# Pressure Underwater of Ship

Discussion in 'Hydrodynamics and Aerodynamics' started by waNsi, Apr 17, 2016.

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### Mr EfficiencySenior Member

He said 20 metres below the ship.

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### TANSLSenior Member

All this does not make any sense. What the OP wants to know is what happens 20 m below the hull. I do not know how sensors in the hull will tell him what happens 20 m below.
On the other hand, the values obtained from a test tank are not directly comparable with those obtained by direct measurement of the actual ship. How do you think you can set the "comparison"?. I know you're not going to answer but I try.
It is also very likely that the OP wants to have data before building his boat.

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### cmckessonNaval Architect

Are you trying to measure this pressure in calm water or in waves?

My thought is to make a CFD model of both the full scale problem and the small scale problem (including modeling the tank boundaries) and see if this will give you insights into the magnitudes of the scale effects. But my fear is that the CFD model itself may be pretty challenging, potentially needing a RANS solution. And then, if you are interested in waves and not calm water, that adds the need for an unsteady solution and I'm not sure that's even practical in CFD.

My gut feel - decidedly not science! - is that the pressure field should indeed be a potential flow result and that this result will correctly Froude scale from model tests. But the right solution is to sit down with the equations and study them, not on 'how to solve this', but instead only on 'does this equation scale linearly with size?' And it sounds like you are already doing that.

Best of luck with this interesting project! What is the research topic, if you're allowed to say?

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### NavalSArtichokeSenior Member

A ship of 150 m length is only of modest size nowadays. Large tankers can run a minimum of 250 m and container ships are being built which are 400 m long.

It's also not clear why the pressure at a depth of 20 m beneath the ship is of particular interest. That can only be obtained if the ship is sailing in a certain minimum depth of water. A more interesting problem is when the water is shallow and there is minimal under keel clearance when a large vessel passes. Things like squat and bank effect occur, making ship maneuvering quite tricky.

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### gonzoSenior Member

WaNsi: Is the measurement 20 meters below the waterline or below the keel?

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### waNsiJunior Member

There is another vessel at this depth, and it should define that is there any floating object on the surface or not (by using the pressure field as this PDF http://www.navlab.net/Publications/...ater_Vehicles_for_Improved_Seabed_Imaging.pdf). We do not want to use a sensor from the ship.

For comparison we want to scale the hull for example for a hull as length 200m and average speed of 20m/s we want to use a hull with 2m length with speed of 2m/s so that the Fr number would be the same.

We do not design a ship with this data, just define the length and speed of the ship at the surface

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### waNsiJunior Member

You are quite clever cmckesson and thank you for your post.

The situation in which we want to measure the pressure is in real sea or oceans with Bft between 3-8 so the wave has many patterns but at the first step Bft 6 is considered. So, we have waves with wave length 60-100m and wave height 3-6m.

As you mentioned CFD is much difficult than experiments. Your instinct is very good. At the first step we managed the linear potential equations (Laplace equation) for solving the pressure and before us scientists in NavLab company done that http://www.navlab.net/Publications/...ater_Vehicles_for_Improved_Seabed_Imaging.pdf which is not same as ur case but the purpose is some kind of similar. At the first step we should read data to define the environment, in the second step we should solve the equations and find the potential field, after the apparatus read any consistence data it should solve the equations again and the different between two pressure field is a new term in the equations which is UX (U is the ship speed and X length).

The topic is simple defining the any object on the surface. With this method we cannot define small boats or objects with the length smaller than 50m because their dynamic pressure field is small and would be damped in the depth of 5-15m.

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### waNsiJunior Member

I explained the purpose at the above post. Shallow water can be defined with the length and other parameters of the ship. Although the equations are slightly different compared to deep water but for the simplicity deep water is considered for the first step.

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### waNsiJunior Member

Our measurement is base on the keel.

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### Leo LazauskasSenior Member

My program "Michlet" allows you to calculate the sea-bed pressure
made by a monohull or multihull vessel travelling at depth-based
Froude numbers below 1.0.

The pressure signatures in the attached report were produced by
Michlet.

The inverse problem (identifying a ship from a given pressure
signature) has applications to designing pressure mines that can be
triggered when they detect a ship of a particular type. I leave
that as an exercise for interested students.

#### Attached Files:

• ###### nibberluna0.pdf
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451.9 KB
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### Mr EfficiencySenior Member

The OP has been less than forthcoming about what the purpose of this exercise is. Maybe there is a clue in Leo's mention of mines !

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### Leo LazauskasSenior Member

I very much doubt that there is anything sinister in it.
"Real" pressure mine R&D wouldn't be a student project, and the
researchers wouldn't be looking for information on public forums.

Michlet implements a very fast numerical technique developed by Ernie
Tuck around 1963 when he was at the David Taylor Model Basin in Maryland.
Tuck was impressed with how well he could predict the length, displacement,
prismatic coefficient, and a few other geometrical parameters from the
pressure distribution, and pinned a printout of one calculation on a
board in a common area, along with some notes explaining the method. A
vice-admiral who came down to the lab one day saw it and angrily
tore it down because it could be used against US ships in Vietnam.

In the 50+ years since then, methods for detecting ships have advanced a
lot and it is highly unlikely that anyone would still be using (autonomous)
mines as a defence measure.

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### Mr EfficiencySenior Member

You may well be right, but can you see any other utility in having the data he is looking for ?

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### Leo LazauskasSenior Member

It might well be a part of a system to automatically detect ships
passing through remote locations, but that doesn't necessarily mean that
the research will be used for pressure mines.

I've always thought that for every military application of research, there
are usually many more non-military applications.

For example, boats and ships in shallow water can damage sea-grassed
ecosystems, mussel beds, and other aquaculture activities.

Disturbance of mud and silt can cause eutrophication and other problems.
Mud layers near industrial sites can contain toxic chemicals; they can also
harbour viruses and other nasties whch you don't want to kick up.

Some reservoirs and lakes in Australia have large quantities of copper
in the mud because large amounts of copper sulphate were used for several
decades in an effort to control blue-green algae. (It's now banned in
most states.) Kicking up that mud could render water undrinkable.

There is a strange phenomenon called "over-turning" where the top few metres
of water can become colder than deeper water, especially during clear spring
and autumn nights. The heat absorbed by the top layers of water during the
day radiates out into space at night (according to the black body radiation
law of Stefan-Boltzmann). That cold top layer can be very unstable and the
pressures created by wind waves or ships, can cause it to suddenly flip over
and sink because it is heavier than the warmer water below.
The sudden over-turning can kick up mud and render the water undrinkable for
considerable time. Reservoir managers try to avoid that occuring, and you
will often see large air compressors near Australian reservoirs. They are
used to mix the top layers to prevent large density differences. A large
amount of energy is used around the world to mix (and oxygenate) water!

I can't remember where you are in Australia, but I'm sure you can imagine what
heavy metals and other toxins are in the mud in Sydney Harbour and some of
the nearby coastal regions after decades of uncontrolled industrial activity.
Knowing the pressures caused by ships is useful because it might help to impose
reasonable speed limits to prevent disturbing the crap. Or you could blow them
up before they get there, I guess.

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### Mr EfficiencySenior Member

The OP may like to clear up the "mystery" of what it is intended for, in his case.

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