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interFoam (HELYX-OS) pressure boundary conditions |
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May 11, 2016, 04:49 |
interFoam (HELYX-OS) pressure boundary conditions
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Join Date: May 2016
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Hello:
I’m using OpenFOAM (2.4.0) with interFoam via the HELYX-OS GUI (version 2.3.1) in Linux Ubuntu 14.04. A summary of my problem is that when I use pressure boundary conditions, I get what seem like surprisingly low velocities compared to another code. I don't have gravity in my problem, so I don't think this discrepancy can be due to hydrostatic pressure. I want to model two-phase fluid flow out of a tank. The fluid leaves the tank through a narrower outlet. There are two layers of fluid. An overview of the geometry is attached. I need to use a pressure boundary condition at the bottom of the tank. As a first step, I have tried to compare results with results from a simple example calculated using another code (an older code based on Galerkin finite elements), to check that everything is in order. The example is as simple as possible: incompressible flow, no gravity, the same density and viscosity values in the two fluid layers (1 kg m-3 and 1 Pa s, respectively; see geometry diagram). I’m not experienced in fluid dynamics or OpenFOAM, so I chose these simple inputs to avoid any possible misunderstandings on my part about p_rgh, etc. Results for a velocity inlet boundary condition are pretty similar for the two codes. However, there is a major difference for a pressure inlet boundary condition. The other code calculates that the interface between the two fluid layers moves significantly faster compared to interFoam. If I want the interface to move at a similar velocity in the two codes, I have to decrease the inlet pressure about three orders of magnitude in the other code, which seems extreme. I have tried all combinations of pressure boundary conditions at the inlet and outlet that are relevant to pressure and available in HELYX-OS (e.g., total pressure or fixed value at the inlet with total pressure, fixed value, or zero-gradient at the outlet; alpha1/phase fraction at inlet of “Inlet Outlet” or fixed value, alpha1/phase fraction at outlet of “Inlet Outlet” or fixed value or zero gradient). However, no matter which combination I use, there is no significant difference in the calculated velocities. An example of what I have tried is: Inlet: alpha1/Phase Fraction = Inlet Outlet, inlet value 1 U = Pressure Inlet Outlet Velocity p_rgh = Fixed Value, pressure 10 m2 s-2 Outlet: alpha1 = Zero Gradient U = Pressure Inlet Outlet Velocity p_rgh = Fixed Value, 0.0 m2 s-2 I created my geometry in Blender and have ensured that units are in meters. I have not found any existing examples/benchmarks that use interFoam with pressure boundary conditions. Also, I have seen a few comments in posts here that allude to possible complications with pressure boundary conditions and interFoam. I would be very surprised if the problem is with interFoam and is not some simple mistake I'm making somewhere, but I don't see where I'm going wrong. Over the last couple of months, I have looked at the other code and even asked its developer to take a look at my problem (in case the issue is with the other code and not OpenFOAM/HELYX-OS/interFoam), but we have not spotted anything that seems problematic. Does anyone have any hints or knowledge about possible pressure boundary condition problems in interFoam? Or anything else that might be the cause of my problem? Are there any other solvers that have been benchmarked for two-phase flow that I could try instead? As a side question, I understand that pressure boundary conditions must be entered as pressure/density for interFoam. I don’t understand how this would work for a layered fluid in which the two fluids have distinct densities. What would be the value of density for determining the pressure? For my current very-simple comparison, I avoid this issue by using a value of 1 kg m-3 for the density of both fluid phases. Thank you very much. Last edited by SFr; May 27, 2016 at 07:57. Reason: Clarification. I added a summary in case people are getting scared off answering my question because of all the detail/text. |
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Tags |
boundary condition, helyx-os, interfoam, openfoam, pressure |
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