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An object in a fluid

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rynhardt
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An object in a fluid

Postby rynhardt » Wed May 29, 2019 9:02 am

What happens when you place a real, physical, rigid object with non-zero volume and density into a fluid like water?
Well, the object and the fluid cannot occupy the same space at the same time, so the object displaces water.

Let's assume for the time being that the relative velocity between the object and the water is zero.

If the density of the object is lower than water, it will float, and displace it's weight in the equivalent weight of water volume.
If the density of the object is higher than water, it will sink, and displace it's entire volume in water volume.
If the object floats, it displaces air as well as water, but let's ignore the air for the sake of simplicity.

So at zero relative velocity, all real, physical, rigid objects are displacement hulls. :D

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Re: An object in a fluid

Postby rynhardt » Wed May 29, 2019 9:35 am

When the relative velocity between the object and the fluid is non-zero, the object will need to displace water in the direction it is moving, because once again it can't occupy the same space as the water at the same time :-)
Displacing the water requires pressure, which, depending on the object's geometry,will generate forces on the water and equal but opposite forces on the object.

Under steady state conditions, i.e. constant velocity (and keeping all other variables fixed, ideally), the forces will be in balance.
The geometry of the object will dictate the direction and magnitude of the forces. Let's ignore drag forces for the time being and only look at lift.

Thought exercise: Using a cube as a candidate object, let's say this cube has neutral buoyancy (i.e. same density as water).
Put this cube under water, and drag it with a zero-width non-stretchy ideal cable attached to the center of the front face at a constant velocity.
As constant velocity, water will flow around the cube. A pressure gradient will exist with the highest pressure in the center of the front face and lowest pressure at the center of the rear face.
Under these conditions the cube will experience no net lift orthogonal to the direction of pull. The lift forces on the left and right will be in balance, as will the lift forces on top and bottom.

(In reality there will be a von karman vortex street behind the cube causing it to oscillate, but let's ignore that for the time being.) :o

The point of this thought exercise is that, at constant velocity, there will be a pressure gradient determined by the object geometry. The nature of this geometry will determine in which direction the forces act on the object.

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Re: An object in a fluid

Postby Regis-de-giens » Wed May 29, 2019 9:53 am

I do not undertand to which question this message answers, but I like it ! I guess you will speak of the vertical lift induced by a non-symetrical object in the water, caused by a non-zero angle of attack...

it is rare that these notions of water displacements are described ; this is great since it impact all movement equations for hulls in the water

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Re: An object in a fluid

Postby rynhardt » Wed May 29, 2019 10:06 am

Regis-de-giens wrote:
Wed May 29, 2019 9:53 am
I do not undertand to which question this message answers, but I like it ! I guess you will speak of the vertical lift induced by a non-symetrical object in the water, caused by a non-zero angle of attack...

it is rare that these notions of water displacements are described ; this is great since it impact all movement equations for hulls in the water
Yes, exactly where I'm heading with this 8)

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Re: An object in a fluid

Postby lederhosen » Wed May 29, 2019 10:26 am

If you dont care about the eddies you can calculate the forces on the object with a random average navier stokes RANS model in steady flow in a fluid solver

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Re: An object in a fluid

Postby rynhardt » Wed May 29, 2019 10:50 am

lederhosen wrote:
Wed May 29, 2019 10:26 am
If you dont care about the eddies you can calculate the forces on the object with a random average navier stokes RANS model in steady flow in a fluid solver
Are you volunteering? :naughty:

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Re: An object in a fluid

Postby rynhardt » Wed May 29, 2019 11:02 am

Anyway, back to my little cube, being dragged under water.

In the first case, where it is symmetrical in all respects orthogonal to the direction of travel, the net lift forces are zero.
I'm using red arrows to indicate the force the water is exerting on the object.

In a second case, it's no longer a cube but a prism with the bottom surface forming an angle of attack (AoA) of 10 deg to the direction of travel. The pressure on the bottom surface is higher than on the top surface so the object will experience a net lift force upwards.
Attachments
cube_aoa.png

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Re: An object in a fluid

Postby grigorib » Wed May 29, 2019 1:24 pm

Well....until someone comes and tells you those are STRUTS and nothing else!
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Re: An object in a fluid

Postby plummet » Wed May 29, 2019 6:30 pm

Whats your point of this thought experiment. Seems you are just openly considering fluid dynamics. But to what end? what do you hope to achieve?

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Re: An object in a fluid

Postby rynhardt » Thu May 30, 2019 7:15 am

plummet wrote:
Wed May 29, 2019 6:30 pm
Whats your point of this thought experiment. Seems you are just openly considering fluid dynamics. But to what end? what do you hope to achieve?
Mostly I like the sound of my own voice :D
There was a discussion around planing vs displacement hulls. I want to write down my understanding of it and see if anybody can point out any glaring flaws.


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