Magnetohydrodynamic entrance flow in a channel

A MHD entrance flow in a channel is studied in the Prandtl approximation. It is shown that consistency with the approximation requires an appropriate change in the boundary conditions which hold for the original, unsimplified equations. The correct boundary conditions are established and used to repete the numerical calculations in a few cases studied by other authors with the unmodified boundary conditions. In the cases here considered the numerical differences do not amount in practice to more than 10%, though they grow with increasing Hartmann number. Gabinete de Aplicaciones Nucleares de O.P. and Centro Coordinado de Fisica (C.S.I.C.-U.A.M.). Present address: Departamento de Física, Facultad de Ciencias, Universidad Autónoma de Madrid.