# [aspect-devel] boundary velocity function expressions

John Naliboff jbnaliboff at ucdavis.edu
Mon Oct 10 21:54:06 PDT 2016

```Hi Payman,

> However, how can I prescribe velocity to a segment of a boundary and let the rest of that boundary be tangential?

In effect, you would like part of a vertical wall (right boundary) to be free-slip, while the other part has the vertical velocity prescribed?  Hmm, I’m not sure ASPECT is setup to do a ‘mixed’ velocity boundary.  In fact, I’m having a hard time thinking of a study that does this.

In the case you describe below, do you need to prescribed both the vertical velocity and not just the horizontal velocity?

Cheers,
John

*************************************************
John Naliboff
Assistant Project Scientist, CIG
Earth & Planetary Sciences Dept., UC Davis

> On Oct 10, 2016, at 7:22 PM, Payman Janbakhsh <payman.janbakhsh at mail.utoronto.ca> wrote:
>
> Thank you John,
> Actually the continental extension prm file that you sent to Mohamed helped me to figure out the velocity prescription to different boundaries using functions.
>
> However, how can I prescribe velocity to a segment of a boundary and let the rest of that boundary be tangential?
> For instance, considering a right boundary, a mantle lithospheric slab having its own westward prescribed velocity, but the rest of that boundary (mantle) be tangential.
>
> Thanks again
>
> Payman
>
> From: Aspect-devel [mailto:aspect-devel-bounces at geodynamics.org] On Behalf Of John Naliboff
> Sent: October 10, 2016 12:25 PM
> To: aspect-devel at geodynamics.org
> Subject: Re: [aspect-devel] boundary velocity function expressions
>
> Hi Payman,
>
> Specifying the velocity on the boundaries works a bit differently than what you have outlined below.
>
> In 2D you need to specify velocity components for x and y, which you do via the function.  You can control how the velocity on different boundaries (ex: left & right) by including conditional statements in your functions based on the position (defined by 'x' and 'y' in 2D).
>
> You can also specify that only certain components of the function apply to a certain boundary (i.e. only x velocity from functions is applied to left/right boundary).
>
> If there is a specific setup you are thinking of, can you sent over a basic parameter file that we can look over?
>
> Cheers,
> John
>
> *************************************************
> John Naliboff
> Assistant Project Scientist, CIG
> Earth & Planetary Sciences Dept., UC Davis
> On 10/09/2016 03:08 PM, Payman Janbakhsh wrote:
> Hi,
> I’m trying to implement the following for a 2D problem:
>
> Set Prescribed velocity boundary indicators = left : function  ,  right : function
>
> But get error when trying to give  Function expressions in:     set Function expression =
> I have tried separating 2 velocity components with semicolon and separate function expressions with comma .
> Example:
>                                                             Left boundary function                  right boundary function
> Set Function expression =             if(z>=1000 , 1, 0) ; 0           ,           if(z>=1000, -1,0) ; 0
>
> I’ve tried many ways including below but all give errors or nonsensical velocity Glyphs:
> Set Function expression =          left:   if(z>=1000 , 1, 0) ; 0           ,     right:      if(z>=1000, -1,0) ; 0
> Set Function expression =           left =   if(z>=1000 , 1, 0) ; 0           ,    right=       if(z>=1000, -1,0) ; 0
> Set Function expression =           (  if(z>=1000 , 1, 0) ; 0     )      ,       (    if(z>=1000, -1,0) ; 0 )
> …
>
> How can I separate multiple function expressions each for its corresponding boundary indicator ?
>
> Thanks
>
> Payman
>
>
>
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