# Quick start

VisualPDE is a web-based set of tools for solving partial differential equations (PDEs) via an interactive, easy-to-use simulation. To get started, try playing with some of the linear examples, or read on for some quick tips for using the solver.

### Interacting with the simulation

Clicking/pressing on the simulation draws values right onto the domain. You can customise exactly what this does under **Brush** For example, the default settings in the → heat equation example allow you to paint ‘heat’ of value 1 onto the domain, which acts like an initial condition for the rest of the simulation.

### The equations panel

Pressing **equations panel**.

Here you can:

- See the equation being simulated, here $\pd{u}{t} = \vnabla\cdot(D_u\vnabla u) + f_u$.
- Set the named functions in the equations, here $D_u$ and $f_u$, under
**Definitions**. These can be functions of any of the unknowns, space, and time (here $u$, $x$, $y$, and $t$), and of any parameters that will be defined further down the panel. - Set the value of any extra parameters.
- Set the boundary conditions.
- Set the initial conditions.
- Set the number and type of equations to be solved.

### Domain shape

The default **domain** for solving PDEs is a 2D rectangle, $\domain = [0,L_x]\times[0,L_y]$, which fits the size of your browser window or phone screen. Throughout VisualPDE, we use coordinates $x\in[0,L_x]$ and $y\in[0,L_y]$.

You can force the domain to be a square, $\domain = [0,L]\times[0,L]$, by toggling off **Domain** → **Fill screen** →

### Boundary conditions

The following **boundary conditions** are available to allow you to set the value of the function, or the value of its derivative, along the boundary $\boundary$ of the domain $\domain$:

- Periodic
- Dirichlet (e.g. $u\onboundary = 0$)
- Neumann (e.g. $\pd{u}{n}\onboundary = 0$)
- Robin (e.g. $(u + \pd{u}{n})\onboundary = 0$)

You can swap between boundary conditions by choosing **Boundary conditions** and selecting from the list for each variable. →

### Initial conditions

You can specify the values to which the unknowns ($u$, $v$, $w$) are initialised when resetting the simulation. These expressions can be functions of $x$, $y$, the special string ‘RAND’ that assigns a random number in [0,1] to each point in the domain, along with any user-defined parameters and the images $I_S$ and $I_T$ (see the advanced documentation for more details). You can also use $L$, $L_x$ and $L_y$.

### Changing the equations

The simplest system VisualPDE can solve is a single PDE,

\[\pd{u}{t} = \vnabla \cdot (D_u \vnabla u) + f_u,\]where $D_u$ and $f_u$ are functions of $u$, $x$, $y$, and $t$ that you can specify.

The most complicated type is a coupled system of PDEs in four unknowns, $u$, $v$, $w$ and $q$:

\[\begin{aligned} t_u\pd{u}{t} &= \vnabla \cdot(D_{uu}\vnabla u+D_{uv}\vnabla v+D_{uw}\vnabla w+D_{uq}\vnabla q) + f_u,\\ \text{one of}\left\{\begin{matrix}\displaystyle t_v\pd{v}{t} \\ v\end{matrix}\right. & \begin{aligned} &= \vnabla \cdot(D_{vu}\vnabla u+D_{vv}\vnabla v+D_{vw}\vnabla w+D_{vq}\vnabla q) + f_v \vphantom{\displaystyle t_v\pd{v}{t}}, \\ &= \vnabla \cdot(D_{vu}\vnabla u+D_{vw}\vnabla w+D_{vq}\vnabla q) + f_v, \end{aligned}\\ \text{one of}\left\{\begin{matrix}\displaystyle t_w\pd{w}{t} \\ w\end{matrix}\right. & \begin{aligned} &= \vnabla \cdot(D_{wu}\vnabla u+D_{wv}\vnabla v+D_{ww}\vnabla w+D_{wq}\vnabla q) + f_w \vphantom{\displaystyle t_w\pd{w}{t}}, \\ &= \vnabla \cdot(D_{wu}\vnabla u+D_{wv}\vnabla v+D_{wq}\vnabla q) + f_w, \end{aligned}\\ \text{one of}\left\{\begin{matrix}\displaystyle t_q\pd{q}{t} \\ q\end{matrix}\right. & \begin{aligned} &= \vnabla \cdot(D_{qu}\vnabla u+D_{qv}\vnabla v+D_{qw}\vnabla w+D_{qq}\vnabla q) + f_q \vphantom{\displaystyle t_q\pd{q}{t}}, \\ &= \vnabla \cdot(D_{qu}\vnabla u+D_{qv}\vnabla v+D_{qw}\vnabla w) + f_q, \end{aligned} \end{aligned}\]where $D_{uu}, \dots, D_{qq}$, $f_u, \dots, f_q$ and $t_u, \dots, t_q$ are functions of $u$, $v$, $w$, $q$, $x$, $y$ and $t$ that you can specify.

- You can change the number of unknowns by choosing
**Advanced options**→**Num. species**→ - In systems of multiple unknowns, you can include terms representing cross-diffusion (e.g. $D_{uv}$, $D_{vu}$) by toggling
**Advanced options**→**Cross diffusion**→ - In systems of multiple unknowns, you can choose between a differential or algebraic equation for some of the species (e.g. ‘$\partial w/\partial t=$’ or ‘$w=$’) by toggling
**Advanced options**→**Algebraic w**(or**v**or**q**) →

### More VisualPDE

For a comprehensive list of all the options that you can set in VisualPDE, check out the Advanced documentation, or discover what VisualPDE can solve in our brief summary.