Numerical Simulation Validation

Muhammad Shoaib · March 29, 2018, 06:23

Hi Everyone! I am working on ANSYS Fluent for porous media fluid flow.. I am trying to do some simple cases to know about all the possible options and their importance. I am trying to simulate multiphase horizontal flow in a 2D parallel plate (Dimensions - 2microns x 100micons). Inlet and Outlet pressures are 0 Pa. VOF model is used, interfacial tension water/oil equal to 0.03N/m and contact angle 30deg (water-wet). Initialization with 100% oil. Water is allowed to enter from inlet due to interfacial tension force only (spontaneous imbibition). Velocity vs. flow time was obtained from numerical simulation, and later compared with analytical solution.

Different mesh sizes were used, 2 microns was divided into 5, 7, 10, 11, 20 meshes. Velocity vs flow time from analytical and numerical simulation matches exactly with 7 meshes in vertical direction. Increasing number of meshes in vertical direction, explicit VOF shows high residual of continuity and interface curvature is not smooth. I also tried to use Implicit VOF, interface curvature becomes smooth but velocity simulated is lower compared to analytical as in the previous case of explicit VOF.

Can someone tell me why continuity residual is higher in case of large number of meshes in vertical direction for explicit VOF, as I was expecting the opposite? Any other help will be highly appreciated.

Thanks!

LuckyTran · March 30, 2018, 16:22

Continuity residual is not scaled in a smart way. It is normalized by the worst residual in first 5 iterations. So depending your initial guess and solver settings, you can make the continuity residual be anything. I would compare something else and not the continuity residual, it is quite useless except for seeing if the solution is more-like-converging or diverging.

March 29, 2018, 06:23	Numerical Simulation Validation	#1
Muhammad Shoaib New Member Muhammad Shoaib Join Date: Mar 2018 Posts: 1 Rep Power: 0	Hi Everyone! I am working on ANSYS Fluent for porous media fluid flow.. I am trying to do some simple cases to know about all the possible options and their importance. I am trying to simulate multiphase horizontal flow in a 2D parallel plate (Dimensions - 2microns x 100micons). Inlet and Outlet pressures are 0 Pa. VOF model is used, interfacial tension water/oil equal to 0.03N/m and contact angle 30deg (water-wet). Initialization with 100% oil. Water is allowed to enter from inlet due to interfacial tension force only (spontaneous imbibition). Velocity vs. flow time was obtained from numerical simulation, and later compared with analytical solution. Different mesh sizes were used, 2 microns was divided into 5, 7, 10, 11, 20 meshes. Velocity vs flow time from analytical and numerical simulation matches exactly with 7 meshes in vertical direction. Increasing number of meshes in vertical direction, explicit VOF shows high residual of continuity and interface curvature is not smooth. I also tried to use Implicit VOF, interface curvature becomes smooth but velocity simulated is lower compared to analytical as in the previous case of explicit VOF. Can someone tell me why continuity residual is higher in case of large number of meshes in vertical direction for explicit VOF, as I was expecting the opposite? Any other help will be highly appreciated. Thanks!

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March 30, 2018, 16:22		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	Continuity residual is not scaled in a smart way. It is normalized by the worst residual in first 5 iterations. So depending your initial guess and solver settings, you can make the continuity residual be anything. I would compare something else and not the continuity residual, it is quite useless except for seeing if the solution is more-like-converging or diverging.