Adiabatic boundary condition at channel flow outlet

[ramirofreile]

Hello everyone,

I am currently doing a phase change simulation using the Total Enthalpy method, to study blocking lengths as a function of different dimensionless numbers.

The current geometry of interest is a rectangular channel and it consists of an inlet, an outlet, and top and bottom walls.

For the fluid lattice (D2Q9), I set the velocity at the inlet and set the pressure at the exit. No-slip at the walls using bounce back.

For the advection diffusion lattice (D2Q5), the top and bottom walls are subdivided into three segments: an inlet adiabatic segment, a cooled segment and an exit adiabatic outlet segment. For the boundary conditions I set a hot inlet temperature (enthalpy) at the inlet using setRegularizedTemperature and I set a cold temperature in the cooled segments of the top and bottom wall also with setRegularizedTemperature. For the remaining adiabatic segments (top and bottom inlet segment, top and bottom outlet segment and outlet) I am applying bounce back boundary conditions.

After running the simulation, solid is formed in the cooled segment, which makes sense and it is what we expect. However, I am observing that at the outlet there is an inward heat flux that heats up the fluid from the outlet to the bulk. The temperature at the outlet is even larger than that of the inlet temperature, which should physically be the maximum of the domain. This is not what I would expect.

I am wondering if I am prescribing the adiabatic boundary condition incorrectly. I have also tried using the setRegularizedHeatFlux function to prescribe a zero flux, although this also failed.

There may be better way to prescribe adiabatic boundary conditions at the outlet and walls which I might not be aware of. Is someone familiar with a possible solution to this problem? Any insight is highly appreciated. Thanks!

Ramiro

[mgaedtke]

Hi Ramiro,

great to hear, that you had success in setting up the total enthalpy method to at least some extent.

For adiabatic outlet boundaries openlb provied a special convection boundary type, which you can set via setAdvectionDiffusionConvectionBoundary. For the other, stationary adiabatic boundaries, bounce back is what I used in my simulations, too.

Best regards,
Max

[ramirofreile]

Dear Max,

Thank you for your answer!

1) I just noticed that the setAdvectionDiffusionConvectionBoundary is only coded for 3D (correct me if I am wrong).

2) I am just wondering if you could help me understand what is the difference between the bounce back and the setAdvectionDiffusionConvectionBoundary. I am guessing that this comes from the fact that in the bounce back the equilibrium functions are set with a zero velocity. Could you guide me towards the answer maybe with a reference of the equations?

Thank you again for your time.

Ramiro

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