I have solved the unsteady diffusion equation using piece-wise linear Finite elements(triangles) for spatial discretisation and Forward Euler for temporal discretisation. I have the following mesh convergence results. I'm not sure how to interpret them. The error was calculated using an analytical solution at time = 2 seconds. Time step, dt of 0.04 was used.
How do I verify the order of convergence, does the slope have to be 2 for second order accuracy in space?
Do we have anything like Courant number ($\frac{\Delta t}{\Delta h^2}\leq1/2$) to restrict the time step?