Pressure distribution on a hull in a slanted flow ...

[ropf]

... is there a "easy to use" software providing this?

Easy to use means like michlet or flotilla. I tried to get into openfoam, but the dozens of different solvers and the correct formulation of the boundary conditions are a bit overwhelming for me. A potential based code for gnu octave or matlab would also help.

 

[gonzo]

Did you mean slanted flow?

 

[ropf]

Did you mean slanted flow?

Yes, sorry.

... how can i correct this?

 

[Boat Design Net Moderator]

Thread title updated now.

 

[jehardiman]

I know of no "easy to use" software that will do it for you. AeroHydro can be forced to do it.

What you are actually looking for is sometimes called the "alpha flow" (i.e. flow due to the angle of attack) or the "Munk Moment". Generally this is never calculated directly, only to relatively to get the directional stability coefficients (see any good NA text on maneuvering). Sailing vessels, airships, and maneuvering rockets/missiles are the only bodies that need to deal with, and balance, these pressures/moments long term.

I think only Munk in his airship work (1924), Davidson (1936), Halsey Herreshoff and Kerwin (1966) or Larsson (1990) ever directly addressed pressure on the hull/body due to non-axial incipient flow.
Remarks on the pressure distribution over the surface of an ellipsoid, moving translationally through a perfect fluid : Munk, M. M : Free Download, Borrow, and Streaming : Internet Archive https://archive.org/details/nasa_techdoc_19800006782/page/n1/mode/2up

 

[ropf]

@jehardiman: thanks for your answer. Aerohydro is unfortunately three sizes too big for me ...

No, i'm not looking for the Munk Moment - i am directly interested in the forces to the hull through the "alpha-flow" - how strong they are, how they are distributed, and how I can influence them by hullshape.

Generally this is never calculated directly ... Sailing vessels, airships, and maneuvering rockets/missiles are the only bodies that need to deal with

Now i'm really surprised. We are in a boatesign forum, arn't we? With a lot of sailboat designers, design-interested sailors, bright minds with knowledge of hydrodynamics and programming skills - always going slanted (sometimes slunted, sometimes under attack of an angel ) - and no one interested on a deeper look???

In the meantime I dug out a german book that addresses exactly this problem: "Berechnung der Potentialströmung um einen schräg fahrenden Schiffsrumpf auf tiefem und flachem Wasser" (Calculating the potential flow around a ship's hull moving at an angle on deep and shallow water).

The abstract is in english, from here: https://tore.tuhh.de/entities/publication/581d9662-0e9b-4f3b-90c4-41886a7ea2db

A Rankine panel method for calculating lifting potential flow about three-dimensional surface-piercing bodies in deep and/or shallow water is developed and applied to ships in steady motion at small drift angles. The disturbance velocity potential is represented by a source distribution on the hull surface and on a horizontal plane above the free surface, and by a normal dipole distribution on the centerplane of the ship and downstream of the trailing edge of the hull. The singularity strengths are determined so that the boundary conditions on the hull surface and free surface and the Kutta condition at the trailing edge are satisfied at the corresponding collocation points. Moreover, the radiation condition is satisfied by using the numerical technique of "staggered grids" suggested by Jensen, while the shallow water effect is taken into account by the method of images.

 

[jehardiman]

@jehardiman: Now i'm really surprised. We are in a boatesign forum, arn't we? With a lot of sailboat designers, design-interested sailors, bright minds with knowledge of hydrodynamics and programming skills - always going slanted (sometimes slunted, sometimes under attack of an angel ) - and no one interested on a deeper look???

<shrug> First of all, the Munk Moment is just the integration of the pressures you are looking for, caused by flow, over the surface.

Second, people tried to analytically determine what you are looking for BPC with only Munk and Lamb developing general solutions for elliptical bodies of revolution. Much like the Wigley Hull, when theory was compared to model tests (rotating arm tests) it was determined that there where only small tweaks to coefficients needed (i.e. big long equation*c). Since the mid 1960's and especially the mid-1970's when standard hull form construction became the norm, the only need to investigate this devolved to the sailing and rocket community, and the occasional grad student study. The GRIMCRACK tests at the Davidson Lab and the 12m work by MIT in the 1960's showed that a well designed sailing hull should have little Munk Moment while heeled and the, now obliquitous, fin keel should do the lifting work (as demonstrated by the modern IMOCA 60's). This carried on into the work by Pierre de Saix and Peter van Oossanen. As far as the pressure on the hull goes, it was readily determined that seaway has more effect on a well designed hull than the alpha flow, so unless you are in either theoretical or perfect water determining the pressures is just trying to take a pencil to a line marked by a big piece of chalk.

 

[ropf]

As far as the pressure on the hull goes, it was readily determined that seaway has more effect on a well designed hull than the alpha flow, so unless you are in either theoretical or perfect water determining the pressures is just trying to take a pencil to a line marked by a big piece of chalk.

The problem is - the big marked line in the theory - simply doesnt match (my) reality.

There are lots of different waters here (around berlin, germany), they are neither theoretical nor perfect. The majority is very shallow, often we have underwater obstacles like tree trunks, and regulary we have a problem with see gras. On the other side we have virtually no Seaway. So the theoretical (and in other places maybe practical too) perfect boat - nice rounded body, high aspect ratio fin, vanishing Munk Moment if you want - is nothing else then perfect useless in THIS reality.

The boat I want is of shallow draft, easy to beach, robust against minor rammings, not collecting grass ... this means deep-V hulls, or maybe rounded with very shallow long keels with a very(!) smooth entry, not to forget a skeg rudder with a smooth entry too. So the Munk Moment, as the integration of forces you want to vanish - turns in to the driving forces of my vessel.

