[aspect-devel] Couette flow solution

[Juliane Dannberg]

Hi Felipe,

What happens in your model is that because you did not prescribe 
boundary conditions for the side walls, these boundaries are basically 
open boundaries, where free in- and outflow can occur, but the normal 
stress applied at these side walls is zero everywhere. Because the 
pressure in your model is the hydrostatic pressure, you get an outflow 
at the bottom and an inflow at the top.

What you probably want to do is to apply traction boundary conditions 
(you can find the available options in the manual under "Boundary 
traction model"). These plugins allow you to prescribe a traction that 
is applied at your model boundary. At the moment you can prescribe a 
function, or read in a data file, but Anne Glerum is also working on a 
1D lithostatic pressure profile as traction boundary condition (see pull 
request #1044 <https://github.com/geodynamics/aspect/pull/1044>).

Best,
Juliane


On 01/05/2017 09:54 PM, FELIPE ORELLANA ROVIROSA wrote:
>
>  Hi all,
>
>    I have a question on Aspect Stokes solver.
>
>    I am simulating something that includes a Couette flow, where my 
> model box is a control volume and the top boundary (plate) moves 
> creating a velocity boundary layer by viscous dragging.
>    Velocity BCs: I am keeping the front (y=0) and back (y=8e5) walls 
> as tangential, the bottom (z=0) fixed, and prescribing a velocity on 
> top (z=5e5, where V= 0.1 m/yr ; 0 ; 0). My experiment: I am NOT 
> writing anything about the left and right walls (upstream receiver 
> (x=0) and downstream window (x=1e6) of the flow directed by the X 
> velocity BC on top.
>
>    What I get are fluid velocities ~2 orders of magnitude above the 
> top BC imposed velocity, and what happens at both the left and right 
> walls can be interpreted as local mass conservation: the upper halves 
> has an inflow, the lower halves an outflow, they vary continuously 
> having a zero in the middle.
>
>   What is Aspect doing here? correct me if I am wrong, as I am trying 
> to understand:
>
>    Because I did not provide a BC on those walls, the system is 
> UNDER-determined? Then, it solves the unknowns via a compensated 
> solution containing inflow and outflow along the X coordinate (and 0 s 
> on the other coordinates)?
>
>    But why does not Aspect compensates the inflow /outflow between the 
> walls?, say inflow at left, outflow at right, and instead it does 
> inflow/outflow on each wall.
>
>    Is there any way to instead, provide BCs for pressure (p), plus 
> Vy=0, Vz=0, and let Aspect find Vx on those walls (and on the 
> interior)? or maybe Aspect just can not compute variables on the 
> boundaries..
>
> thanks,
>
> Felipe
>
>
>
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