Tearing Velocity?

[Leo Lazauskas]

Hello Strangers!
A few years ago I produced some wave patterns for the 2 vol. book:
"Hydrodynamics of High-Performance Marine Vessels", by Lawrence J. Doctors.

One of the figures showed the "Transverse Perturbation Velocity" of a Wigley
Catamaran. As you can see the pattern is anti-symmetric, with positive
velocities to starboard and negative velocities to port.

In some other work, Dr. Stuart Anderson called this velocity component the
"tearing velocity", which I thought was an excellent descriptive term. He said
he came across it in a plasma physics paper.

Has anyone ever seen the term in ship hydrodynamics?

I'm putting together some youtube videos where I show some "cartoons". See:

I needed to add links to my social media pages but I didn't have any. However,
they were happy to accept my BDN account, so here I am again!

Leo.

 

[montero]

My advice: don't disturb the calm waters.
Slender WP hulls and foils are what I want to do.

 

[jehardiman]

Leo, do you have a photo of this effect IRL? There should be lots of tank data if this is true....or go after it with a Grad slave...uh...student project. Otherwise, could it be a CFD calculation artifact? Have you reversed the element order?

 

[Ad Hoc]

Hello Strangers!...

G'day mate....hope all is well?

 

[Ad Hoc]

...One of the figures showed the "Transverse Perturbation Velocity" of a Wigley
Catamaran. As you can see the pattern is anti-symmetric, with positive
velocities to starboard and negative velocities to port....

Did this 'wave pattern' analysis also produce the very long period soliton wave as well?

What Fn and depth was this conducted at?...and does the LD ratio influence what you 'found'...? ...is it over a range of at one specific condition?

 

[Leo Lazauskas]

Leo, do you have a photo of this effect IRL? There should be lots of tank data if this is true....or go after it with a Grad slave...uh...student project. Otherwise, could it be a CFD calculation artifact? Have you reversed the element order?

The three perturbation velocities (u,v,w) are defined in terms of the gradient of the velocity potential.
So the image I showed is as "real" as the y-derivative of the velocity potential.

Only the u-component is required for wave resistance and far field wave elavations.
However, the other two components v and w are also required because of the Laplace equation.
(Sorry, but I don't know how to post partial derivatives here.)

Here are the 3 velocity components and the wave elevation which is just the u-velocity
scaled by -U/g, where U is the ship speed and g is gravitational acceleration.
Leo.

 

[Leo Lazauskas]

Did this 'wave pattern' analysis also produce the very long period soliton wave as well?

What Fn and depth was this conducted at?...and does the LD ratio influence what you 'found'...? ...is it over a range of at one specific condition?

Hi AdHoc-san!
All is well here.

No, solitons are not part of this work (yet). Dr Tim Gourlay is the goto guy for that.
See his papers on the bore produced between catamaran hulls and also the paper
about solitons in tank tests at:

https://www.researchgate.net/profile/Tim-Gourlay

My calculations were for Fn = 0.5 in (infinitely) deep water for one separation distance.
I think Lawry Doctors wanted to show the perturbation velocity components because
(as far as I know) nobody has ever shown them in such detail. They are calculated using
Flotilla 15.(?) and SWPE 5,

The v and w components might have some applications to ship identification. I'm still
trying to see how the v-component affects the wave wake when there are surfactants
present on the free-surface,

Leo.

 

[jehardiman]

The three perturbation velocities (u,v,w) are defined in terms of the gradient of the velocity potential.
So the image I showed is as "real" as the y-derivative of the velocity potential.

Only the u-component is required for wave resistance and far field wave elavations.
However, the other two components v and w are also required because of the Laplace equation.
(Sorry, but I don't know how to post partial derivatives here.)

Here are the 3 velocity components and the wave elevation which is just the u-velocity
scaled by -U/g, where U is the ship speed and g is gravitational acceleration.
Leo.

View attachment 209532

Ahh, sorry I misinterpreted the figure (vweb.p). So this is the v axis velocity...which should be opposite and equal on each side of the hulls.....an artifact of a fixed frame of reference. Not a hump and hollow plot.

Edit to add: I did some work on the effect of the body on the diverging wave a couple of decades ago about how and when a bow wave breaks and how that affects how the energy is distributed through the fluid. And you were involved in this old post.

Thread 'Wakes'

Apr 24, 2014

https://en.wikipedia.org/wiki/Wake

This Wikipedia page seems to contain a good mathematical discussion about the usual V-shaped boat wakes; but I still need some information; each V is composed of an imbricated succession of parallel waves waves successively starting and finishing; but what wavelength are those waves (measured at right angle to that arm of the wake V) for each speed in knots? Does that wavelength vary as the square root of the speed of the boat, or what?

• Anthony Appleyard

 

[Skip Johnson]

Hello Strangers!

Welcome back!

 

[jehardiman]

Would that everything be so simple...

... in real life....

(Edit due to IRL photo getting pulled for copyright concerns)

 

[Leo Lazauskas]

Would that everything be so simple...
View attachment 209539

... in real life....

Of course, that's why I put scare quotes around "real" in my first reply to you.
My plots are of potential flow, you know, where water is dry.

You are showing me the situation at the stern where there is, among many other
complications (i.e. "reality"), a thick separated boundary layer.

The term "tearing velocity" is probably a little more appropriate for the flow
near a sharp bow. And of course it is for potential flow. I'm an applied
mathematician, not an engineer, so solution of the flow equations is paramount.

Another aspect you haven't taken into account (and this is no way a snipe at
you or your excellent observations!) is the scaling of the three components in
the plots. They have all been normalised. In "reality" (Oh how we LOL at that
one here at chez Leo&Kit) the v- and w-velocities are much smaller than
the u-velocity.

I'm now concerned that you will have conniptions when I announce that, using an
"exact" method and 72 decimal place computations, the lift slope of a circular
planform wing is 1.79002 30364 50297 17315 (to 20 decimal places).

Anyway, back to my original question: Has anyone come across the term tearing
velocity" before?

 

[Leo Lazauskas]

My advice: don't disturb the calm waters.
Slender WP hulls and foils are what I want to do.

Glad you clarified that in the last line.
Otherwise I would have suggested that you build yourself an artifical flounder.

 

[montero]

Ha ha

Glad you clarified that in the last line.
Otherwise I would have suggested that you build yourself an artifical flounder.

Ha ha ...

 

[jehardiman]

I'm now concerned that you will have conniptions when I announce that, using an
"exact" method and 72 decimal place computations, the lift slope of a circular
planform wing is 1.79002 30364 50297 17315 (to 20 decimal places).

Nah, no real grievance with over precise numerical calculations... usually because I'll tack on "+5/-3 percent" or some such...

Anyway, back to my original question: Has anyone come across the term tearing
velocity" before?

I never heard that term. Realizing that my CFD days go back before the original beige dinosaurs, when as a student you paid for CPU seconds, transverse velocity was inferred out of the movement of the equipotential lines around the source-sinks. I never went to the grad courses of the time were N-S stress tensor systems for free surface solutions were being examined, and eventually my needs turned towards deep submergence and breaking waves which rendered most of that moot.

 

[Ad Hoc]

...Anyway, back to my original question: Has anyone come across the term tearing
velocity" before?

No, I haven't.
I've also not observed the "issues" you note in your fancy colour plots either, when performing physical model testing.
Thus is this a quirk of the mathematical modelling, or just so minuscule, in scale, that it is not observed in real-life tank testing?

.... the lift slope of a circular
planform wing is 1.79002 30364 50297 17315 (to 20 decimal places).

Why be accurate when you can have trends