cross-section shape of the scoop will create a large area of separated flow along the whole rear part of it.
2. The inwards-bent duct at the tips of the scoop was designed to turn the flow towards the inlet at the main body. The mass of water ingested by the scoop and sent towards the central body will be a missing water mass in the longitudinal direction behind the scoop. This missing mass will have to be replaced by the surrounding water, which will hence tend to rush towards the centerline of the vessel immediately behind the scoop.
A solution to this is, imo, hidden right there in the problem. There is a missing mass of water behind the scoop which has to be replaced by the surrounding water? And we don't like it? Ok, then let's avoid creating a missing mass in first place. You can achieve it by expelling the ingested and filtered water right where we have taken it from the ambient - along the rear part of the scoop.

Therefore, if you suck a mass of water M in front of the scoop and along its whole length, you have to expel the same mass M behind the scoop and along its whole length, like in the attached pic. The whole trick now consists of designing a correct ductwork to obtain this result.

Cheers

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• ###### Scoop.gif
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### Eric SponbergSenior Member

Good points. Are the scoops nets, letting the water pass through, but not the algae? I agree that you'll have at least some void behind the scoops, or collectors as I called them, so yes, that would tend to suck water from above and below the collectors. I still think you may have some eddy effects off the tips. Again, scale pool tests will be bear all this out.

Eric

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### FredrikAJunior Member

Hi guys and thank you for your answers! I really appriciate the great help!

Right now i´m doing scale testes in a aquarium...i dont have the access to a pool but i think it works ok. The project is still a vision for 2030 but i want the shape to be as good as possible. I´ll try different mock-ups with your ideas to see if i can get a better result. I use tiny pieces of styrofoam to symbolize the algae and as i tested my design on the attached images i saw a great amount of "algae" sucked in behind the "wings" so clearly there's a problem with my design.

The machine wings works as an oil boom that first concentrates the algae mass....then with nozzle on the upper and lower edge of the wings sucks them in to the machines body, separates the algae from water and uses the outflow of water as thrust to drive forward like a jetski. The first drawing had two propellers in the rear but i´ve choosen this other idea now so sorry if i mislead you. It would be awesome to let the water out from the wings after the separation as you showed but then i cant use it as thrust?

The lines in blue on the attached image in this post shows how the water behaves with the current design. the green lines shows how the water sucks into the machine and The hole lenght of the wings function as inductions.

Sorry for my bad english! Hope you´ll understand.

I´m open to any suggestions of form. if you have an idea it´ll be awesome if you could make an outline drawing of it.

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### Leo LazauskasSenior Member

It's a lovely design project aimed at a very important problem in
many regions around the world. As you know, blue-green algae are
remarkably difficult to eliminate: they managed to survive all 5
major extinction events!

Sorry for the long lesson, but I think it might give you something
else to think about...

Several years ago, my colleague and I came up with a system we
thought would be useful to get rid of blue-green algae in fresh-
water reservoirs. It was basically a very large diameter (about
8 metre) vertical tube of about 20 m length. Our idea was to very
slowly skim the surface water and pump it downwards, thereby
setting up a slow circulation in the dam. The idea was to keep the
algae below the "photic layer", i.e. below the level where there
is a enough light for the algae. If you can keep them out of light
for more than about 24 hours or so, they die.

So far, so good. Then I realized that what we had actually invented
was an excellent system for killing off the least buoyant algae. They
would be unable to rise quickly enough, but the survivors would be
more buoyant and rise to the surface more quickly. These survivors
would then pass on those characteristics to the next generation.

Remember: you are setting yourself up as a predator, and your prey
will evolve to avoid you.

Your invention should work well at picking up the topmost layer of
algae, but you might find that there will still be large numbers
below the reach of your device and the bottom of the photic layer.

Of course, you probably don't have time to go into all that in
your project, but it might make a nice paragraph to show you have
at least thought about it.

One minor correction to your first post: waves will "disperse", not
"dissolve" algae.

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### SukiSoloSenior Member

Is there a reason for the long booms other than to get width? I assume the algae wanted is at one height in the water?

My gut feel would be a shape more like a wide mouthed whale or shark type shape. Like a squashed version of Thunderbird 2! Of course the 'mouth' area could be concave in plan not convex.

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### DCockeySenior Member

The upstream cross-sectional area of the streamtube which flows into the system is determined by the rate of flow into the system. This is simple continuity/conservation of mass.

Upstream cross-sectional area of stream tube * Forward velocity of the system = Volumetric flow rate into the system​

Rearranging:

Upstream cross-sectional area of stream tube = Volumetric flow rate into the system / Forward velocity of the system​

The shape of the upstream cross-sectional area and consequently the width at the surface will be affected by the shape and size of the inlet.

daiquiri's point that water need to flow back out on the backside of the scoop is excellent.

In general some sort of "pump" will be needed to add energy/total pressure/"head" to the water moving through the system if upstream cross-sectional area is to equal or be greater than the exit area. This is due to losses as the water flows through the system.

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### DCockeySenior Member

A semi-submerged body or even a submerged body (unless deeply submerged) will still generate waves, and the magnitude of the wake will depend in part on the Froude number of the body, Fn
Fn = Velocity / Square root of (gravitational constant * length of the body)

For Fn much less than 0.3 the wakes can be negligible. As Fn approaches 0.3 the magnitude of the waves will increase significantly.

