# Nonlinear finite elements/Steady state heat conduction

 Subject classification: this is a physics resource .

## Contents

If the problem does not depend on time and the material is isotropic, we get the boundary value problem for steady state heat conduction.

{ \begin{align} & &\mathsf{ The~ boundary~ value~ problem ~for~ steady~ heat ~conduction}\\ & & \\ & \text{PDE:}~~~ &~~~- \frac{1}{C_v~\rho} \boldsymbol{\nabla} \bullet (\boldsymbol{\kappa}\bullet\boldsymbol{\nabla T)} = Q ~~\text{in}~~\Omega\quad\\ & \text{BCs:}~~~ &~~~ T = \overline{T}(\mathbf{x})~~\text{on}~~\Gamma_T ~~\text{and}~~ \frac{\partial T}{\partial n} = g(\mathbf{x})~~\text{on}~~\Gamma_q\quad\\ \end{align} }


### Poisson's equation

If the material is homogeneous the density, heat capacity, and the thermal conductivity are constant. Define the thermal diffusivity as

$k := \frac{\kappa}{C_v~\rho}$

Then, the boundary value problem becomes

{ \begin{align} & &\mathsf{ Poisson's~ equation}\\ & & \\ & \text{PDE:}~~~ &~~~- k \nabla^2 T = Q ~~\text{in}~~\Omega\quad\\ & \text{BCs:}~~~ &~~~ T = \overline{T}(\mathbf{x})~~\text{on}~~\Gamma_T ~~\text{and}~~ \frac{\partial T}{\partial n} = g(\mathbf{x})~~\text{on}~~\Gamma_q\quad\\ \end{align} }


where $\nabla^2 T$ is the Laplacian

$\nabla^2 T := \boldsymbol{\nabla} \bullet \boldsymbol{\nabla T}$

### Laplace's equation

Finally, if there is no internal source of heat, the value of $Q$ is zero, and we get Laplace's equation.

{ \begin{align} & &\mathsf{ Laplace's~ equation}\\ & & \\ & \text{PDE:}~~~ &~~~\nabla^2 T = 0 ~~\text{in}~~\Omega\quad\\ & \text{BCs:}~~~ &~~~ T = \overline{T}(\mathbf{x})~~\text{on}~~\Gamma_T ~~\text{and}~~ \frac{\partial T}{\partial n} = g(\mathbf{x})~~\text{on}~~\Gamma_q\quad\\ \end{align} }


## The Analogous Membrane Problem

The thin elastic membrane problem is another similar problem. See Figure 1 for the geometry of the membrane.

The membrane is thin and elastic. It is initially planar and occupies the 2D domain $\Omega$. It is fixed along part of its boundary $\Gamma_u$. A transverse force $\mathbf{f}$ per unit area is applied. The final shape at equilibrium is nonplanar. The final displacement of a point $\mathbf{x}$ on the membrane is $\mathbf{u}(\mathbf{x})$. There is no dependence on time.

The goal is to find the displacement $\mathbf{u}(\mathbf{x})$ at equilibrium.

 Figure 1. The membrane problem.

It turns out that the equations for this problem are the same as those for the heat conduction problem - with the following changes:

• The time derivatives vanish.
• The balance of energy is replaced by the balance of forces.
• The constitutive equation is replaced by a relation that states that the vertical force depends on the displacement gradient ($\boldsymbol{\nabla} \mathbf{u}$).

If the membrane if inhomogeneous, the boundary value problem is:

{ \begin{align} & &\mathsf{ The~boundary~ value~ problem~ for~ membrane~ deformation}\\ & & \\ & \text{PDE:}~~~ &~~~- \boldsymbol{\nabla} \bullet (E~\boldsymbol{\nabla \mathbf{u})} = Q ~~\text{in}~~\Omega\quad\\ & \text{BCs:}~~~ &~~~ \mathbf{u} = \bar{\mathbf{u}}(\mathbf{x})~~\text{on}~~\Gamma_u ~~\text{and}~~ \frac{\partial u}{\partial n} = g(\mathbf{x})~~\text{on}~~\Gamma_t\quad\\ \end{align} }


For a homogeneous membrane, we get

{ \begin{align} & &\mathsf{ Poisson's~ Equation}\\ & & \\ & \text{PDE:}~~~ &~~~- E \nabla^2 \mathbf{u} = Q ~~\text{in}~~\Omega\quad\\ & \text{BCs:}~~~ &~~~ \mathbf{u} = \bar{\mathbf{u}}(\mathbf{x})~~\text{on}~~\Gamma_u ~~\text{and}~~ \frac{\partial u}{\partial n} = g(\mathbf{x})~~\text{on}~~\Gamma_t\quad\\ \end{align} }


Note that the membrane problem can be formulated in terms of a problem of minimization of potential energy.