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Fix stress #3897

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merged 7 commits into from Nov 20, 2020
Merged

Fix stress #3897

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ce3b6f1
Fix sign of shear stress
anne-glerum Nov 18, 2020
c5b4afe
Fix sign surface stress plus comments
anne-glerum Nov 19, 2020
76b3a7d
Update descriptions
anne-glerum Nov 19, 2020
9881812
Add changelog
anne-glerum Nov 19, 2020
9444fef
Indent
anne-glerum Nov 19, 2020
d2c0c63
Update test comment
anne-glerum Nov 20, 2020
fcb1a85
Update tests
anne-glerum Nov 20, 2020
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6 changes: 6 additions & 0 deletions doc/modules/changes/20201119_glerum
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@@ -0,0 +1,6 @@
Fixed: The visualization postprocessors that output stresses now use the
correct sign for the shear stress term (under the convention of positive
compressive stress), i.e. -2*eta*strain_rate+pressure*I instead of
2*eta*strain_rate+pressure*I.
<br>
(Anne Glerum, Sungho Lee, 2020/11/19)
10 changes: 6 additions & 4 deletions source/postprocess/visualization/shear_stress.cc
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Expand Up @@ -73,7 +73,8 @@ namespace aspect

const double eta = out.viscosities[q];

const SymmetricTensor<2,dim> shear_stress = 2*eta*compressible_strain_rate;
// Compressive stress is positive in geoscience applications
const SymmetricTensor<2,dim> shear_stress = -2.*eta*compressible_strain_rate;
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The "compressible_strain_rate" variable is actually the incompressible part of the strain rate. Ditto for the other two files. Suggest changing the name to incompressible_strain_rate?

This is my only quibble :)

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Thanks for having a look. It depends on what the material model returns for (this->get_material_model().is_compressible() whether the strain rate in compressible_strain_rate is in fact compressible or the incompressible part. So the name is not always correct. But that's not really related to this PR, so I'll leave that for now, OK?

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Hmm, the relevant code above is:

= (this->get_material_model().is_compressible()
?
strain_rate - 1./3 * trace(strain_rate) * unit_symmetric_tensor()
:
strain_rate);

Even if the material is compressible, the pressure is removed (-1/3 * trace(strain_rate)) and thus you have the deviatoric strain rate. If an incompressible formulation is used, then should the strain rate by default be deviatoric?

If the above is correct, would deviatoric_strain_rate be the most accurate name?

No preference on whether this is resolved now or in a follow-up PR, but will leave that up to @bobmyhill.

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@naliboff Yes, that's what I was trying to say. Thanks for clarifying!
I'm fine for the rename to be a separate PR.


for (unsigned int d=0; d<dim; ++d)
for (unsigned int e=0; e<dim; ++e)
Expand Down Expand Up @@ -103,13 +104,14 @@ namespace aspect
"A visualization output object that generates output "
"for the 3 (in 2d) or 6 (in 3d) components of the shear stress "
"tensor, i.e., for the components of the tensor "
"$2\\eta\\varepsilon(\\mathbf u)$ "
"$-2\\eta\\varepsilon(\\mathbf u)$ "
"in the incompressible case and "
"$2\\eta\\left[\\varepsilon(\\mathbf u)-"
"$-2\\eta\\left[\\varepsilon(\\mathbf u)-"
"\\tfrac 13(\\textrm{tr}\\;\\varepsilon(\\mathbf u))\\mathbf I\\right]$ "
"in the compressible case. The shear "
"stress differs from the full stress tensor "
"by the absence of the pressure.")
"by the absence of the pressure. Note that the convention "
"of positive compressive stress is followed. ")
}
}
}
10 changes: 6 additions & 4 deletions source/postprocess/visualization/stress.cc
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Expand Up @@ -73,7 +73,8 @@ namespace aspect

const double eta = out.viscosities[q];

const SymmetricTensor<2,dim> stress = 2*eta*compressible_strain_rate +
// Compressive stress is positive in geoscience applications
const SymmetricTensor<2,dim> stress = -2.*eta*compressible_strain_rate +
in.pressure[q] * unit_symmetric_tensor<dim>();
for (unsigned int d=0; d<dim; ++d)
for (unsigned int e=0; e<dim; ++e)
Expand Down Expand Up @@ -103,11 +104,12 @@ namespace aspect
"A visualization output object that generates output "
"for the 3 (in 2d) or 6 (in 3d) components of the stress "
"tensor, i.e., for the components of the tensor "
"$2\\eta\\varepsilon(\\mathbf u)+pI$ "
"$-2\\eta\\varepsilon(\\mathbf u)+pI$ "
"in the incompressible case and "
"$2\\eta\\left[\\varepsilon(\\mathbf u)-"
"$-2\\eta\\left[\\varepsilon(\\mathbf u)-"
"\\tfrac 13(\\textrm{tr}\\;\\varepsilon(\\mathbf u))\\mathbf I\\right]+pI$ "
"in the compressible case.")
"in the compressible case. Note that the convention of positive "
"compressive stress is followed. ")
}
}
}
10 changes: 6 additions & 4 deletions source/postprocess/visualization/surface_stress.cc
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Expand Up @@ -63,7 +63,8 @@ namespace aspect

const double eta = out.viscosities[q];

const SymmetricTensor<2,dim> stress = 2*eta*compressible_strain_rate +
// Compressive stress is positive in geoscience applications
const SymmetricTensor<2,dim> stress = -2.*eta*compressible_strain_rate +
in.pressure[q] * unit_symmetric_tensor<dim>();
for (unsigned int i=0; i<SymmetricTensor<2,dim>::n_independent_components; ++i)
computed_quantities[q](i) = stress[stress.unrolled_to_component_indices(i)];
Expand Down Expand Up @@ -144,11 +145,12 @@ namespace aspect
"on the surface of the domain "
"for the 3 (in 2d) or 6 (in 3d) components of the stress "
"tensor, i.e., for the components of the tensor "
"$2\\eta\\varepsilon(\\mathbf u)+pI$ "
"$-2\\eta\\varepsilon(\\mathbf u)+pI$ "
"in the incompressible case and "
"$2\\eta\\left[\\varepsilon(\\mathbf u)-"
"$-2\\eta\\left[\\varepsilon(\\mathbf u)-"
"\\tfrac 13(\\textrm{tr}\\;\\varepsilon(\\mathbf u))\\mathbf I\\right]+pI$ "
"in the compressible case.")
"in the compressible case. Note that the convention of positive "
"compressive stress is followed. ")
}
}
}