A question about Submarines

Hi,

I was wondering if anybody could offer me some clarification on a subject.

Surface vessels travelling in shallow water close to a critical speed ( related to the depth of the water), will experience a change in the wave pattern ( wave angle and wave resistance) as well as the phenomenon of squat ( faster flow underneath the hull, lower pressure less upwards force). One decreases resistance and the other increases?

It is my understanding that these are not the same.

Now assuming a fully submerged vessel such as a submarine, if it was travelling close to the bottom, it would also at a certain speed and depth, feel the effects of squat. How would this effect the drag of the submarine? Physically, what happens to the flow?

Do you know of any documentation I can find on the subject, either on the Net or references to journals and books. I am also looking for any past CFD analysis done on the effect as well.

Thank you.

 

The quick answer is yes, and because it is such an esoteric subject you will not find much, if any, papers on the subject in the open literature. I'd go look in Saunders first.

As for the differences between critical speed and squat, you need to fully grasp the hydromechanics that is taking place. First, stop thinking about the effects of the hydrodynamics and focus on the causes. Second, remember that there is no flow in the fluid as most CFD programs define it. In the absence of waves and current,the water is not moving, and the vessel causes all fluid movement due to it's presence. Many CFD analysis trip up on this point WRT boundary effects.

Critical speed limitations occur because the pressure disturbance caused by the vessel moving in the fluid has traveled to the bottom and has been slowed and reflected back in an adverse interference pattern. Critical speed limitations require the vessel to be at or very near the free surface.

Squat (or suction) occurs because chokeing between the hull and the bottom/side/surface causes increased flow velocity in a localized reigon which results in a low pressure over a small area of the hull pulling the vessel toward the boundary. This is a function of distance to the boundary (solid or free surface).

 

Squat is the change of the vessel's attitude with respect to its at rest position. It does not always lead to either an increase or a decrease in total resistance.

There are some wave patterns for ships in shallow water at:
www.cyberiad.net/wake.htm
See the section "Wake Images".

Squat in finite depth water is not an easy subject. Google around for recent publications by Tim Goulay and his PhD thesis. There also some interesting papers on zero wave drag catamarans travelling in finite width, finite depth canals by Sharma and Chen.

To estimate the forces and moments on a submarine in shallow water, but far from the free surface so it does not make waves, you can treat the air-water surface as a rigid lid, and use a method such as the one developed by Hess and Smith. In the first instance, you could also approximate the submarine by an ellipsoid. In fact, I think J. Hess and A.M.O. Smith looked at that problem in their paper. If the gap between the bottom and the submarine is very small, you might be able to treat the problem using lubrication theory.

There is also a bibliography at the www site I quoted earlier which give you a start.

All the best!
Leo.

 

All I know about submarines is that 50 men go down, 25 couples come up.

It's just a joke a Navy guy (submarine) told me 22 years ago.

 

Thanks for your replies.

To put things into context, im looking at the drag and lift of a towed submersible. This submersible is in the shape of a upside down wing to maximised downwards lift.

Experimental tow tank testing has been done as well as CFD analysis.

The CFD analysis of the lift forces is, within very small error, close to the lift forces found experimentally .

However the drag forces found using CFD are significantly smaller than those found in the tow tank. I am trying to find out why.

Some hypotheses :

problems with experimental:
- the model was close to the tank bottom ?? effects? less drag?
- small differences in trim causing a very different form drag


problems with the CFD
- the turbulence model (k-E high reynolds model) not appropriate
- appendages not simulated (at the time assumed to be negligeable, now rethinking that)

...So far that is where i am coming from , and what i am looking into.

if you have any brainstorm, i would welcome any extra information. I am still a student and becoming more familiar with marine design, so I fully admit that my in experience in the subject could cause me to not see some obvious solution. But I am researching away....

Cheers.

 

Appendages are never negligeable in a fully submerged body moving above 1 knot and may (especially with interference effects) account for 30-50% for the drag total.

 

Can't resist...
Why do you need to know?

 

What is the (planform) shape of this wing? What's the aspect ratio?

Does your CFD code estimate induced drag?

Leo.

 

Drag of Submarines

There are studies being done in this field as it is an important topic in the UDT (Underseas Defense Technologies).

