J . Fluid Mech. (1983), vol. 135, pp. 261-282 Printed in Great Britain 26 1 Stratified flow over three-dimensional ridges By I. P. CASTRO, Department of Mechanical Engineering, University of Surrey, Guildford, Surrey GU2 5XH, U.K. W. H. SNYDERt Meteorology and Assessment Division, US. Environmental Protection Agency, Research Triangle Park, NC 27711, U.S.A. AND G. L. MARSH Northrop Services Inc., Research Triangle Park, NC 27711, U.S.A. (Received 4 October 1982 and in revised form 7 June 1983) An experimental study of the stratified flow over triangular-shaped ridges of various aspect ratios is described. The flows were produced by towing inverted bodies through saline-water solutions with stable (linear) density gradients. Flow-visualization techniques were used extensively to obtain measurements of the lee-wave structure and its interaction with the near-wake recirculating region and to determine the height of the upstream dividing streamline (below which all fluid moved around, rather than over the body). The Froude number F( = U/Nh) and Reynolds number (Uh/v), where U is the towing speed, N is the BrunGVaisala frequency, h is the body height, and v is the kinematic viscosity, were in the nominal ranges 0.2-1.6 (and 00) and 200&16000 respectively. The study demonstrates that the wave amplitude can be maximized by ‘tuning’ the body shape to the lee-wave field, that in certain circumstances steady wave breaking can occur or multiple recirculation regions (rotors) can exist downstream of the body, that vortex shedding in horizontal planes is possible even at E’ = 0.3, and that the ratio of the cross-stream width of the body to its height has a negligible effect on the dividing streamline height. The results of the study are compared with those of previous theoretical and experimental studies where appropriate. 1. Introduction The flow of a continuously stratified fluid over two-dimensional obstacles has been a subject of study for many years. Theoretical studies have generally been based on either small-perturbation, linearized types of model (Lyra 1943 ; Queney 1948 ; McIntyre 1972) or models that assume a steady flow with, generally, a particular form of the upstream boundary conditions, which results in a linear equation for the streamline displacements (Long 1953 ; Davis 1969). Using various obstacle shapes, both Long and Davis conducted experiments which, as far as the basic features of the lee-wave field behind the obstacles were concerned, demonstrated reasonable agreement with the predictions of ‘Long’s model ’. The important dimensionless parameters are K = ND/nU, F = U/Nh and t Permanent address : National Oceanic and Atmospheric Administration, U.S. Department of Commerce.