Valve Rigid Body Modelling

mo-ca · February 9, 2017, 11:11

Hi,
I'm working on a valve plate simulation and want to start with and 10° wedge of a fully rotational symmetric setup. Currently, there is one rigid body in my Pre setup. This is, where the first questions arise [at this point, a little more detail in the cfx help would be nice]:

1) When I only compute 10°, my mass of the rigid body is 10 / 360 times the original one, correct?
2) The spring force - as not area dependant - should be the full constant, not scaled, correct?

Thanks in advance

[edit: for ease of reading]
due to the fact, that my body moves, when it should not move as the driving pressure difference is much to small -> more precise: it should get pushed into its seat with the conditions applying

ghorrocks · February 9, 2017, 16:36

I do not really understand your question - computers understand nested parenthesis just fine but it overloads a human front side bus

.

If you are modelling a 10 degree wedge of a axisymmetric model with a rigid body simulation then yes, the 10 degree wedge should have 10/360 of the mass but the spring should have 10/360 of the spring constant.

The natural frequency of a spring mass system is f = k sqrt(k/m). So to keep the frequency the same with 10/360 of the mass the spring constant will also need to be 10/360.

mo-ca · February 10, 2017, 07:51

thanks so far - and especially for the explanation ... I edited the first post for the ease of reading.

mo-ca · February 16, 2017, 11:26

Hi again & sorry for the double post, but as this belongs to the same case, perhaps its ok to ask follow-ups.

I created the mentioned rigid body system with mass and spring as my measurable system. Now the piston moves out of the working chamber, expanding the air inside. That works so far, but when comparing measured data to simulation, the expansion is way to slow and uses a far to low pressure difference to open. As this simulation is adiabatic as well as air with ideal gas, it should be closer to an ideal expansion with lower pressure difference.
[click pictures for larger version]

If I put the same system into matlab, solve the ode for a simple force induced mass-spring-system, I get a reasonable lift with oscillation

Has anyone experienced such things with cfx rigid body modelling?

[edit] ok .. I files a service request and will share a possible solution afterwards

February 9, 2017, 11:11	Valve Rigid Body Modelling	#1
mo-ca New Member Join Date: Aug 2013 Posts: 23 Rep Power: 12	Hi, I'm working on a valve plate simulation and want to start with and 10° wedge of a fully rotational symmetric setup. Currently, there is one rigid body in my Pre setup. This is, where the first questions arise [at this point, a little more detail in the cfx help would be nice]: 1) When I only compute 10°, my mass of the rigid body is 10 / 360 times the original one, correct? 2) The spring force - as not area dependant - should be the full constant, not scaled, correct? Thanks in advance [edit: for ease of reading] due to the fact, that my body moves, when it should not move as the driving pressure difference is much to small -> more precise: it should get pushed into its seat with the conditions applying Last edited by mo-ca; February 10, 2017 at 01:47. Reason: ease of reading

February 10, 2017, 07:51		#3
mo-ca New Member Join Date: Aug 2013 Posts: 23 Rep Power: 12	thanks so far - and especially for the explanation ... I edited the first post for the ease of reading. Last edited by mo-ca; February 11, 2017 at 08:34.

February 16, 2017, 11:26		#4
mo-ca New Member Join Date: Aug 2013 Posts: 23 Rep Power: 12	Hi again & sorry for the double post, but as this belongs to the same case, perhaps its ok to ask follow-ups. I created the mentioned rigid body system with mass and spring as my measurable system. Now the piston moves out of the working chamber, expanding the air inside. That works so far, but when comparing measured data to simulation, the expansion is way to slow and uses a far to low pressure difference to open. As this simulation is adiabatic as well as air with ideal gas, it should be closer to an ideal expansion with lower pressure difference. [click pictures for larger version] If I put the same system into matlab, solve the ode for a simple force induced mass-spring-system, I get a reasonable lift with oscillation Has anyone experienced such things with cfx rigid body modelling? [edit] ok .. I files a service request and will share a possible solution afterwards Last edited by mo-ca; February 17, 2017 at 08:02. Reason: Matlab Update, Service Request filed...

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February 9, 2017, 16:36		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I do not really understand your question - computers understand nested parenthesis just fine but it overloads a human front side bus . If you are modelling a 10 degree wedge of a axisymmetric model with a rigid body simulation then yes, the 10 degree wedge should have 10/360 of the mass but the spring should have 10/360 of the spring constant. The natural frequency of a spring mass system is f = k sqrt(k/m). So to keep the frequency the same with 10/360 of the mass the spring constant will also need to be 10/360.