Sorry if this is boring for you. For me is the question - stick with the V-Hulls for the next boat, or change to shallow keels? Is a possible performance advantage worth the extra effort and the higher sensitivity - with a depth limit of say 50cm on an 8m boat? Wich effect has the smooth entry angel of the keel, do i get get same edge vortex as a delta wing? Why are the Wharrams so tricky to tack, is this a general property of the V-hull, or does the rudder skeg slow down the rotation? ...

In short, i have 1000 questions, want to see the flow and the pressures in detail - and if the big white line doesnt help i have to continue with pencil strokes

 

[Ad Hoc]

....I think only Munk in his airship work (1924), Davidson (1936), Halsey Herreshoff and Kerwin (1966) or Larsson (1990) ever directly addressed pressure on the hull/body due to non-axial incipient flow....

Correct.
But the Munk moment only applies to a fully submerged object....ergo, not applicable to a shape that is in the air-water interface.

I never understand why those designing yachts, or attempting to, always refer to the Munk monet in their designs.
Unless it is a submarine, that value it is nowt.

....So the Munk Moment, as the integration of forces you want to vanish - turns in to the driving forces of my vessel.....

You need to read and digest the full text and what the Munk moment is, when you do, you'll understand it is not applicable to surface vessels like a yacht.

 

[jehardiman]

Correct.
But the Munk moment only applies to a fully submerged object....ergo, not applicable to a shape that is in the air-water interface.

I never understand why those designing yachts, or attempting to, always refer to the Munk monet in their designs.
Unless it is a submarine, that value it is nowt.

Designing submarines....yeah, pretty much what I did for 33 of my 34 years... But the basis Lamb and Munk laid down still works pretty good for analyzing alpha flow on submarines and airplanes.

 

[ropf]

You need to read and digest the full text and what the Munk moment is

I did, and understood that it provides the theoretical explanation why an ellipsoidal body always wants to turn its nose out of the wind. This is NOT what I am looking for, at most as a side effect:

No, i'm not looking for the Munk Moment - i am directly interested in the forces to the hull

you'll understand it is not applicable to surface vessels like a yacht.

Interestingly, you can observe the effect directly on a boat on the surface ... when it is anchored, it always tends to break out to one side, ;-)

But again - I am not interested in the Munk effect at all, not on the water and not under water. Instead, I am looking for a (simple) simulation for the flow around a boat in respect(!) of free surface waves. The book I linked above describes a panel-based potential flow method that is suitable for this - and is clear enough to code it yourself if necessary.

 

[jehardiman]

ropf;
Actually, where you may want to go is over to Heorner's Fluid Dynamic Lift and Drag. He does a fair amount on the whole aircraft including attachments to the skin, and I have often used some of his reference works for what I had need for. You may also want to check Blevins Applied Fluid Dynamics Handbook, it has a fair amount on flow around items on a planar interface. Either of those two books should have reference to other studies and papers.

 

[CarlosK2]

"I never understand why those designing yachts, or attempting to, always refer to the Munk monet in their designs."

---

because a sailboat/yacht is subject to three variants or first cousins of the Munk Moment family

M1, due to Yaw
M2, due to Roll
M3, due to Pitch

The Aero- and Hydromechanics of Keel Yachts https://link.springer.com/book/10.1007/978-3-319-13275-4

---

I think that naval architects should not design sailboats and should dedicate themselves to their own thing: motorboats, destroyers, fishing boats, tugboats, merchant ships, oil tankers ...

The design of sailboats would be better in the hands of aeromodelling enthusiasts

.

 

[Ad Hoc]

....because a sailboat/yacht is subject to three variants or first cousins of the Munk Moment family
M1, due to Yaw
M2, due to Roll
M3, due to Pitch

...

The Munk moment is the separation of flow of a submerged body to an inclined flow.
The separation of flow, aft of the lead edge, and at the corresponding after body, leads to a destabilising pitch moment.
Nothing else.

A yacht is not a submerged body.

QED.

What is incorrectly being "referenced" as the Munk Moment, is what is called D'Alembert's Principle.

 

[ropf]

What is incorrectly being "referenced" as the Munk Moment, is what is called D'Alembert's Principle.

I had to think about it. Both Munk and D'Alembert ignore viscosity. Neither produces drag in a 2D flow. But one produces lift, the other a momentum. What is the basic difference?

The answer is of course trivial - it is the sharp trailing edge of the foil that forces circulation. But have you ever thought about it in the context of hull design? Not me. From this perspective, the lateral line of historical ships with a constant depth from the main frame to the rudder, suddenly gives sense.

Furthermore, with a V-shaped hull with appropriate volume distribution, at least the entire rear body is somehow trailing. And even further, the separation of the edge vortex probably takes place much earlier, similar to a delta wing...

... the authors of the textbook I mentioned in number 3 explicitly do NOT take this effect into account, but still calculate a lift - making some waves. In general, they achieved significantly higher values for lift and moment than predicted by the theory of slender bodies. Here the wave pattern of a wigley ship at FN 0.25 and the the lift coefficient over the Froud number:


None of the tools and codes i found address early separation of the tip vortex, which is simply not of interest for aerodynamics where the chords are short and the wingspans are long. But on a V-hull it must exist, it changes the entire flow pattern - and therefore these simulations are useless if not present in the model.

And now? There are some publications that deal with the modelling of edge vortices on delta wings using a potential flow methods. I have to read them and maybe I can combine the ideas with wave modelling ...