dear all,what is the periodic boundry condition,and how to set it.thanks a lot.

A periodic boundary condition requires the value of the variable of interest to be the same at the opposite boundaries of the domain. A typical example of a periodic boundary condition is often showed CFD books for a 1d uniform velocity advection of a pulse signal dc/dt + udc/dx=0 If you specify the periodic bc you will see the wave travel from left to right and re-enter from the left in your domain. It is a good way to implement an infinite domain. In other terms it work as if your physical domain had been bended and joined as a Moebius strip. As for the numerical realization of a periodic bc it will depend on the method you are using but for finite differences or finite volumes you will have to grab at the boundary the value at the other end of the domain. This will modify the shape and properties of you matrices (if you are using implicit or semi-implicit methods) so watch out. For spectral methods I guess the periodic boundary coundition would be a relief. Another typical field of application of a periodic bc is in the DNS or LES where a turbulent inflow bc is required (see for instance Kim and Moin DNS of channel flow) Hope it helps Luca