How to extract right- and left-moving waves

Steve

This is a post-pro question, so may be OT here. However...

If I have a 1D section of a model and full access to transient time-based results for all the relevant information (pressure, velocity, temperature, etc), how can I extract left-travelling and right-travelling pressure waves? To make things worse, the system is by no means linear.

Any references or explanations welcome.