Two quick simple questions: creating a second time derivative kernel and a partial derivative kernel

[john.m...@uconn.edu]

I'm looking to write a kernel that just takes a second time derivative \frac{\partial^2 u}{\partial t^2} and was a bit confused as to go about this. I would have to create a new TimeDerivative.h and TimeDerivative.C file right?

In a seperate issue I'm a bit stumped when I want to do something like \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} as well or mix the derivatives \frac{\partial^2 u}{\partial x \partial y}. The introductory material slide(78/313) with (Some) values available to Kernels does not have a partial derivative with respect to one spatial variable listed but does say there are more options available. Is there a full list somewhere?

Thanks for the help.

John Mangeri
Graduate Student
University of Connecticut

[Daniel Schwen]

An alternative to creating a d^2/dt^2 kernel (which may not even be
easily possible) would be to split your equation into two first order
time derivatives, coupled through a "velocity" variable.

[Peterson, JW]

On Tue, Jun 17, 2014 at 3:38 PM, <john.m...@uconn.edu> wrote:
>
> In a seperate issue I'm a bit stumped when I want to do something like
> \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} as
> well or mix the derivatives \frac{\partial^2 u}{\partial x \partial y}. The
> introductory material slide(78/313) with (Some) values available to Kernels
> does not have a partial derivative with respect to one spatial variable
> listed but does say there are more options available. Is there a full list
> somewhere?

What equation are you solving, exactly?

If you actually need second derivatives, you might need to use C1 elements...

--
John

[Derek Gaston]

To see an example of doing the second derivative  in time you can look at moose/modules/solid_mechanics/src/kernels/SecondDerivativeImplicitEuler.C .  You can inherit from that and multiply by your own coefficients if you need to.

_grad_u is the spatial derivative of your unknown.  If you want just the "x" component of that it would be _grad_u[_qp](0) ... "y" is 1 and "z" is 2.  Normally... we don't access components like this though because we write everything in non-dimensional form.

You can get access to the second derivatives of your unknown with respect to space (x,y,z).  In order to do this you must do these things:

1.  Use shape functions that support higher derivatives (like THIRD order HERMITE ).

2.  Call "second()"  in your constructor initialization list ( http://mooseframework.com/docs/doxygen/moose/classMooseVariableInterface.html#a067843442f1d79be80a9ba6facd9e1ac )

For an example of #2 look at  moose/modules/phase_field/src/kernels/CHInterface.C

Finally: What equations are you solving?  We may be able to point you to existing implementations that are similar that you can either use directly or work with those developers to modify to fit your needs.

Derek

--
You received this message because you are subscribed to the Google Groups "moose-users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to moose-users...@googlegroups.com.
Visit this group at http://groups.google.com/group/moose-users.

To view this discussion on the web visit https://groups.google.com/d/msgid/moose-users/CANj%2BmwCO5gYpTA0zDBkk%2BousCgWodcTBxMobiiW3%3Dm8ZhDQgvA%40mail.gmail.com.

For more options, visit https://groups.google.com/d/optout.

[john.m...@uconn.edu]

Thank you Derek. The first solution was easily implemented. I was merely trying to create a wave equation example which worked out perfectly.

For the second problem I was trying to solve some elasticity equations(simple Hooke's law for a cube) but I've recently realized I can use kernels in the tensor_mechanics module instead of building the kernels myself. Is there any sort of documentation on implementing this module?

[Andrew....@csiro.au]

Hi John,

If you look in the tests (tensor mechanics tests, or combined tests) you will see some examples of linear elasticity.  Sorry I can’t specify more precisely - I’m in the middle of a meeting!

a

To view this discussion on the web visit https://groups.google.com/d/msgid/moose-users/cdb6213d-e4c0-47c3-889a-4d97121ca412%40googlegroups.com.

[Tonks, Michael R]

There is no documentation on the mooseframework.org wiki yet, but I will put some on soon. For now, I am attaching some slides on the tensor mechanics system that is part of the MARMOT training.

Mike

To view this discussion on the web visit https://groups.google.com/d/msgid/moose-users/CFC85DD6.139ED%25Andrew.Wilkins%40csiro.au.

For more options, visit https://groups.google.com/d/optout.

--

*************************

Michael R. Tonks

Fuel Modeling and Simulation Department

Idaho National Laboratory

208-526-6319

michae...@inl.gov

[john.m...@uconn.edu]

Excellent this is precisely what I was looking for.

Thanks!
John

[Daniel Schwen]

An alternative to creating a d^2/dt^2 kernel (which may not even be
easily possible) would be to split your equation into two first order
time derivatives, coupled through a "velocity" variable.

On Tue, Jun 17, 2014 at 3:38 PM, <john.m...@uconn.edu> wrote:

> --
> You received this message because you are subscribed to the Google Groups
> "moose-users" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to moose-users...@googlegroups.com.
> Visit this group at http://groups.google.com/group/moose-users.
> To view this discussion on the web visit

> https://groups.google.com/d/msgid/moose-users/bf15424a-d5f6-45c2-b71e-96de90b605b5%40googlegroups.com.

[jamesowl]

Hi,

I'm a new to MOOSE and I've also got questions about 2nd time derivative -- d^2 u / d t^2.

I'm splitting 2nd time derivative into two equations, coupled by a velocity variable "v"

d u / d t = v
d v / d t = ...

For this coupling, do I need to build a new TimeDerivative kernel that includes "coupling parameters" or "coupling in kernel" as those in Example03? or do I just use existing TimeDerivative kernel twice for each variable "u" and "v" in Kernel block in the input file?

Thanks in advance,
Xikai

[Peterson, JW]

On Thu, Mar 12, 2015 at 1:50 PM, jamesowl <james...@gmail.com> wrote:

Hi,

I'm a new to MOOSE and I've also got questions about 2nd time derivative -- d^2 u / d t^2.

I'm splitting 2nd time derivative into two equations, coupled by a velocity variable "v"

d u / d t = v
d v / d t = ...

For this coupling, do I need to build a new TimeDerivative kernel that includes "coupling parameters" or "coupling in kernel" as those in Example03? or do I just use existing TimeDerivative kernel twice for each variable "u" and "v" in Kernel block in the input file?

I don't think you need to write new code to achieve what you want, as written these are both simply TimeDerivative kernels (one for u and one for v).

 

--

John

[jamesowl]

Hi John,

Thanks for your quick help. I'm just started to build my first application.

Xikai