The Roe approximate Riemann solver, devised by Phil Roe, is an approximate Riemann solver based on the Godunov scheme and involves finding an estimate for the intercell numerical flux or Godunov flux
A non-linear system of hyperbolic partial differential equations representing a set of conservation laws in one spatial dimension can be written in the form Applying the chain rule to the second term we get the quasi-linear hyperbolic system where
The Roe method consists of finding a matrix
The Riemann problem can then be solved as a truly linear hyperbolic system at each cell interface.
The Roe matrix must obey the following conditions: Phil Roe introduced a method of parameter vectors to find such a matrix for some systems of conservation laws.
[1] Once the Roe matrix corresponding to the interface between two cells is found, the intercell flux is given by solving the quasi-linear system as a truly linear system.