However, the majority of submarines cannot go deep enough to get close at the ocean's bottom so this query is actually a hypothetic question. The modern submarine can get go down till 500-600 metres ( 1800' roughly) with some exeptions like the titanium skinned sub's out of the cold war. They were equipped with a 2 x 2" titanium alloy skin and therefore they were able to go at (unconfirmed) dephts of 1200 metres (3700'). The ex USSR design office 'Rubin' designed this boat that became infamous for the noise it produced.

The modern submarine has about 16% positive buoyancy and does not require similar formdynamics as for example an airplane or a fast car. There have been many studies carried out by many institutions, but the design of the sub's hull actually never have changed dramatically.

If you look at the present changes in military power, the immense problem how to get rid of the nuclear warheads that are still drifting along, the importance of the submarine is becoming less and less important, if you weigh that against the tremendous costs of maintainance etc.

In view of this, you have to check the seminars that have been held at the UDT's conferences, and if you have access to them, you might learn a lot.

 

I think you are very wrong in this, just read most major seapowers (including the Netherlands) white papers on future force projection. Most countries expect modern submarine warfare to occur in water less than 200m deep. Because of modern search sensors, it is unviable to project unseen power off a coast from the surface. The submarine with SLCMs and SPECOPs is the "gunboat diplomacy" of the 21st century.

 

It depends from which direction you are looking at this topic. Actually, I forgot to include in my reply that I had the Nuclear propulsed Sub in mind and that the popularity of the diesel-electric is gaining because the lower (much lower) costs of maintainance and the soundlevel is a fraction of that of a Nuclear Sub.

However, the rest of your reply is not correct. After 2010, all the Dutch Sub's are fodder for the cutters and only a few countries, like China and India are yet buying the newly developed Russian designed and built Diesel-Electrics.

One fired ISBN with a nuclear warhead by the US will create WWIII - sometimes I tend to believe that the US still live in the days of the cold war and that they do not know how to reverse. It seems also that the political union between the Eurpoean Union and the US is growing apart. At this precise moment, the only country with readily available nuclear power that may destroy a good part of the world is the U.S. The other countries hardly have the power or the aim to hurt the US badly.
America maintained its full strength of Nuclear Warheads whilst those of the ex-USSR are derelict, beyond repair and rotting and gradually deteriorating in their silo's.
In other words: the present danger is not coming from Russia, or China or whatever any other country. And that is well known.

As Jeff says, boatdesign.net is not the place for this kind of issue, so let's quit the "Sub" topic, this leads to this unwanted kind of exchanges of views.

 

D'Artois, what I just wanted to point out is that "littoral" is where it is at today. Bluewater thinking like what you wrote is at least 15 years out-of-date. AMGL is asking a real question that the general comunity is just beginning to approach.

BTW, SLCM's are not nukes and the only person likely to throw a nuke these days is some PALTPD.

 

I agree with Leo that your CFD code should be able to simulate the bottom effect as a reflection plane.

For experimental confirmation you might look at the literature for ships near the sidewalls of a waterway and two ships steaming side-by-side. Besides the force wanting to suck the craft toward the bottom, the Munk moment will be increased and affect the pitch stability. Yaw stability and turning radius will also be affected - turning radius on ships is increased significantly in shallow water. Take a look at Principles of Naval Architecture, Vol III. There are charts showing the magnitude of these effects.

Aircraft ground-effects data may also be relevant, since you are fully submerged.

As for the drag, the excressence drag can be very high. You might use a source like Hoerner's "Fluid Dynamic Drag" to estimate the parasite drag of all the pieces you left out of the CFD calculation. You have to do a very thorough job of bookkeeping all the possible contributions to drag.

You haven't mentioned the drag of the cable for this towed submersible. If you measure the angle between the cable and the submersible at the attachment point (possibly from photographs) you can get the lift/drag ratio of the submersible alone and separate its drag from that of the cable. If you tow the cable by itself, it will stream in a straight line and you can estimate the crossflow and tangential drag coefficients from the tension and angle of the cable. See http://www.marlab.ac.uk/FRS.Web/Uploads/Documents/No%2016.pdf.

Strumming (vortex-induced vibration) of the cable is also a source of drag. See http://ocw.mit.edu/NR/rdonlyres/Ocean-Engineering/13-49Maneuvering-and-Control-of-Surface-and-Underwater-VehiclesFall2000/2ABF5978-7154-4520-885B-117773724F36/0/chap13.pdf