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Solve nonlinear elliptic PDE problem

`pdenonlin`

is not recommended. Use `solvepde`

instead.

solves
the nonlinear PDE`u`

= pdenonlin(`model`

,`c`

,`a`

,`f`

)

$$-\nabla \cdot \left(c\nabla u\right)+au=f$$

with geometry, boundary conditions, and finite element mesh
in `model`

, and coefficients `c`

, `a`

,
and `f`

. In this context, "nonlinear"
means some coefficient in `c`

, `a`

,
or `f`

depends on the solution `u`

or
its gradient. If the PDE is a system of equations (`model.PDESystemSize`

> 1), then `pdenonlin`

solves
the system of equations

$$-\nabla \cdot \left(c\otimes \nabla u\right)+au=f$$

,
for any previous arguments, modifies the solution process with `u`

= pdenonlin(___,`Name,Value`

)`Name`

, `Value`

pairs.

Solve a minimal surface problem. Because this problem has a nonlinear `c`

coefficient, use `pdenonlin`

to solve it.

Create a model and include circular geometry using the built-in `circleg`

function.

model = createpde; geometryFromEdges(model,@circleg);

Set the coefficients.

```
a = 0;
f = 0;
c = '1./sqrt(1+ux.^2+uy.^2)';
```

Set a Dirichlet boundary condition with value .

boundaryfun = @(region,state)region.x.^2; applyBoundaryCondition(model,'edge',1:model.Geometry.NumEdges,... 'u',boundaryfun,'Vectorized','on');

Generate a mesh and solve the problem.

generateMesh(model,'Hmax',0.1); u = pdenonlin(model,c,a,f); pdeplot(model,'XYData',u,'ZData',u)

Solve the minimal surface problem using the legacy approach for creating boundary conditions and geometry.

Create the geometry using the built-in `circleg`

function. Plot the geometry to see the edge labels.

g = @circleg; pdegplot(g,'EdgeLabels','on') axis equal

Create Dirichlet boundary conditions with value .Create the following file and save it on your Matlab™ path. For details of this approach, see Boundary Conditions by Writing Functions.

function [qmatrix,gmatrix,hmatrix,rmatrix] = pdex2bound(p,e,u,time)

ne = size(e,2); % number of edges qmatrix = zeros(1,ne); gmatrix = qmatrix; hmatrix = zeros(1,2*ne); rmatrix = hmatrix;

for k = 1:ne x1 = p(1,e(1,k)); % x at first point in segment x2 = p(1,e(2,k)); % x at second point in segment xm = (x1 + x2)/2; % x at segment midpoint y1 = p(2,e(1,k)); % y at first point in segment y2 = p(2,e(2,k)); % y at second point in segment ym = (y1 + y2)/2; % y at segment midpoint switch e(5,k) case {1,2,3,4} hmatrix(k) = 1; hmatrix(k+ne) = 1; rmatrix(k) = x1^2; rmatrix(k+ne) = x2^2; end end

Set the coefficients and boundary conditions.

```
a = 0;
f = 0;
c = '1./sqrt(1+ux.^2+uy.^2)';
b = @pdex2bound;
```

Generate a mesh and solve the problem.

[p,e,t] = initmesh(g,'Hmax',0.1); u = pdenonlin(b,p,e,t,c,a,f); pdeplot(p,e,t,'XYData',u,'ZData',u)

Solve a nonlinear 3-D problem with nontrivial geometry.

Import the geometry from the `BracketWithHole.stl`

file. Plot the geometry and face labels.

model = createpde(); importGeometry(model,'BracketWithHole.stl'); figure pdegplot(model,'FaceLabels','on') view(30,30) title('Bracket with Face Labels') figure pdegplot(model,'FaceLabels','on') view(-134,-32) title('Bracket with Face Labels, Rear View')

Set a Dirichlet boundary condition with value 1000 on the back face, which is face 4. Set the large faces 1 and 7, and also the circular face 11, to have Neumann boundary conditions with value `g = -10`

. Do not set boundary conditions on the other faces. Those faces default to Neumann boundary conditions with value `g = 0`

.

applyBoundaryCondition(model,'Face',4,'u',1000); applyBoundaryCondition(model,'Face',[1,7,11],'g',-10);

Set the `c`

coefficient to 1, `f`

to 0.1, and `a`

to the nonlinear value `'0.1 + 0.001*u.^2'`

.

```
c = 1;
f = 0.1;
a = '0.1 + 0.001*u.^2';
```

Generate the mesh and solve the PDE. Start from the initial guess `u0 = 1000`

, which matches the value you set on face 4. Turn on the `Report`

option to observe the convergence during the solution.

generateMesh(model); u = pdenonlin(model,c,a,f,'U0',1000,'Report','on');

Iteration Residual Step size Jacobian: full 0 3.6247e-01 1 9.9800e-02 1.0000000 2 2.9717e-02 1.0000000 3 8.5756e-03 1.0000000 4 1.8635e-03 1.0000000 5 1.5256e-04 1.0000000 6 1.2428e-06 1.0000000

Plot the solution on the geometry boundary.

```
pdeplot3D(model,'ColorMapData',u)
```

`model`

— PDE model`PDEModel`

objectPDE model, specified as a `PDEModel`

object.

