# Implicit finite difference scheme for LWR-v PDE

I want to discretize the LWR(Lighthill-Whitham-Richards)-v partial differential equation (as shown in 1) $$\frac{\partial u}{\partial t} + \frac{\partial R(u)}{\partial x} = 0 \quad \text{i.e.,} \quad \frac{\partial u}{\partial t} + \frac{dR}{du}\,\frac{\partial u}{\partial x} = 0 \quad \dots\dots \quad \text{(1)}$$ In the equation $u(x,t) \equiv u_x^t$ is the speed at time $t$ at section $x$ and $R(u)$ is given by Greenshield’s speed-density relationship $$R(u) = u^2 - u\,u_f \quad \implies \quad \frac{dR}{du} = 2u - u_f =: a(u)$$ where $u$ is the stream speed and $u_f$ is the free flow speed or the maximum speed in the section.

I want to use an implicit finite difference scheme (backward time, back ward space as shown in (2) to discretize equation (1): $$\frac{u_x^{t+1}-u_x^t}{\Delta t} + a(u_x^{t+1})\,\frac{u_x^{t+1}-u_{x-1}^{t+1}}{\Delta x} = 0 \quad \dots\dots \quad \text{(2)}$$

In literature I could find solutions for explicit schemes alone like upwind scheme, downwind scheme, Godunov scheme etc. Can anyone suggest a good discretization scheme or some references in literature for the implicit case shown above in equation (2)?

Thanks for the help!

• To build on what DavidKetcheson has said, the standard advice would be to use something developed for hyperbolic problems, which tend to use explicit schemes. You could try something like a second-order Lax-Wendroff approach with an explicit SSP time integrator (forward Euler, for instance) and a limiter of some sort for the shocks. If you want to go higher-order in space and time, you could use ENO/WENO methods in space and higher-order SSP integrators in time. – Geoff Oxberry Mar 14 '14 at 8:31