resistance test in wave conditions

good day!

i wanna ask one question..if using maxsurf hullspeed, it can predict the resistance in calm water right.. but how about the resistance in wave conditions, which software can i use to predict it?

-ty-

 

The U.S. Navy's Ship Motion Program (SMP) is one that you might be able to find on the net.
The open source code PDStrip calculates ship motions, but I'm not sure if or how it estimates added resistance.

Good luck!

 

pdStrip (and consequently openDynamics) is a frequency-domain strip theory method. It will give you enough information to determine the response of a vessel to incident waves, from which you can infer the added resistance.

For yacht hull forms, it is worth looking up Keuning and Gerritsma's work on the systematic delft series. I think they looked at added resistance in waves.

Tim B.

 

Hi Tim,
Does that mean that PDstrip doesn't actually calculate the added resistance?
I'm not sure what you mean by "infer".

Some people might find it difficult to go from the response in waves to the added resistance.

 

thanks guys

 

Sometimes maybe If one of the proffered series of resistance curves matches your hullform correctly and you choose the right one. Even then there's usually a big error margin.

 

Sounds like you are skeptical about naval architecture magic.

 

Also an important thing to remember about "speed in waves" is that the actual wetted surface propulsion load only increases by 7-10% if all you consider is "waves" of small celerity. As the seaway increases though, the wave slap and slamming loads on the other hand may exceede the total propulsion (and possibly structural) loads, and bring her up "dead" on occasion. Wave celerity can exceede Va and cause overspeeding of the prop and loss of thrust. Additionally, depending on hull form and case, wind loading will become 20-30% of the total resistance. So speed in a seaway is dependent upon more than simple factors that can be approximated in tank tests/CFD.

 

Thanks for those many good points.
One factor that still puzzles me is the "effective" kinematic viscosity of churned-up water as might be found in a large sea. If it is much higher, then the effective Rn is much lower ,and the skin-friction will also be higher than in calm water.
That's something that might also be difficult to scale correctly in a tank.

 

Leo, I assume when you say "effective" kinematic viscosity you are talking about averaging the small scale eddies, etc along the lines of what's done for Reynold's Averaged numerical models. What is the maximum of eddies to be included when considering a boat or ship? Any articles or similar which discuss the concept in the context you are using it?

 

There are many references around that discuss "eddy viscosity" and its effect on skin-friction, wave generation, and wave decay. A Google search for "eddy viscosity skin friction" brings up thousands of hits.

My source of bewilderment stems from a commonly seen statement that:
"molecular viscosity is a property of water, whereas eddy viscosity is a property of the flow".

That seems quite clear and fair to me. I can appreciate that the eddy viscosity concept is sometimes useful (because of our ignorance of turbulence) and sometimes strained. Whether it is useful for determining an effective Reynolds number is not clear to me.

Eddy viscosity can vary by about six orders of magnitude in the ocean, so it could be very important for some calculations such as wave decay, and immaterial or inappropriate for others, such as skin-friction or ship-wave generation.

Maybe I should just stick to inviscid theoretical calculations and leave the hard stuff to others

 

My assumption is that the scales of turbulence to be lumped into "eddy viscosity" depends on the relevant length scales of the flow in general which for our purposes in turn depends on the size of the watercraft. Following this logic the eddy viscosity relevant to calculations of a 1 meter model boat might be very different than that of a 400 meter ship. So perhaps ambient eddy viscosity needs to be estimated in part based on the size of the watercraft.

This may be one of those concepts which is a great aid for modeling and calculations, but occasionally if pushed too far may obscure the physical reality.

 

I have tried using large values of the eddy viscosity in an attempt to smooth out the humps and hollows in wave resistance curves (as did Prof. Ada Gotman in her studies). Molecular values are far too small to have any effect, but values of 1,000 - 10,000 times the molecular viscosity do reasonably well.
I fully admit that it's a desperate act to throw lumps of viscosity at a problem and hope it works.

I agree, but you're working my side of the street!

 

Big whirls have little whirls,
Which feed on their velocity.
Little whrils have smaller whirls,
And so on to viscosity.

L. F. Richardson: English mathematician, physicist, and meteorologist who pioneered modern mathematical techniques of weather forecasting

Have you seen "Vortex Flow in Nature and Technology" (Hans Lugt, Wiley & Sons, 1983, ISBN 0-471-86925-2)? There is a fairly good discussion on turbulence and how it is formed and it effects. In the end though, I think each hydrodynamicist needs to come to his own understanding on the interplay of the energy transfer between viscosity and flow because different needs will cause the boundary to be a moving point.

Edit: X-post with the two above items.

 

I've certainly seen the Richardson quote many times.
It is a nice companion to Box's "All models are wrong. Some are useful".

I've read dozens of books and chapters on turbulence and its causes over the last 25 years, but I can't remember the Lugt one specifically.
Remind me - does he devote a section or two to negative eddy viscosity flows in Nature?

No argument there, except I'd put it more crudely: each has to find a level of ******** with which they feel comfortable.

Experimentalists (and the users of their results) have their own problems.
For example, most researchers tend to give greater credence to low turbulence wind tunnel tests for skin-friction, performance of wind turbines and propellers etc, and to calm undisturbed water in towing tanks for resistance and powering estimates of ship hulls. How relevant are they to real field conditions?

I've tried to find a way of introducing repeatable turbulence in a towing tank, but I suspect it's a hopeless cause.