I have a question about indices of strain rate tensor.

I tried to calculte ISA rotation timescale, grain lag angle in Postprocess section, all of these values were 0.

So, I tried to calculate these values myself. But I couldn’t find its indices.

Fllowing Kaminski and Ribe (2002) or Conrad et al. (2007), velocity gradient tensor defined in spherical coordinate system.

But indices of output strain_rate_tensor from ASPECT were 0, 1, 2, 3, 4, 5, 6, 7, 8.

What id the meaning of each index?
Is it x,y,z coordinates (Cartesian)?
If it is the cartesian coordinates, how I can convert it to spherical coordinates in ASPECT software?

@kjun825 I assume that you mean the strain rate as output into a visualization file (.vtu)? If so, the 9 components of the strain rate tensor are ordered as xx, xy, xz, yx, yy, yz, zx, zy, zz – i.e., row-wise through a 3x3 tensor in Cartesian coordinates.

ASPECT itself does not currently have a way to output these tensors in spherical coordinates. But you can probably convert to spherical coordinates during visualization in Paraview or Visit. You might have to read through the manuals of these software packages on how to define derived quantities.