Floating Object: Mass Body gives a wrong value

Asalih3d

Hello!

I am using a floating object and defining the massbody property. So say I have rho_reference = 1000 kg/m^3, then I define the massbody = 50 kg for a cylinder. When I then look into the xml file and say masspart*count I get that the real weight of the particles is 80 kg.

What am I doing wrong? I am doing exactly as shown in the guide:

Kind regards

Alex

Can you paste here the piece of XML you are using, please?

Asalih3d

https://forums.dual.sphysics.org/discussion/comment/4807#Comment_4807

Of course!

I am using this piece of XML:

        <geometry>
            <definition dp="0.04">
                <pointmin x="-2.00" y="-2.25" z="-2.00" />
                <pointmax x="2" y="2.5" z="2.00" />
            </definition>
            <commands>
                <mainlist>
                    <setmkbound mk="10" />
                    <drawcylinder radius="1">
                        <point x="0" y="0" z="0" />
                        <point x="0" y="0" z="1" />
                    </drawcylinder>
                    <shapeout file="" />
                </mainlist>
            </commands>
        </geometry>
        <floatings>
            <floating mkbound="10">
                <massbody value="50"/>
            </floating>
        </floatings>

Which then outputs the _out.xml:

            <floating mkbound="10" mk="20" begin="0" count="50882">
                <massbody value="50" units_comment="kg" />
                <masspart value="0.064" units_comment="kg" />
                <center x="-0.000858457" y="-0.00828309" z="0.5" units_comment="metres (m)" />
                <inertia units_comment="kg*m^2">
                    <values v11="16.9362" v12="-0.0314687" v13="2.01831e-17" />
                    <values v21="-0.0314687" v22="16.9834" v23="-2.19778e-17" />
                    <values v31="2.01831e-17" v32="-2.19778e-17" v33="24.9196" />
                </inertia>
            </floating>

And the only thing I care about for now is the mass of the object. It says the object has a masspart of 0.064, which equals m_i = rhop*dp^3 = 1000*0.04^3 = 0.064. This means that masspart * count gives:

MassOfParticles = masspart*count = 0.064*50882 = 3.256,448 kg

But in MassBody I specified 50 kg.

What is my misunderstanding?

Kind regards

Asalih3d

And also note that the mass of the particles when extracted by partvtk is equal to masspart.

Kind regards

Alex

That is correct.

Massbody (50 kg in this case) will be used to compute the weight or in Chrono when solving collisions between objects.

However the masspart (0.064) that by default is the same as mass fluid is used only for particle interactions to compute force exerted by the fluid.

Regards

Asalih3d

https://forums.dual.sphysics.org/discussion/comment/4815#Comment_4815

Thanks for the clarification Alex. My concern would be though that this seems to be wrong?

This means that in the cfd it will see a gravitational force of; 3256,448 kg * 9.81 m/s^2, while in reality it should only be 50*9.81?

I know that I have gotten correct force results from floating bodies before so something must be missing in my theoretical understanding.

What I am pretty sure of though is that this approach makes it impossible to trust the volume of the body? For example for this simple cylinder with radius = 1 and height = 1, then volume must be:

V = pi*R^2 * height = pi = 3.14

But in reality from the particles then the volume must be:

V_particles = MassOfParticles/rho_reference = 3256,448 kg / 1000 kg/m^3 = 3.25

So it is much more bouyant than it should be.

I feel I understand more, but meanwhile my confusion has also grown.. have a good weekend, looking forward to your response

Kind regards

Alex

Why do you think that: This means that in the cfd it will see a gravitational force of; 3256,448 kg * 9.81 m/s^2, while in reality it should only be 50*9.81?

I am lost there, sorry.

Asalih3d

https://forums.dual.sphysics.org/discussion/comment/4819#Comment_4819

It is because in my case I had 50882 particles which each had a mass of 0.064 kg. That equals 3256 kg, so I would assume that the gravitational force of the particles would be 3256 * gravitational acceleration?

Through mass body I specify 50 kg, but that is not transferred to the individual particles.

Kind regards

Alex

Do not worry.

The total mass is only used to determine the final motion of the rigid body and is also considerered by Chrono if needed.

The individual mass of the particles of the floating body are used only when interacting with surruonding fluid particles.

Right now, in the code during the simulation, by default, there is no summation of the individual masses.

Regards