**Example: **`model = createpde(1)`

`c`

— PDE coefficientscalar or matrix | character array | coefficient function

PDE coefficient, specified as a scalar or matrix, as a character
array, or as a coefficient function. `c`

represents
the *c* coefficient in the scalar PDE

$$-\nabla \cdot \left(c\nabla u\right)+au=f$$

or in the system of PDEs

$$-\nabla \cdot \left(c\otimes \nabla u\right)+au=f$$

You can specify`c`

in various ways, detailed
in c Coefficient for Systems. See
also Specify Scalar PDE Coefficients in Character Form, Specify 2-D Scalar Coefficients in Function Form,
and Specify 3-D PDE Coefficients in Function Form.

**Example: **`'cosh(x+y.^2)'`

**Data Types: **`double`

| `char`

| `function_handle`

**Complex Number Support: **Yes

`a`

— PDE coefficientscalar or matrix | character array | coefficient function

PDE coefficient, specified as a scalar or matrix, as a character
array, or as a coefficient function. `a`

represents
the *a* coefficient in the scalar PDE

$$-\nabla \cdot \left(c\nabla u\right)+au=f$$

or in the system of PDEs

$$-\nabla \cdot \left(c\otimes \nabla u\right)+au=f$$

You can specify`a`

in various ways, detailed
in a or d Coefficient for Systems.
See also Specify Scalar PDE Coefficients in Character Form, Specify 2-D Scalar Coefficients in Function Form,
and Specify 3-D PDE Coefficients in Function Form.

**Example: **`2*eye(3)`

**Data Types: **`double`

| `char`

| `function_handle`

**Complex Number Support: **Yes

`f`

— PDE coefficientscalar or matrix | character array | coefficient function

PDE coefficient, specified as a scalar or matrix, as a character
array, or as a coefficient function. `f`

represents
the *f* coefficient in the scalar PDE

$$-\nabla \cdot \left(c\nabla u\right)+au=f$$

or in the system of PDEs

$$-\nabla \cdot \left(c\otimes \nabla u\right)+au=f$$

You can specify`f`

in various ways, detailed
in f Coefficient for Systems. See
also Specify Scalar PDE Coefficients in Character Form, Specify 2-D Scalar Coefficients in Function Form,
and Specify 3-D PDE Coefficients in Function Form.

**Example: **`char('sin(x)';'cos(y)';'tan(z)')`

**Data Types: **`double`

| `char`

| `function_handle`

**Complex Number Support: **Yes

`b`

— Boundary conditionsboundary matrix | boundary file

Boundary conditions, specified as a boundary matrix or boundary file. Pass a boundary file as a function handle or as a file name.

A boundary matrix is generally an export from the PDE app. For details of the structure of this matrix, see Boundary Matrix for 2-D Geometry.

A boundary file is a file that you write in the syntax specified in Boundary Conditions by Writing Functions.

