Three-Dimensional Effects in Flow over Mountains

Abstract

An investigation is made of three-dimensional effects on the flow upstream of a mountain whose shape varies in a horizontal direction transverse to the wind. To generate the mountain, a dipole whose strength varies sinusoidally in the spanwise direction is used. The basic wind is assumed to be a uniform stream with constant stability and the disturbances are assumed to be small. This corresponds roughly to a mountain range with a semi-circular cross-sectional profile whose height varies periodically from a maximum of h to zero over a distance W. The results show that significant three-dimensional effects persist right down to those values of the ratio r=h/W where one would normally be inclined to believe that a two-dimensional treatment suffices. This is a consequence of the strong tendency of a stratified wind to flow around rather than over a mountain peak. Two-dimensional approximations which preclude such motions should, therefore, be used with added caution.