# Pressure boundary conditions #1896

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merged 7 commits into from Aug 21, 2017

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### fparisio commented Aug 3, 2017

 Normal mechanical stress at external boundaries. Tests include thick walled cylinder and sphere in bi-dimensional, tri-dimensional conditions and with elastic and plastic material models.

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### wenqing reviewed Aug 7, 2017

 // // Special boundary conditions // if (type == "Pressure")

#### wenqing Aug 7, 2017

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Pressure is a general name, and this sort of BC is only for normal stress or normal traction BC. Therefore, I think it would be good to use NormalTraction or NormalStress for this specific BC type.

#### wenqing Aug 21, 2017

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Still, I think Pressure may not be a good BC type name. There might be fluid pressure on boundaries in coupled THM processes, which is Dirichlet type. It could be MechanicalPressure.

#### nagelt Aug 21, 2017

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+1

 GlobalDimVectorType element_normal(GlobalDim); // TODO Extend to rotated 2d meshes and line elements.

#### wenqing Aug 7, 2017

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Extend to axisymmetry? That case could be included in shape_matrices_u[ip].integralMeasure.

#### endJunction Aug 7, 2017

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The rotation does not refer to the axisymmetry but to 2D-meshes in non-xy-plane.

 } } void assemble(std::size_t const id,

#### wenqing Aug 7, 2017

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What is this member for? I can only see the calculation of \sum {w * Nu * p} without detJ.

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#### wenqing Aug 7, 2017

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I see that PressureBoundaryConditionLocalAssembler calculates Nu*n and w, and assemble() computes the traction BC along with time.

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### xingyuanmiao commented Aug 7, 2017

 @endJunction @fparisio Deviation between the numerical and analytical results is found in plane strain elastic cylinder, axisymmetric elastic sphere (where notable difference is found in comparison of displacement u_r), 3d elastic sphere, ..., any idea about the difference? Also, typos are found in document.
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### fparisio commented Aug 7, 2017

 @xingyuanmiao We discussed today with @chleh, @nagelt and @endJunction and probably the cause is related to the fact that pressure is normal to the linearized boundary, not to the "real" curved one. Partly the difference is due to interpolation (a small deviation, as confirmed by axisymmetric pipe test with a linear boundary) and partly to the non-linear geometry. Next fix of pressure_bc with quadratic elements should fix the issue. I will check for typos.
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### xingyuanmiao commented Aug 8, 2017

 Thank you! Waiting for the update. :)

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### renchao-lu commented Aug 16, 2017

 @fparisio I have read your documentation and have had the following major comments so far There shows great discrepancy at r=1.0 mm in the elastic cylinder case under plane strain conditions (see Figure 1) while under axisymmetric conditions the numerical simulation reproduces the analytical solution (see Figure 2). If the discrepancy arises from the issue of nodal interpolation, why isn't the discrepancy shown in the latter? Could you please use another legend in Figure 6? Using a solid line with empty circles as a legend may make readers confused that the discrepancy shown in the elastic and plastic cases has been resolved, as the solid line and the empty circle, representing the numerical and analytical results respectively, are used in the preceding result comparison.

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### endJunction commented Aug 16, 2017

 Set this to WIP because there is an update needed after merging of the non-uniform BCs.
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### fparisio commented Aug 16, 2017

 @renchao-lu Thank you for the comments. Question 1 was already answered and is related to curved boundary. In the documentation, a final note was also added. I understand question 2 but I do not agree, the legend is clear and symbols are just helpful to identify at which nodes quantities are computed. Also, the caption and the reference text clearly explain the figure, so that the attentive reader could not misunderstand it.

### endJunction and others added some commits Dec 2, 2016

 [PL] BCs: Relax requirement of <component> tag. 
Some boundary conditions might not be applicable to a specific component
but to all components. In this case the optional component id is not
initialized and it is no longer an error.
 4cdbf04 
 Add pressure BC ctests. 
 915db79 
 [PL] BCs: Implement pressure boundary condition. 
 9c21b9d 
 Added documentation for Pressure_BC 
 8cde9a8 
 Smaller mesh for the plastic sphere 
 fb146ec 
 [PL] PressureBC: Document prj file parameter. 
 8ceae79 
 Updated documentation 
 d62d689 

### endJunction merged commit 5f69185 into ufz:master Aug 21, 2017 3 checks passed

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### nagelt commented Aug 21, 2017 • edited

 @fparisio @xingyuanmiao @renchao-lu @endJunction @chleh As the issue was raised repeatedly please find attached some results from another FE simulator which reproduces the behaviour Francesco observed (sigma_zz increasing on the inside despite the constant analytical solution for a plane-strain pipe). I used other material parameters, so don't pay attention to the absolute numbers. As correctly stated in the above discussion, this is due to the linear approximation of the curved boundary. Also note that these are linear elements and stresses are proportional to the gradient of the primary variable. If we refine the mesh (second image) the effect decreases as we approximate gradients better as well as the curved boundary.
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### renchao-lu commented Aug 21, 2017

 @fparisio @xingyuanmiao @nagelt @endJunction @chleh @wenqing It can be seen from the tests with both coarse and fine mesh grids that a limited error is distributed along the radial direction. The relative error is less than 1%. Using fine mesh grids for sure improves the simulation result. However, in the present plane strain case (implemented by ogs6), there shows a relative error which is clearly visible to the naked eye (grossly estimated greater than 10%) and merely appears at the inner boundary node rather than at all the nodes along the radial direction. Actually this is the point which I cannot understand. I have doubts if such a behaviour can be also attributed to the linear approximation of the curved boundary? Maybe I am too stupid.
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### nagelt commented Aug 22, 2017 • edited

 @renchao-lu nobody here is stupid. It's a valid point. See the reply regarding the extrapolation residual (in conjunction with nodal averaging). We have another test coming up that will hopefully clarify this! @chleh I have another question regarding the extrapolation residual. Let's discuss once you're here.

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