  The appropriate boundary condition between an unconfined incompressible
viscous fluid and a porous medium is given by the law of Beavers and Joseph.
The latter has been justified both experimentally and mathematically, using the
method of periodic homogenisation. However, all results so far deal only with
the case of a planar boundary. In this work, we consider the case of a curved,
macroscopically periodic boundary. By using a coordinate transformation, we
obtain a description of the flow in a domain with a planar boundary, for which
we derive the effective behaviour: The effective velocity is continuous in
normal direction. Tangential to the interface, a slip occurs. Additionally, a
pressure jump occurs. The magnitude of the slip velocity as well as the jump in
pressure can be determined with the help of a generalised boundary layer
function. The results indicate the validity of a generalised law of Beavers and
Joseph, where the geometry of the interface has an influence on the slip and
jump constants.
