Turbulent boundary-layer flow over low and moderate slope hills

Selected aspects of turbulent boundary-layer how over low hills and other topography are reviewed and some basic ideas related to modelling are presented. Some simple considerations, based on the inner-layer concept, lead to rules of thumb or guidelines for estimating speed-up on hilltops in neutrally stratified flow. More detailed modelling studies based on a mixed spectral finite difference (MSFD) model are described, including recent studies with an extension of the model to include stably-stratified flow and studies of the development of 3D longitudinal vortices in the flow over 2D waves. It should be noted that the paper is not intended as a comprehensive review of the field, but more of a "retrospective" of some of the work in which the author has been involved. (C) 1998 Elsevier Science Ltd. All rights reserved.