Stratified Flow over Two-Dimensional Topography in Fluid of Infinite Depth: A Laboratory Simulation

Peter G. Baines CSIRO Division of Atmospheric Research, Aspendale, 3195, Australia

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Klaus P. Hoinka DFVLR D-8031 Oberpfaffenhofen, West Germany

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Abstract

This paper describes some laboratory experiments with two-dimensional stratified flow over isolated topography, in which a novel configuration simulating a radiating upper boundary condition is employed. Several experimental tests show that the upper boundary is quite effective in absorbing energy. The properties of flow over five different obstacle shapes were obtained for a merge of values of the parameter Nh/U(h is obstacle height, N Brunt-Väisälä frequency and U towing velocity) from 0 to 4 (approximately). The main results of the study are 1) for 0 ≤ Nh/U ≤ 0.5 (±0.2), the flow is consistent with linear theory and Longs model; 2) for 0.5 ≲ Nh/U ≲ 2.0, upstream columnar disturbances are found which apparently propagate arbitrarily far upstream in an inviscid system. 3) overturning and rotors in the lee wave field occur for Nh/U ∼ 1.5; and 4) for Nh/U ∼ 2.0, blocked fluid is present upstream, and in some cases is also apparent downstream. This upstream blocking is due to the super-position of the propagating columnar disturbances; it will similarly extend arbitrarily far upstream given sufficient time.

Abstract

This paper describes some laboratory experiments with two-dimensional stratified flow over isolated topography, in which a novel configuration simulating a radiating upper boundary condition is employed. Several experimental tests show that the upper boundary is quite effective in absorbing energy. The properties of flow over five different obstacle shapes were obtained for a merge of values of the parameter Nh/U(h is obstacle height, N Brunt-Väisälä frequency and U towing velocity) from 0 to 4 (approximately). The main results of the study are 1) for 0 ≤ Nh/U ≤ 0.5 (±0.2), the flow is consistent with linear theory and Longs model; 2) for 0.5 ≲ Nh/U ≲ 2.0, upstream columnar disturbances are found which apparently propagate arbitrarily far upstream in an inviscid system. 3) overturning and rotors in the lee wave field occur for Nh/U ∼ 1.5; and 4) for Nh/U ∼ 2.0, blocked fluid is present upstream, and in some cases is also apparent downstream. This upstream blocking is due to the super-position of the propagating columnar disturbances; it will similarly extend arbitrarily far upstream given sufficient time.

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