Dear all,
I am writing to ask for some suggestion regarding bc’s for a model which reproduces a mid-oceanic ridge in a domain which is 1000 km wide and 150 km deep.
Velocity bc’s are set in such a way so that there is a spreading at the top. At the left, right and bottom of the domain open bc’s are simulated using traction bc’s (i.e., initial lithostatic pressure). Problems arise when I try to add a migrating bottom, changing its bc’s to prescribe a velocity towards the right side of the model using the function:

In fact, I am expecting a counterflow within the mantle in the left side of the numerical domain, but I don’t get why I have a counterflow in the right side of the numerical domain (see attached picture), where I am expecting a coherent mantle/lithosphere flow towards the right side of the numerical domain.
I have the suspect that there is something wrong in the function I wrote, but I cannot find where is the error.

I think this may be related to the viscosity structure of your model. The upper half of your model is cold and hard to deform, so it moves with a similar velocity as what your prescribe at the surface. In the bottom half, the material is easier to deform. So material can flow in there to balance the mass in the model: If you only had outflow on the right, there would be more mass flowing out of the model than flowing in.
You could try and counterbalance that by leaving the vertical component of the velocity at the bottom free (and using the lithostatic pressure to prescribe a boundary traction) so that material can flow in from the bottom.

Dear Juliane,
thanks for your answer and sorry for the late reply.
I needed some time to think about how to try to do what you suggested, but I finally realized that I could easily manage the different components of velocity at boundaries using the bottom x, bottom y names.
The model is currently running, but the velocity fields seem correct now!