Nonlinear periodic drift waves are investigatcd on the basis of a simple perturbation scheme for both the amplitude and inverse frequency. The coefficients for the generation of the forced harmonics are derived, a nonlinear dispersion relation is suggested and a criterion for the onset of the modulational instability is obtained. The results are compared with those obtained with the help ofa standard KBM ireaimeni. Moreover, cnoidai driii waves are suggested and compared !O an experimentai observation.

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An approximate solution is presented for the problem of the thickness of the diffusion boundary layer on solidification of a killed-steel ingot under conditions of natural thermal-melt convection.