Hello all: I'm a newbie, trying to write/use code for solving a 2D advection-diffusion problem. I'm not sure how many boundary conditions I should have for the property that is being transported. In my problem, I have diffusion switched off (advection only). The property being transported (from left to right) is a chemical element. I have its initial mass as an initial condition. I have specified its mass at the left boundary. Do I need a second boundary condition at the right? Thanks for any advice.