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A question about volume field representations of forces

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Old   August 11, 2022, 12:18
Default A question about volume field representations of forces
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Saddam Hijazi
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Hello everyone,


I have a curiosity regarding the so-called volume field representations of forces when computing the forces. I set the writeFields in the forces dictionary to yes




Code:
// Store and write volume field representations of forces and moments
    writeFields     yes;
Then I ran the problem of the flow around a cylinder in 2D. I got the forces fields where I have in each output folder a forces file named forces:force (also a moment field)


My question is how one should understand these fields ?


I would like to understand how I could compute the overall force vector that is acting on the cylinder given these forces fields representations. I thought that I only have to compute the sum of the forces after projecting them on x and y component using the angle that the cell (at which I have the forces value) is making with center of the cylinder. Is that correct ?

Last edited by SaddamH; August 11, 2022 at 18:02.
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Old   August 11, 2022, 12:50
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Domenico Lahaye
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force (N) =
mass (kg or [1 0 0 0 0 0 0 0]) * acceleration (m/s^2 or [0 1 -2 0 0 0 0 0])
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Old   August 11, 2022, 18:01
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Saddam Hijazi
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Thank you dlahaye, I am sorry, I don't know why I messed up in understanding the unit.


So my question is only about how one could compute the resultant force on the whole body given these forces values on the cells which are NOT evenly distributed on the cylinder boundary.


Thanks.
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Old   August 12, 2022, 04:49
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Domenico Lahaye
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No worries. We are all learning here.

Force-vector on patch = surface-integral stress-tensor * normal-on-patch.

See e.g. Section Simple Stress of https://en.wikipedia.org/wiki/Stress_(mechanics)
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Old   August 12, 2022, 18:59
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Thanks dlahaye, I am still a bit confused of what I should do with fields named forces:force do you mean that I have to compute the integral of these fields over the surface of the cylinder after taking the scalar product with the normal vector ?
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force vector, forces and force coeff., forcesincompressible

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