Menu

Discussion Forum

New Discussion
查找

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

seismic acceleration time history input

Posted 2018年9月12日 GMT-4 23:37 Structural & Acoustics, Geomechanics, Structural Mechanics Version 5.3a 1 Reply

Shigang Shi
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

I am modeling structure fixed on the ground，the acceleration time history need to be apllied to the boundary .So I have to set prescribed displacement/rotation and prescribed acceleration to the boundary to the same region simultaneously.But they cannot work together, the former is overriden by the latter. I want to know if the comsol can realize my idea,how. If it could not do this in this in my idea, some other mehod is also OK.

1 Reply Last Post 2018年9月13日 GMT-4 17:53
Henrik Sönnerlind COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 6 years ago 2018年9月13日 GMT-4 17:53

Hi,

For physical reasons, it is not possible to prescribe displacement and acceleration simultaneously for the same degree of freedom, since one is the second time derivative of the other.

If, however, you have a known displacement in one direction and a known acceleration in an orthogonal direction, it would be admissible from a physical point of view. Then you must use some kind of trick to circumvent the override rules. Some suggestions:

a) Integrate the acceleration twice, and enter both as prescribed displacements.
b) Compute the second time derivative of the displacement, and enter both as accelerations. This is less numerically stable, and also computationally more expensive that a)
c) You can also use a more raw approach, where you add a Pointwise Constraint in which you set the prescribed displacement. A Pointwise Constraint does not override other boundary conditions, so there are no checks against conflicts with a prescribed acceleration.

Regards,
Henrik

-------------------
Henrik Sönnerlind
COMSOL
Hi, For physical reasons, it is not possible to prescribe displacement and acceleration simultaneously for the same degree of freedom, since one is the second time derivative of the other. If, however, you have a known displacement in one direction and a known acceleration in an orthogonal direction, it would be admissible from a physical point of view. Then you must use some kind of trick to circumvent the override rules. Some suggestions: a) Integrate the acceleration twice, and enter both as prescribed displacements. b) Compute the second time derivative of the displacement, and enter both as accelerations. This is less numerically stable, and also computationally more expensive that a) c) You can also use a more raw approach, where you add a **Pointwise Constraint** in which you set the prescribed displacement. A **Pointwise Constraint** does not override other boundary conditions, so there are no checks against conflicts with a prescribed acceleration. Regards, Henrik

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content