**Example: **`b = 'circleb1'`

or equivalently ```
b
= @circleb1
```

**Data Types: **`double`

| `char`

| `function_handle`

`p`

— Mesh pointsmatrix

Mesh points, specified as a 2-by-`Np`

matrix
of points, where `Np`

is the number of points in
the mesh. For a description of the (`p`

,`e`

,`t`

)
matrices, see Mesh Data.

Typically, you use the `p`

, `e`

,
and `t`

data exported from the PDE app, or generated
by `initmesh`

or `refinemesh`

.

**Example: **`[p,e,t] = initmesh(gd)`

**Data Types: **`double`

`e`

— Mesh edgesmatrix

Mesh edges, specified as a `7`

-by-`Ne`

matrix
of edges, where `Ne`

is the number of edges in the
mesh. For a description of the (`p`

,`e`

,`t`

)
matrices, see Mesh Data.

Typically, you use the `p`

, `e`

,
and `t`

data exported from the PDE app, or generated
by `initmesh`

or `refinemesh`

.

**Example: **`[p,e,t] = initmesh(gd)`

**Data Types: **`double`

`t`

— Mesh trianglesmatrix

Mesh triangles, specified as a `4`

-by-`Nt`

matrix
of triangles, where `Nt`

is the number of triangles
in the mesh. For a description of the (`p`

,`e`

,`t`

)
matrices, see Mesh Data.

Typically, you use the `p`

, `e`

,
and `t`

data exported from the PDE app, or generated
by `initmesh`

or `refinemesh`

.

**Example: **`[p,e,t] = initmesh(gd)`

**Data Types: **`double`

Specify optional comma-separated pairs of `Name,Value`

arguments.
`Name`

is the argument
name and `Value`

is the corresponding
value. `Name`

must appear
inside single quotes (`' '`

).
You can specify several name and value pair
arguments in any order as `Name1,Value1,...,NameN,ValueN`

.

`'Jacobian'`

— Approximation of Jacobian`'full'`

(3-D default) | `'fixed'`

(2-D default) | `'lumped'`

Approximation of Jacobian, specified as `'full'`

, `'fixed'`

,
or `'lumped'`

.

`'full'`

means numerical evaluation of the full Jacobian based on the sparse version of the`numjac`

function. 3-D geometry uses only`'full'`

, any other specification yields an error.`'fixed'`

specifies a fixed-point iteration matrix where the Jacobian is approximated by the stiffness matrix. This is the 2-D geometry default.`'lumped'`

specifies a "lumped" approximation as described in Nonlinear Equations. This approximation is based on the numerical differentiation of the coefficients.

**Example: **`u = pdenonlin(model,c,a,f,'Jacobian','full')`

**Data Types: **`char`

`'U0'`

— Initial solution guess0 (default) | scalar | vector of characters | vector of numbers

Initial solution guess, specified as a scalar, a vector of characters, or a vector of numbers. For details, see Solve PDEs with Initial Conditions.

A scalar specifies a constant initial condition for either a scalar or PDE system.

For scalar problems, use the same syntax as Specify Scalar PDE Coefficients in Character Form.

For systems of

*N*equations, write a character array with*N*rows, where each row has the syntax of Specify Scalar PDE Coefficients in Character Form.For systems of

*N*equations, and a mesh with`Np`

nodes, give a column vector with*N**`Np`

components. The nodes are either`model.Mesh.Nodes`

, or the`p`

data from`initmesh`

or`meshToPet`

. See Mesh Data.The first

`N`

elements contain the values of component 1, where the value of element_{p}`k`

corresponds to node`p(k)`

. The next`N`

points contain the values of component 2, etc. It can be convenient to first represent the initial conditions_{p}`u0`

as an`N`

-by-_{p}`N`

matrix, where the first column contains entries for component 1, the second column contains entries for component 2, etc. The final representation of the initial conditions is`u0(:)`

.

**Example: **`u = pdenonlin(model,c,a,f,'U0','x.^2-y.^2')`

**Data Types: **`double`

| `char`

**Complex Number Support: **Yes

`'Tol'`

— Residual size at termination1e-4 (default) | positive scalar

Residual size at termination, specified as a positive scalar. `pdenonlin`

iterates
until the residual size is less than `'Tol'`

.

**Example: **`u = pdenonlin(model,c,a,f,'Tol',1e-6)`

**Data Types: **`double`

`'MaxIter'`

— Maximum number of Gauss-Newton iterations`25`

(default) | positive integerMaximum number of Gauss-Newton iterations, specified as a positive integer.

**Example: **`u = pdenonlin(model,c,a,f,'MaxIter',12)`

**Data Types: **`double`

`'MinStep'`

— Minimum damping of search direction`1/2^16`

(default) | positive scalarMinimum damping of search direction, specified as a positive scalar.

**Example: **`u = pdenonlin(model,c,a,f,'MinStep',1e-3)`

**Data Types: **`double`

`'Report'`

— Print convergence information`'off'`

(default) | `'on'`

Print convergence information, specified as `'off'`

or `'on'`

.

**Example: **`u = pdenonlin(model,c,a,f,'Report','on')`

**Data Types: **`char`

`'Norm'`

— Residual norm`Inf`

(default) | p value for L`'energy'`

Residual norm, specified as the `p`

value for
L^{p} norm, or as `'energy'`

. `p`

can
be any positive real value, `Inf`

, or `-Inf`

.
The `p`

norm of a vector `v`

is `sum(abs(v)^p)^(1/p)`

.
See `norm`

.

**Example: **`u = pdenonlin(model,c,a,f,'Norm',2)`

**Data Types: **`double`

| `char`

`u`

— PDE solutionvector

PDE solution, returned as a vector.

If the PDE is scalar, meaning only one equation, then

`u`

is a column vector representing the solution*u*at each node in the mesh.`u(i)`

is the solution at the`i`

th column of`model.Mesh.Nodes`

or the`i`

th column of`p`

.If the PDE is a system of

*N*> 1 equations, then`u`

is a column vector with*N**`Np`

elements, where`Np`

is the number of nodes in the mesh. The first`Np`

elements of`u`

represent the solution of equation 1, then next`Np`

elements represent the solution of equation 2, etc.

To obtain the solution at an arbitrary point in the geometry,
use `pdeInterpolant`

.

To plot the solution, use `pdeplot`

for
2-D geometry, or see Plot 3-D Solutions and Their Gradients.

`res`

— Norm of Newton step residualsscalar

Norm of Newton step residuals, returned as a scalar. For information about the algorithm, see Nonlinear Equations.

If the Newton iteration does not converge,

`pdenonlin`

displays the error message`Too many iterations`

or`Stepsize too small`

.If the initial guess produces matrices containing

`NaN`

or`Inf`

elements,`pdenonlin`

displays the error message`Unsuitable initial guess U0 (default: U0 = 0)`

.If you have very small coefficients, or very small geometric dimensions,

`pdenonlin`

can fail to converge, or can converge to an incorrect solution. If so, you can sometimes obtain better results by scaling the coefficients or geometry dimensions to be of order one.

The `pdenonlin`

algorithm is a Gauss-Newton
iteration scheme applied to the finite element matrices. For details,
see Nonlinear Equations.

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