sponge_distance: $D_{sponge}\ge 0$

An example of how to implement a sponge layer is shown in phys.c in the form

$$\frac{u^{n+1}-u^n}{\Delta t} = -\frac{1}{\tau_{sponge}}\exp\left(-\frac{x}{D_{sponge}}\right) ,$$

where the $\tau_{sponge}$ timescale is defined by the value of sponge_decay. If $D_{sponge}=0$ then no sponge layer is employed. Note that this sponge layer assumes the boundary is located at $x=0$ and that the sponge layer is roughly $D_{sponge}$ m thick and extends in the positive $x$ direction!