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setting "air" pressure
Posted May 26, 2015, 5:11 p.m. EDT MEMS & Nanotechnology, MEMS & Piezoelectric Devices, Structural Mechanics Version 5.1 1 Reply
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This is a really basic question and I apologize for asking it, but have mercy on me.
I want to emulate vacuum for electromechanical actuation, so I use an "air" material. The "air" material properties include air density, which is a function of pressure ( "pA") and temperature ("T"). If I wish to set these variables, I set them in Global Definitions -> Parameters, for example, here I'm setting the air pressure to 1 µPa:
i.imgur.com/nBKBRU7.png
Is this correct?
Further, in the solid mechanics module, I need to set the Young's modulus of the gas to "pA". This is consistent with the definition of pressure of an ideal gas: reducing the length of a gas cylinder of length L to length L - dL increases the pressure from pA to pA L / (L - dL) ≈ pA ( 1 + dL / L ). The "strain" in this case is -dL/L, and the "stress" is dL/L pA, so the effective Young's modulus is the ratio = pA. It's curious this isn't built into the model and needs to be specified, but in any case my pressures are so low it's effectively zero (exactly zero generates a singular matrix).
Further, with solid mechanics and air there is no need to use the moving mesh module, if I read the documentation correctly.
Whoops -- I've deviated substantially from my original question, which was whether "pA" is specified as a global parameter definition.
I want to emulate vacuum for electromechanical actuation, so I use an "air" material. The "air" material properties include air density, which is a function of pressure ( "pA") and temperature ("T"). If I wish to set these variables, I set them in Global Definitions -> Parameters, for example, here I'm setting the air pressure to 1 µPa:
i.imgur.com/nBKBRU7.png
Is this correct?
Further, in the solid mechanics module, I need to set the Young's modulus of the gas to "pA". This is consistent with the definition of pressure of an ideal gas: reducing the length of a gas cylinder of length L to length L - dL increases the pressure from pA to pA L / (L - dL) ≈ pA ( 1 + dL / L ). The "strain" in this case is -dL/L, and the "stress" is dL/L pA, so the effective Young's modulus is the ratio = pA. It's curious this isn't built into the model and needs to be specified, but in any case my pressures are so low it's effectively zero (exactly zero generates a singular matrix).
Further, with solid mechanics and air there is no need to use the moving mesh module, if I read the documentation correctly.
Whoops -- I've deviated substantially from my original question, which was whether "pA" is specified as a global parameter definition.
1 Reply Last Post May 28, 2015, 2:58 p.m. EDT
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Posted:
9 years ago
Hello Daniel,
If I understand your system correctly, you have some flexible component (A membrane, say) that deforms because a vacuum condition is applies to one side of it. If that's the case, you will want to deactivate the structural mechanics physics in the air region altogether, and would model the pull force as a distributed normal force (i.e. a pressure) boundary condition on the membrane.
I hope I didn't completely misunderstand what you're trying to model.
Jeff
If I understand your system correctly, you have some flexible component (A membrane, say) that deforms because a vacuum condition is applies to one side of it. If that's the case, you will want to deactivate the structural mechanics physics in the air region altogether, and would model the pull force as a distributed normal force (i.e. a pressure) boundary condition on the membrane.
I hope I didn't completely misunderstand what you're trying to model.
Jeff