# Stability analysis simplification for PDE

I have the nonlinear PDE $$\frac{\partial U(z,t)}{\partial t} + A(U)\frac{\partial U(z,t)}{\partial z} + B(U)U(z,t) + C(z,t) = 0,$$ where $$A(U)$$ and $$B(U)$$ are guaranteed to be real and positive.

I want to use the MacCormack finite difference method to numerically solve it. For (Von Neumann) stability analysis, I use a linearized version of the PDE without $$C(z,t)$$, as that term will not contribute to instability in the error.

Without $$B$$, I retrieve the expected C.F.L. condition. That case is also a typical example found in texts on the MacCormack method and its stability, but I have not seen any examples where $$B \neq 0$$. If I don't neglect $$B$$, the stability analysis is still doable, just more messy than without it and the resulting condition on $$\Delta t$$ (and $$\Delta z$$) is less obvious.

So, is it safe to neglect $$B$$ for simplicity in the stability analysis?

• Is U in your equation scalar or vector? Also it is not clear how you organize computation with MacCormack method? Did you discretize equation in space and then use predictor-corrector method to solve system of ODEs? Commented Aug 21, 2021 at 2:15
• In principle, $U$ is a vector in my physical set of equations. However, for my question here I mentioned a scalar version on purpose, because even in the scalar case I have not seen analysis where $B \neq 0$. For the computation, indeed I discretize space and follow the predictor-corrector method exactly as written in MacCormacks original paper. Commented Aug 21, 2021 at 8:51

Consider the case where $$A=C=0$$ and, as you stated, $$B>0$$. Then you simply have an ODE at every point $$z$$ with a solution that decays to zero (at least if $$B$$ is bounded away from zero). You can then apply the usual step length criteria developed for ODEs. In other words, you will need $$\Delta t \le \frac{c}{B(z,t)}$$ if you were to use an explicit method (e.g., the forward Euler method, for which $$c=2$$; other methods have other stability constants) for the ODE at $$z$$. Since you want to use the same time step everywhere, you need $$\Delta t \le c \min_{z\in\Omega} \frac{1}{B(z,t)}.$$
• I agree with the approach @Wolfgang Bangerth suggest, although I believe the forward Euler bound should be $\Delta t \leq \frac{\mathbf{2}}{B(U)}$. The step is of the form $U_{n+1} = (1 - \Delta t B(U_n)) U_n$, and we need $|1 - \Delta t B(U_n)| \leq 1$. Commented Aug 20, 2021 at 22:30
• I am not sure I follow the goal of your approach, since in my case $A \neq 0$. Do you mean that I should analyze your suggestion too, and the case with $B = 0$, and then get two upper bounds on $\Delta t$ and make sure I always satisfy both? Commented Aug 20, 2021 at 23:00
• @mcv100 Yes. I was just pointing out that you can't neglect the reaction term with $C$ in your stability considerations. You will have to take into account all of the terms. Commented Aug 20, 2021 at 23:25
• @Wolfgang Bangerth Are you sure $C$ matters for stability? If you write out the discretized set of equations and take the error, $C$ drops out because it does not multiply $U$. See also: scicomp.stackexchange.com/questions/26838/…. Commented Aug 21, 2021 at 8:53