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6DOF solver (buoyancy and hydrostatic force)

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Old   September 4, 2016, 05:57
Default 6DOF solver (buoyancy and hydrostatic force)
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Zachary Hill
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Hi all,

I am a relatively new Fluent user and am trying to simulate a floating object in water with a regular wave input. I am attempting to use the 6DOF solver to set the properties of the object such as mass, moments of inertia, and a spring damper system to represent a tether.

My question is how I deal with the hydrostatic force that acts on the object. As this force is quite large, will it effect my object's or is it ignored by the 6DOF solver? Do I need to remove the value of the hydrostatic force by using an equal and opposite external force acting on the body with prop[SDOF_LOAD_F_Y]?

Additionally, is buoyancy automatically included in the 6DOF solver? Or do I need to include this buoyancy force also with prop[SDOF_LOAD_F_Y]?

I have attached the UDF but it may be difficult to follow, let me know if more info is needed.

Thanks in advance for the help,

-Zac
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File Type: c 6DOF_2D_parallel.c (3.6 KB, 47 views)
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Old   September 7, 2016, 10:21
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ERMACORA Florian
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Quote:
Originally Posted by ZacHillAdl View Post
Hi all,

I am a relatively new Fluent user and am trying to simulate a floating object in water with a regular wave input. I am attempting to use the 6DOF solver to set the properties of the object such as mass, moments of inertia, and a spring damper system to represent a tether.

My question is how I deal with the hydrostatic force that acts on the object. As this force is quite large, will it effect my object's or is it ignored by the 6DOF solver? Do I need to remove the value of the hydrostatic force by using an equal and opposite external force acting on the body with prop[SDOF_LOAD_F_Y]?

Additionally, is buoyancy automatically included in the 6DOF solver? Or do I need to include this buoyancy force also with prop[SDOF_LOAD_F_Y]?

I have attached the UDF but it may be difficult to follow, let me know if more info is needed.

Thanks in advance for the help,

-Zac
Hello,

in 6DOF, the solver take the hydrostatic force but any other force due to the gravity for exemple without you indicate a gravity field in fluent. whats more in your UDF, there is a thing that I think it s wrong.
Code:
	for(i=0;i<3;i++)
		x_cg[i]=DT_CG(dt)[i];
		v_cg[i]=(x_cg[i]-x_cg[i-1])/dtime;
In this code, you indicate the position in 3 dimensions with the two firsts lines. Then when you want to calculate the velocity, it do

for i = 0
x_cg[0]=DT_CG(dt)[0];
v_cg_x = xcg[0]-xcg[-1]

for i = 1
x_cg[1]=DT_CG(dt)[1];
v_cg_x = xcg[1]-xcg[0]

for i = 2
x_cg[2]=DT_CG(dt)[2];
v_cg_x = xcg[2]-xcg[1]

for i = 3
x_cg[3]=DT_CG(dt)[3];
v_cg_x = xcg[3]-xcg[2]

But 0 correspond to x-axis
1 correspond to y-axis
2 correspond to z-axis
3 correspond to nothing

instead of taking the value of positionog your gravity center in the previous time step.
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Old   March 27, 2017, 16:20
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Hi guys,

Probably it is too late now, but I am also testing a body movement inside a fluid (water) and it seems like buoyancy is not automatically included, just like Zachary described. I suppose this can be very tricky in cases including dynamic variation of wetted volume.

I my case the body free vibration is much less damped than I expected. Any tips?
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