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Strange boundary artifacts for two phase level set model

Posted 20.01.2015, 14:30 GMT-5 Fluid & Heat, Computational Fluid Dynamics (CFD), Mesh, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers 5 Replies

Jiangzhi Chen
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I built a model to investigate the case that viscous fluid from a pipe deforms on a horizontal surface. In the 2D axisymmetric model modified from the level set nozzle inkjet model, viscous fluid from a pipe enters a large cylinder which was all air. The inlet fluid velocity has a parabolic profile. The right vertical boundary is an outlet with constant pressure, and other boundaries are no slip walls. I attached two plots, where the red region is the fluid, and the blue region is air. The fluid puddle evolves fine, but at the right boundary, strange artifacts develop at later time, and will be more serious as time goes. I want to know how I can eliminate this artifact. I tried fixed mesh and adaptive mesh, but it does not help. Thanks.

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5 Replies Last Post 26.01.2015, 18:57 GMT-5
Frank van Gool COMSOL Employee

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Posted: 10 years ago 22.01.2015, 05:15 GMT-5
Dear Jiangzhi,

Probably fluid is entering from your outlet, and you have not set the fluid at that boundary.
Try to set it to "air", or close it completely to stop this effect.

Best regards,
Frank
Dear Jiangzhi, Probably fluid is entering from your outlet, and you have not set the fluid at that boundary. Try to set it to "air", or close it completely to stop this effect. Best regards, Frank
Ingo Schaarschmidt
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Posted: 10 years ago 22.01.2015, 06:48 GMT-5
Hello,
i set "supress backflow" to true in my similar simulations. You can find it under the pressure conditions in your outflow section.
Maybe this will fix the Problem, but i dont know.

Greets
Ingo
Hello, i set "supress backflow" to true in my similar simulations. You can find it under the pressure conditions in your outflow section. Maybe this will fix the Problem, but i dont know. Greets Ingo
Jiangzhi Chen
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Posted: 10 years ago 22.01.2015, 15:14 GMT-5
Thanks. I tried setting the boundary to air and suppressing backflow, but it does not help. And since the flow and air are incompressible, close the boundary is not applicable. I attach the model file here.
Thanks. I tried setting the boundary to air and suppressing backflow, but it does not help. And since the flow and air are incompressible, close the boundary is not applicable. I attach the model file here.
Frank van Gool COMSOL Employee

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Posted: 10 years ago 26.01.2015, 07:51 GMT-5
Dear Jiangzhi,
As you have gravity in your model, your outlet which is set to 0 Pa is not consistent. Please try setting the outlet to g_const*(4-z)*1.2[kg/m^3] so that you have the "correct" pressure on that boundary.

Depending on what you want to model, you can also set the topsurface to 0 Pa, and close off the side outlet.

Good luck with your model.

Best regards,
Frank
Dear Jiangzhi, As you have gravity in your model, your outlet which is set to 0 Pa is not consistent. Please try setting the outlet to g_const*(4-z)*1.2[kg/m^3] so that you have the "correct" pressure on that boundary. Depending on what you want to model, you can also set the topsurface to 0 Pa, and close off the side outlet. Good luck with your model. Best regards, Frank
Jiangzhi Chen
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Posted: 10 years ago 26.01.2015, 18:57 GMT-5
Thanks for the help, now the artifact is gone. Quick question, is there an easy way to convert 2D axisymmetric model to 3D model? I want to add some body forces to the fluid.
Thanks for the help, now the artifact is gone. Quick question, is there an easy way to convert 2D axisymmetric model to 3D model? I want to add some body forces to the fluid.

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