Experimental studies of strongly stratified flow past three-dimensional orography

Stably stratified hows past three-dimensional orography have been investiga ted using a stratified towing tank. Flows past idealized axisymmetric orogr aphy in which the Froude number, F-h = U/Nh (where U is the towing speed, N is the buoyancy frequency and h is the height of the obstacle) is less tha n unity have been studied. The orography considered consists of two sizes o f hemisphere and two cones of different slope. For all the obstacles measur ements show that as F-h decreases, the drag coefficient increases, reaching between 2.8 and 5.4 times the value in neutral flow (depending on obstacle shape) for F-h less than or similar to 0.25. Local maxima and minima in th e drag also occur. These are due to the finite depth of the tank and can be explained by linear gravity-wave theory. Flow visualization reveals a lee wave train downstream in which the wave amplitude is O(F(h)h), the smallest wave amplitude occurring for the steepest cone. Measurements show that for all the obstacles, the dividing-streamline height, z(s), is described reas onably well by the formula z(s)/h = 1 - F-h. Flow visualization and acousti c Doppler velocimeter measurements in the wake of the obstacles show that v ortex shedding occurs when F-h less than or similar to 0.4 and that the per iod of the vortex shedding is independent of height. Based on velocity meas urements in the wake of both sizes of hemisphere (plus two additional small er hemispheres), it is shown that a blockage-corrected Strouhal number, S-2 c = f L-2/U-c, collapses onto a single curve when plotted against the effec tive Froude number, F-hc = U-c/Nh. Here, U-c is the blockage-corrected free -stream speed based on mass-flux considerations, f is the vortex shedding f requency and L-2 is the obstacle width at a height z(s)/2. Collapse of the data is also obtained for the two different shapes of cone and for addition al measurements made in the wake of triangular and rectangular hat plates. Indeed, the values of S-2c for all these obstacles are similar and this sug gests that despite the fact that the obstacle widths vary with height, a si ngle length scale determines the vortex-street dynamics. Experiments conduc ted using a splitter plate indicate that the shedding mechanism provides a major contribution to the total drag (N similar to 25%). The addition of an upstream pointing 'verge region' to a hemisphere is also shown to increase the drag significantly in strongly stratified flow. Possible mechanisms fo r this are discussed.