# Preprint 2006-026

# Hyperbolic balance laws: Riemann invariants and the generalized Riemann problem

## Matania Ben-Artzi and Jiequan Li

**Abstract:**
The Generalized Riemann Problem (GRP) for nonlinear hyperbolic system
of *m* balance laws
(or alternatively “quasi-conservative” laws)
in one space dimension is formulated as follows:
Given initial-data which are analytic on two sides of a discontinuity,
determine the time evolution of the solution at the discontinuity.
In particular, the GRP numerical scheme (second-order high resolution)
is based on an analytical evaluation of the first time derivative.
The analytical solution is readily obtained for a single equation
(*m*=1) and, more generally,
if the system is endowed with a complete
(coordinate) set of Riemann invariants.
In this case it can be “diagonalized”
and reduced to the scalar case.
However, most systems with *m*>2 do not admit
such a set of Riemann invariants.
This paper introduces a generalization of this concept:
“Weakly Coupled Systems” (WCS).
Such systems have only “partial set”
of Riemann invariants,
but these sets are weakly coupled in a way which enables
a “diagonalized” treatment of the GRP.
An important example of a WCS is the Euler system of compressible,
nonisentropic fluid flow (*m*=3).
The solution of the GRP discussed here
is based on a careful analysis of rarefaction waves.
A “propagation of singularities” argument
is applied to appropriate Riemann invariants
across the rarefaction fan.
It serves to “rotate” initial spatial slopes
into “time derivative”.
In particular, the case of a “sonic point”
is incorporated easily into the general treatment.
A GRP scheme based on this solution is derived,
and several numerical examples are presented.
Special attention is given to the “acoustic approximation”
of the analytical solution.
It can be viewed as a proper linearization
(different from the approach of Roe) of the nonlinear system.
The resulting numerical scheme is the simplest
(second-order, high-resolution) generalization of the Godunov scheme.