# Clarifications about "Conjugate heat transfer" example

Hi,
i’m working on a project based on the example “conjugate heat transfer” where I have a hot inlet flow that heats a piece placed inside a channel. Since “conjugate heat transfer” is very similar to my project i have some questions about it:

• Why you add outlet constraint “pressure=0”? I thought that the best constraint is relative to have the same volumetric flow in the inlet and in the outlet, so (assuming the area of inlet is the same of the outlet) have something like this:
``````# Note my channel geometry is parallel to z axis
# Inlet
constraint_inlet = PointwiseBoundaryConstraint(
nodes=flow_nodes,
geometry=geo.square_inlet,
outvar={"u": 0, "v": 0, "w": inlet_vel},
batch_size=cfg.batch_size.Inlet,
lambda_weighting={
"u": 1.0,
"v": 1.0,
"w": 1.0,
},
criteria = pow(x, 2) + pow(y, 2) <= pow(geo.channel_radius, 2),
batch_per_epoch=5000,
)

# Outlet
constraint_outlet = PointwiseBoundaryConstraint(
nodes=flow_nodes,
geometry=geo.outlet,
outvar={"w": inlet_vel},
batch_size=cfg.batch_size.Outlet,
lambda_weighting={"w": 1.0},
batch_per_epoch=5000,
)
``````

I ask you if this have sense, and why in the example “conjugate heat transfer” you didn’t write the code like that and you preferred the constraint “pressure=0”.

• Is the use of integral constraint in the example similar to what i have explain in the last question? I ask this because from the example it seems that the integral planes is added with the constraint of have the same “normal_dot_vel”.
• Where i can find more information about the normal_dot_vel parameter and what it represent? If it represent something connected to the volumetric flow why didn’t you insert an integral plane also in the outlet?
• Inside your channel you don’t have constant pressure, but if i want to add this constraint, can this be done using “PointwiseInteriorConstraint”?
``````# Pressure constraint
pressure_constraint= PointwiseInteriorConstraint(
nodes=flow_nodes,
geometry=geo.all_geo,
outvar={"p": 3},
batch_size=cfg.batch_size.Interior,
lambda_weighting={"p": 1.0},
batch_per_epoch=5000,
parameterization=time_range,
)
``````

Thanks to your support I was able to solve various problems and I hope that this time too you can give me some help.

Hi @tom_02

Why you add outlet constraint “pressure=0”?

Since Navier-Stokes depends on the pressure gradient, the outlet defines the value/range of the pressure for the flow. This is not the only approach however, similar to fluid simulations there are multiple inlet, outlet and interior constraint combos that can be used (some with better convergence than others). So don’t interpret whats in the example as the only solution!

Is the use of integral constraint in the example similar to what i have explain in the last question? I ask this because from the example it seems that the integral planes is added with the constraint of have the same “normal_dot_vel”.

The integral constraints greatly help with the convergence of fluid flow problems. For many of the more complex problems, we have found these constraints to be essential to avoid having the network converge to a trivial solution.

Where i can find more information about the normal_dot_vel parameter and what it represent? If it represent something connected to the volumetric flow why didn’t you insert an integral plane also in the outlet?

The normal dot vel returns the velocity dot normal vector. The normal direction is provided by the geometry module.

Inside your channel you don’t have constant pressure, but if i want to add this constraint, can this be done using “PointwiseInteriorConstraint”?

Correct, this is the purpose of the outlet constraint. If you want to have an interior pressure constraint what you have would be the right approach.

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Hi @ngeneva,