SPH particle mass calculation
Hello,
I am generating a SPH mesh in HyperMesh. I set the pitch distance to 0.01 and the density to the value of the material I am using. The particle distribution I am using is simple cubic. Therefore, according to
the mass of the particles should be automatically calculated as m_p = pitch^3 * density. However, the mass displayed in the SPH property that is created after the SPH generation is almost 1.75 times larger than the one expected by the formula above. Any suggestion on this?
Thank you in advance!
Best Answer
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The theoretical calculation in the help assumes that your cubic grid perfectly complies with your volume
i.e. for your pitch example, you should get 1e6 particles in a 1x1x1 cubic containing volume
In practice, the SPH particles will be misaligned with with your volume and if you also specified wall offsets this will make the difference even greater (so the dimensions of your SPH net will be smaller than the containing volume), to account for the 'missing' particles HM makes each one heavier to give you the correct total mass for your sph based on the volume and material density. e.g. if only 900k particles were created in the example volume I just described, each would have a mass of 1.111x the theoretical value.
When you create your mesh, HM will tell you the total number of particles it created in the bottom left status bar, and the SPH property mass auto applied should be very close to:
(geometric volume x material density) / number of particles
which will lead to the correct overall mass when you add them all up
So in your case, I guess your wall offset/geometry, relative sph spacing to part size etc, means the number of particles created is approx 1.75x less than the theory of pitch distance^3, for it to be so far away suggests maybe the SPH pitch you are using is quite big relative to the features of the part, or that you used quite a large wall offset relative to the pitch distance? Experiment with smaller pitches and offsets to see if you will get closer (mp will always be at least a little bigger than the theoretical value, but you ought to be able to get closer than 1.75x)
If you do a mass calc on all the particles you have, it should be about right per your total theoretical mass, if it is slightly out, you can tweak the property mp if needed.
2
Altair EmployeeAnswers
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The theoretical calculation in the help assumes that your cubic grid perfectly complies with your volume
i.e. for your pitch example, you should get 1e6 particles in a 1x1x1 cubic containing volume
In practice, the SPH particles will be misaligned with with your volume and if you also specified wall offsets this will make the difference even greater (so the dimensions of your SPH net will be smaller than the containing volume), to account for the 'missing' particles HM makes each one heavier to give you the correct total mass for your sph based on the volume and material density. e.g. if only 900k particles were created in the example volume I just described, each would have a mass of 1.111x the theoretical value.
When you create your mesh, HM will tell you the total number of particles it created in the bottom left status bar, and the SPH property mass auto applied should be very close to:
(geometric volume x material density) / number of particles
which will lead to the correct overall mass when you add them all up
So in your case, I guess your wall offset/geometry, relative sph spacing to part size etc, means the number of particles created is approx 1.75x less than the theory of pitch distance^3, for it to be so far away suggests maybe the SPH pitch you are using is quite big relative to the features of the part, or that you used quite a large wall offset relative to the pitch distance? Experiment with smaller pitches and offsets to see if you will get closer (mp will always be at least a little bigger than the theoretical value, but you ought to be able to get closer than 1.75x)
If you do a mass calc on all the particles you have, it should be about right per your total theoretical mass, if it is slightly out, you can tweak the property mp if needed.
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Paul Sharp_21301 said:
The theoretical calculation in the help assumes that your cubic grid perfectly complies with your volume
i.e. for your pitch example, you should get 1e6 particles in a 1x1x1 cubic containing volume
In practice, the SPH particles will be misaligned with with your volume and if you also specified wall offsets this will make the difference even greater (so the dimensions of your SPH net will be smaller than the containing volume), to account for the 'missing' particles HM makes each one heavier to give you the correct total mass for your sph based on the volume and material density. e.g. if only 900k particles were created in the example volume I just described, each would have a mass of 1.111x the theoretical value.
When you create your mesh, HM will tell you the total number of particles it created in the bottom left status bar, and the SPH property mass auto applied should be very close to:
(geometric volume x material density) / number of particles
which will lead to the correct overall mass when you add them all up
So in your case, I guess your wall offset/geometry, relative sph spacing to part size etc, means the number of particles created is approx 1.75x less than the theory of pitch distance^3, for it to be so far away suggests maybe the SPH pitch you are using is quite big relative to the features of the part, or that you used quite a large wall offset relative to the pitch distance? Experiment with smaller pitches and offsets to see if you will get closer (mp will always be at least a little bigger than the theoretical value, but you ought to be able to get closer than 1.75x)
If you do a mass calc on all the particles you have, it should be about right per your total theoretical mass, if it is slightly out, you can tweak the property mp if needed.
Hello Paul,
thank you for your detailed reply. I sensed this was the explanation of the mass difference from the theoretical value. Thanks for clarifying!0