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Pressure continuity between two domains

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I am modeling an ice sheet over a basal sediment layer using solid mechanics. Physically, I would expect pressure to be continuous between the top of the sediment layer and the bottom of the ice layer. I’ve been using a continuity boundary condition between these domains, because I also expect displacement to be continuous between them. With this boundary condition, displacement is continuous, but not pressure. Is there a way to also prescribe pressure/stress continuity between domains? I've attached a simplified, scaled down version of the model.



6 Replies Last Post 2021年8月20日 GMT-4 10:21
Edgar J. Kaiser Certified Consultant

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Posted: 3 years ago 2021年8月18日 GMT-4 15:49

Claire,

I can't test the model because I don't have your modules. In solid mechanics you don't need a continuity bc between domains. Comsol takes care of continuity if you finalize the geometry into a union.

Cheers Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Claire, I can't test the model because I don't have your modules. In solid mechanics you don't need a continuity bc between domains. Comsol takes care of continuity if you finalize the geometry into a union. Cheers Edgar

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Posted: 3 years ago 2021年8月18日 GMT-4 16:21

Thanks for your response, Edgar. I tried finalizing the geometry using a union, and the displacement solution is continuous, but there is still a pressure discontinuity between the domains. Here's a further simplified version (no creep nodes, so it doesn't need the geomechanics module anymore) that uses form union.

Thanks for your response, Edgar. I tried finalizing the geometry using a union, and the displacement solution is continuous, but there is still a pressure discontinuity between the domains. Here's a further simplified version (no creep nodes, so it doesn't need the geomechanics module anymore) that uses form union.


Jeff Hiller COMSOL Employee

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Posted: 3 years ago 2021年8月18日 GMT-4 16:56
Updated: 3 years ago 2021年8月18日 GMT-4 16:56

Hi Claire,

Pressure is generally not continuous at the interface between two different materials.

At such an interface with normal direction z, only sigma_zz, sigma_zx and sigma_zy are continous in general, so that doesn't tell you anything about continuity of p=-(sigma_xx+sigma_yy+sigma_zz)/3.

Best,

Jeff

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Jeff Hiller
Hi Claire, Pressure is generally not continuous at the interface between two different materials. At such an interface with normal direction z, only sigma_zz, sigma_zx and sigma_zy are continous in general, so that doesn't tell you anything about continuity of p=-(sigma_xx+sigma_yy+sigma_zz)/3. Best, Jeff

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Posted: 3 years ago 2021年8月19日 GMT-4 13:13

Thanks Jeff. That makes sense, I wasn't thinking closely about that. The sigma_zz, sigma_zx, and sigma_zy components are continuous in the simplified model, so that's good.

The actual problem I'm solving is more complicated however. For the real case, I’m using topography data and importing the mesh. I’ve made an identity pair between the ice and the sediment and set a continuity boundary condition for that pair. In this case, the sigma_zz component is not continuous between the ice and sediment everywhere along the boundary (see attached). The normal direction is not in the z direction everywhere, but it is close near where the stress discontinuities are largest, so I think there is some problem with the solution. Any ideas about that?

Thanks Jeff. That makes sense, I wasn't thinking closely about that. The sigma_zz, sigma_zx, and sigma_zy components are continuous in the simplified model, so that's good. The actual problem I'm solving is more complicated however. For the real case, I’m using topography data and importing the mesh. I’ve made an identity pair between the ice and the sediment and set a continuity boundary condition for that pair. In this case, the sigma_zz component is not continuous between the ice and sediment everywhere along the boundary (see attached). The normal direction is not in the z direction everywhere, but it is close near where the stress discontinuities are largest, so I think there is some problem with the solution. Any ideas about that?


Jeff Hiller COMSOL Employee

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Posted: 3 years ago 2021年8月19日 GMT-4 14:22
Updated: 3 years ago 2021年8月19日 GMT-4 15:38

The thing is, even when the stress tensor component in the (usually unavailable) analytical solution is continuous, the corresponding FEA stress computed from nodal displacements is not continuous across elements (in the general case). The FEA stress component will become closer and closer to being continuous as you refine the mesh. Plotting stress components with any smoothing disabled is actually an old-school way of assessing if the mesh is fine enough: if the stresses are noticeably discontinous across element boundaries, that can be taken as a sign that your mesh is not fine enough, see e.g. "Finite Element Procedures" by (my grad school advisor! - "Hello!" if you read this) K.J. Bathe.

Best,

Jeff

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Jeff Hiller
The thing is, even when the stress tensor component in the (usually unavailable) analytical solution is continuous, the corresponding FEA stress computed from nodal displacements is not continuous across elements (in the general case). The FEA stress component will become closer and closer to being continuous as you refine the mesh. Plotting stress components with any smoothing disabled is actually an old-school way of assessing if the mesh is fine enough: if the stresses are noticeably discontinous across element boundaries, that can be taken as a sign that your mesh is not fine enough, see e.g. "Finite Element Procedures" by (my grad school advisor! - "Hello!" if you read this) K.J. Bathe. Best, Jeff

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Posted: 3 years ago 2021年8月20日 GMT-4 10:21

Ok, that's good to know. I'll focus on refining the mesh then (I've had problems getting a higher resolution mesh that doesn't have too many low quality elements for this geometry, given the high aspect ratio, but that's a whole other topic).

Ok, that's good to know. I'll focus on refining the mesh then (I've had problems getting a higher resolution mesh that doesn't have too many low quality elements for this geometry, given the high aspect ratio, but that's a whole other topic).

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