I have been getting some sliding velocities that look a bit strange to me using the Weertman relation. They are however not completly unrealistic and wanted to hear if other users are seeing similar sliding patterns. It looks similar to the post: Small basal velocity value patch from Adjoin inverse method. In that post however a inverse problem is solved where I am only working on a forward problem solving a diagnostic setup.

The picture below shows norm basal velocity (left) and norm surface velocity (right). I find it strange that sliding velocities are not smooth. At one spot sliding is >100 m/yr while right next to it almost zero. The total domain is 40x20 km with 200 meters between nodes. I would expect sliding velocity to be more like the surface velocity (however smallere in magnitude of course) when nothing but surface and bed topography is given to the model.

I am using the navier-stokes solver (no parstokes) with the following boundary condition,

- Code: Select all
`!! bedrock:`

Boundary Condition 3

Name = "bedrock"

Target Boundaries = 5

Flow Solution Name = string "Flow Solution" ! and that is its name

Normal-Tangential Velocity = Logical True

Flow Force BC = Logical True

Velocity 1 = Real 0.0

Vxb = Equals Velocity 2

Vyb = Equals Velocity 3

Slip Coefficient 2 = Variable Coordinate 1

Real Procedure "ElmerIceUSF" "Sliding_Weertman"

Slip Coefficient 3 = Variable Coordinate 1

Real Procedure "ElmerIceUSF" "Sliding_Weertman"

Weertman Friction Coefficient = Real 0.0283

Weertman Exponent = Real 0.5

Weertman Linear Velocity = Real 0.00001

End

I have tried different Friction Coefficients and Exponents.

To extract basal values I have tried a couple of different methods. Passing the raw vtk data in python and extracting all bed nodes and using the ExportVertically solver to get velocity values at the basal boundary condition. Both methods give the same results.

I am however not sure if the "Normal-Tangential Velocity" call in the BC results in permanent rotation of all velocities components along the bed. Do I have to rotate them back to align with the global coordinate system or are values in the VTK files already aligned with the global coordinate system?

Is this a "normal" sliding pattern for the weertman sliding law in Elmer or do I need to postprocess the solution in some way?

Cheers,

Christian