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Boundary condition confusion. Engine exhaust manifold ventilation.

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Old   July 8, 2016, 19:34
Exclamation Boundary condition confusion. Engine exhaust manifold ventilation.
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Hello all,

First off apologies ahead of time for the large wall of text!

I am hoping someone will be able to give me some guidance on my problem. Currently I have spend weeks and many hours of trial+error, searching the forums, researching the openFOAM user handbook, etc. but alas I have not confidently solved these very important CFD simulations.

A little background:
I am using OpenFOAM 2.4.0 with Helyx-OS 2.3.1 GUI on Linux Ubuntu 16.04.

The Case Model:
A 2D representation of the model may be seen in the "Boundary_Conditions.jpg" attachment. The case involves a manifold with a constant temperature, an inlet, an outlet, and the surrounding engine cover. Both inlet and outlet can be approximated as being atmospheric pressure.
Simulation 1:
This is the easier of the two simulations. The purpose is to find the outlet velocity and temperature by natural convection of the manifold and inlet temperature alone.

Engine Manifold Cover:Patch="wall (no slip)", Thermal = "zero gradient"
Outlet: Patch= Patch, Velocity= "Inlet Outlet Velocity" inlet value = [0,0,0], Pressure= "Fixed value" 0 m^2/s^2, Thermal= "zero gradient"
Inlet: Patch="Patch", Velocity="Pressure Inlet Velocity", Pressure= "zero gradient", Thermal = "Fixed Temperature"
Manifold: Patch = "wall (no slip)", Thermal = "Fixed Temperature"

Simulation 2:
The purpose of this simulation is to find the internal temperature of the engine cover and outlet temperature when a known velocity is applied to the outlet. I understand typically a velocity is applied to the inlet, but for this case a fan will be applied at the outlet to create a vacuum, thus I cannot confidently estimate the inlet velocity (besides possibly approximating using the energy equation/bernoulli's equation but I would rather apply an outlet velocity).

Engine Manifold Cover:Patch="wall (no slip)", Thermal = "zero gradient"
Outlet: Patch= "Patch", Velocity= "Fixed Value", Pressure= fixed value 0 m^2/s^2, Thermal= zero gradient
Inlet: Patch=Patch, Velocity="Pressure Inlet Velocity", Pressure=zero gradient, Thermal = "Fixed Temperature"
Manifold: Patch = "wall (no slip)", Thermal = Fixed Temperature

The model for both simulations are buoyantboussinesqsimplefoam as I have the energy equation and gravity tuned on. The simulation is also at steady state.

Typically I run these cases on Laminar, but have had more success using k-W SST and setting at turbulence to zero gradient (as I have no idea how to work with/apply/approximate turbulence intensity and mixing)

From what I can gather on the forums and research, the mesh is very important to a good simulation. Typically I try to keep the mesh as coarse as possible without compromising the integrity (e.g. holes in the mesh, unrealistic shapes etc.) due to other posts I have read about regarding compounding error with a finer mesh and difficulties converging the solution. I do run the check-mesh command to ensure that there are no other issues such as overly skewed cells. I have spent quite a bit of time trying to learn different functions of SnappyHexMesh that is included with Helyx-os.

Honestly, most of the residuals have been pretty garbage. The density will not regularly converge below 1e-2. I have many examples if you would like to see any. I will adjust the relaxation factors in order to try to converge the residuals quicker, but I have learned that it will not help long term with converging solutions.

As pointed out by the residuals, typically the CFD will not converge. Not only through residuals but by monitoring the variables (variable changes each time-step).

What else I have tried:
There has been a lot of trial and error with this problem. I have tried adjusting the boundary conditions to zero gradient, pressure inlet outlet, pressure directed inlet outlet, inlet outlet velocity, freestream, etc.

I have tried looking up what each boundary condition does, but the documentation is limited or vague so I am not exactly sure which boundary condition is suitable for which scenario.

Anyways, thank you for reading and any assistance would be greatly appreciated. I am more than willing to give any information that I may have missed or help explain my simulation in greater detail. It has been frustrating, but a great learning experience.

Attached Images
File Type: jpg Boundary_Conditions.JPG (23.2 KB, 29 views)
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Old   July 12, 2016, 17:53
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Bump, please.
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boundary conditions, buoyantboussinesqsimple, helyx-os, openfoam, thermal

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