# Example 3 : Multiphysics coupling

## Problem statement

This problem considers a coupled systems of equations on a 3-D domain : find and such that and

,

where on the top boundary and and on the bottom boundary. The remaining boundaries are natural boundaries: and . The domain, , is the same as utilized in Example 2.

The weak form of this equation, in inner-product notation, is given by:

and

,

where are the test functions and and are the finite element solutions.

## Create Convection Kernel

The convection component of the problem requires the creation of a a new Kernel, as described in Example 02. Here the Kernel must utilize a coupled variable rather than a known constant.

The header for this object, "ExampleConvection.h" is little changed from the previous example, with one exception, a member is defined to store the gradient of the coupled variable:

ExampleConvection.h

The source filem "ExampleConvection.C", after including the header, defines the parameters for the kernel. This definition includes adding a parameter that defines the variable to couple into tis kernel.

ExampleConvection.C

Finally, the computeQpResiduals and computeQpJacobian then utilize the coupled value to compute the desired residuals and jacobians.


Real ExampleConvection::computeQpResidual()
{
}


Real ExampleConvection::computeQpJacobian()
{
}


## Register Kernel

As done in Example 2, the newly created object must be registered. This is accomplished by including the "Convection.h" file and the following in ExampleApp::registerObjects method of src/base/ExampleApp.C within the example 3 directory of MOOSE (examples/ex03_coupling).


registerKernel(ExampleConvection);


## Input File Syntax


[Mesh]
file = mug.e
[]


Then, the two variables are defined: "diffused" and "convected", which refer to u and v from the problem statement, respectively. Both variables in this case are assigned to utilize linear Lagrange shape functions, but they could each use different shape functions and/or orders.


[Variables]
[./convected]
order = FIRST
family = LAGRANGE
[../]
[./diffused]
order = FIRST
family = LAGRANGE
[../]
[]


The problem requires three Kernels, two Diffusion Kernels, one for each of the variables and the ExampleConvection Kernel created above. It is important to point out that for the two Diffusion terms, the same code is utilized; two instances of the C++ object are created to application of the code to two variables. Additionally, the actual coupling of the equations takes place in the ExampleConvection object. The some_variable input parameter was created in the ExampleConvection Kernel and here is assigned to utilize the diffused variable.


[Kernels]
[./diff_convected]
type = Diffusion
variable = convected
[../]
[./conv]
type = ExampleConvection
variable = convected
some_variable = diffused #Couple variable into the convection kernel
[../]
[./diff_diffused]
type = Diffusion
variable = diffused
[../]
[]


For the given problem, each of the variables has a DirichletBC applied at the top and bottom. This is done in the input file as follows.


[BCs]
[./bottom_convected]
type = DirichletBC
variable = convected
boundary = 'bottom'
value = 1
[../]
[./top_convected]
type = DirichletBC
variable = convected
boundary = 'top'
value = 0
[../]
[./bottom_diffused]
type = DirichletBC
variable = diffused
boundary = 'bottom'
value = 2
[../]
[./top_diffused]
type = DirichletBC
variable = diffused
boundary = 'top'
value = 0
[../]
[]


Finally, the Executioner block is setup for solving the problem an the Outputs are set for viewing the results.


[Executioner]
solve_type = 'PJFNK'
[]
[Outputs]
file_base = out
exodus = true
[./console]
type = Console
perf_log = true
linear_residuals = true
[../]
[]


## Running the Problem

This example may be run using Peacock or by running the following commands form the command line.


cd ~/projects/moose/examples/ex03_coupling
make -j8
./ex03-opt -i ex03.i


This will generate the results file, out.e, as shown in Figure 1 and 2. This file may be viewed using Peacock or an external application that supports the Exodus II format (e.g., Paraview).

Example 03 Results, "diffused variable"

Example 03 Results, "convected variable"

# 1D exact solution

A simplified 1D analog of this problem is given as follows, where and :

The exact solution to this problem is

## Complete Source Files

ExampleConvection.h

ExampleConvection.C

ExampleApp.C