At 4 knots Froude number vs Length (m) is:
L Fn
1 0.66
2 0.46
3 0.38
4 0.33
5 0.29
6 0.27
7 0.25
8 0.23
9 0.22
10 0.21
11 0.20
12 0.19
13 0.18
14 0.18
15 0.17

A length of 10 m or so will be needed to minimize wave formation, and a longer length will be better.

A slender body will generate considerably smaller waves than a fat body. A center body Length to Beam ratio of 5 or more would be good and a ratio of 10 or more would be even better.

Also, minimizing the weight of the body and therefore the displaced volume (Archimedes) will also reduce wave formation.

So a long, narrow, light body will generate the smallest waves.

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### DCockeySenior Member

As daiquiri explained above it is inevitable that water and "algae" will be sucked in behind the "wings" unless there is water flowing out the back of the wings.

Water flowing out of the wings will generate thrust if it's exit speed is greater than the forward speed. The exit speed will depend on the size of the exit. Consider a long but narrow exit.

Nothing to apologize about your English. It's better than that of many people for whom English is their only language.

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### tspeerSenior Member

You can control the flow at the ends by putting a flap on the trailing edge of the end panel. The main structural part of the end panel can be fixed. If you angle the flap outboard, that will generate circulation around the end that will pull more water in from the sides. If the flap is angled inboard, that will force water to go around the end, with less being captured.

Sensors on the leading edge of the end panel can provide feedback to the flap actuators so as to maintain the stagnation point at the leading edge of the end panel.

The same thing goes for the horizontal wings. If the trailing edges are deflected so as to open up the exit area, that will suck more water through the device. And vice versa if the exit area is closed down.

As for waves, I'd be more concerned with the waves in which the craft has to operate than waves generated by its motion through the water. You can divide the waves into general classes - those with wave lengths smaller than the size of the craft (chop) and waves larger than the size of the craft (swell). It has to accommodate the chop and ride the swell. The difficult waves to handle will be those that are on the order of the same size as the craft. Those are the ones that will tend to make one end emerge from the water, etc. And may be the ones that induce the largest structural loads.

Another thing you might want to revisit is having the strainer at right angles to the flow. Such a strainer would become clogged very quickly. It would be better to use something like a fine net that streams back in a V-shape to a collector at the apex. The algae can then be removed from the collector continuously as the algae is channeled by the water flow along the surface of the net to the collector. Similarly, the collectors at the apex can be angled back to a single collection point on the centerline. The collectors could be something like a solid half-round trough. You'd want a fairly acute angle with the flow to encourage the algae to flow along it to the apex.

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### tspeerSenior Member

At 4 kt, the propulsive efficiency of a high speed jet is very low. Propellers are a better way to go. And large-ish propellers, too, to be efficient at pushing the draggy device at low speed.

When you asked about waves earlier, I think people literally thought in terms of surface waves. However, what you were really talking about the mass flow through the device. As you've sketched it, no flow goes through it except for what is pumped out through the collectors at top and bottom. Most of the water will flow around it because the volume of water you're pumping is much smaller than the volume of water approaching between the upper and lower sides. In order for the water to come in, you need to accept all the water approaching it. Otherwise, you'll just get high pressure building up inside that will force the flow to go around it.

Here's a plankton net, which already does the kind of thing you want to do:

The plankton is captured in the jar at the apex. There are a couple of things to notice about this. The first is because it is netting, the water that comes in through the hoop can escape. This satisfies the mass flow requirement. If it used a solid cone, no water would flow through it and it would collect very little.

The second thing to notice is the very shallow angle the net makes to the flow. This keeps the plankton moving down the net to get concentrated at the apex. I think you should be aiming for a two-dimensional version of this.

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### daiquiriEngineering and Design

IMO Tom, and with all due respect, the flap-system you are proposing is a technical overkill for this kind of boat. Too many submerged moving parts, sensors and automation for what is essentially a workboat. Simplicity is a virtue here.
The point about angled strainer with angled vanes is indeed a good one, as well as the point about insufficient inflow - which can be regulated with a variable-rpm pump.
Cheers

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### tspeerSenior Member

True. A simpler version is to use a piece of sheet metal at the trailing edge. It can be bent to the angle that results in the desired capture flow.

But the real problem is the solid blockage at the back of the wings. I had thought the backs were open in the original sketches. The concept as he's drawn it won't result in any concentration of the algae in the wings. All the water would have to be sucked into the collectors and pumped out the nozzle. That's way too much flow through the piping of the system.

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### FredrikAJunior Member

Hi again! Thank you for all your answers! I´m super happy to get this much great feedback on the project and all of you have really great thoughts on how i can do this.

I really like the idea of water coming out from the wings since youre also saying that a jetengine wont give that much of thrust at 4knots. I might have to give up the idea of using the water mass as thrust. As i discussed this with marine biologist they told me that we want the outlet of water to be as far away as possible from the algae. so i dont know how the water affects the algae if it comes from the back of the wings. But maybe it could work. All of you give great answers and i´m taking everything into count.

Maybe i´ll explain the concept little further if it helps.
Algae have a negative effect in the baltic sea with to much nutrient in the water and by haversting some of the algea that are full of nutrients, the amount of nutrients in the ocean should be reduced. The idea is that the machine sucks water and algae into the vessel/machine. in the body of the machine theres a seperator which dewaters the algae and stores them in a tank. A service boat will empty this container/tank when the machine is full and later process them to make e.g biofuel and plastics.

Maybe i could also test the different designs and hydrodynamics in a 3d software if there is one for free?

Kind regards! and thank you all!!!

/Fredrik

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