# Convection diffusion reaction equation (stiffness, solver)

I am trying to solve the CDR-Equation in 2D:

$$\frac{\partial c(x,y)}{\partial t} + \nabla \cdot ( -d\nabla c(x,y) + \vec{v}(x,y) c(x,y))+ a c(x,y)=0\,,$$ with Boundary Conditions (length of square is $L$): $$c(0,y)=0$$ $$-d\nabla c(L,y) + v(L,y) (c(L,y))=0\,.$$. $$c(x,y=0)=0$$ $$c(x,y=L)=0$$

1.) Why exactly is this equation stiff? Does it depend on the reaction term $ac(x,y)$?

2.) Can I solve the equation with the Crank-Nicholson method? Is the error huge? If yes, what is the best method?

• Please correct the second boundary condition and add some boundary condition on the other two sides of the square domain. Crank-Nicolson with a correct treatment of reaction term and advection term (the last one maybe with an upwind method) shall give an accurate scheme. For too large time steps you can have some unphysical oscillations in numerical solution, but if you would like to have an accurate numerical solution, you should avoid such large time steps. – Peter Frolkovič Jan 11 '16 at 20:03
• Stiffness is generally regarded as a property of the ODEs you get by semi-discretizing, not of the PDEs. Do you have a semi-discretization in mind? – David Ketcheson Jan 12 '16 at 6:36
• I am asking for stiffness, because the CDR equation is often given as an example for a stiff equations. And there are multiple papers discussing different solvers because of the difficulties in handling it. – Paulinchen2 Jan 12 '16 at 13:01

1. Mathematically speaking, stiffness is meaningless for a single differential equation, and is rather attributed to a set of differential equations that have different time-scales (e.g. when trying to solve two coupled equations with time-scales of 1 second and 1 day, respectively). However, a single equation can also be referred to as stiff if certain numerical integration methods are numerically unstable. Source/sink terms can give rise to stiffness, for instance: $$\frac{dy}{dt}=-1000y$$ is a good example where sink term has given rise to stiffness. That being said, $ac(x,y)$ might be causing the stiffness in your PDE.