ABSTRACT
I t is easy to obtain from ( 1 . 1 ) pw2 rot u = 0 in Q. Therefore we have two types of solutions : one corresponding to the vortex motions (rot u f:. 0 ) , for which the frequen cies are zero and another one corresponding to the irrotational motions (rot u = 0) for which the frequencies are not zero. Since in the frequency analysis , only the irrotational modes are interesting, we require that rot u = O .
