Near-critical orographic forcing of stratifed flow
Johnson, E. R. ; Esler, J. G. ; Paci, Alexandre ; Cazin, S. ; Cid, E. ; Eiff, Olivier ; Lacaze, Laurent
Two inter-related aspects of flow over orography have proved difficult to model simply -- the structure of the flow-field and the pressure drag exerted on the oncoming flow - yet these are of great interest. Rising and falling air affects cloud cover and rainfall distribution, and drag is of particular importance to oceanographers, climate modellers and esearchers involved in the development of numerical weather-prediction models, because of the need to parameterize the drag exerted by orography with spatial scales below the model grid scale.
Analytical studies show that the dynamics of the flow of two fluid layers of almost equal depths and densities differs significantly from those where the densities are similar but the depths differ. In particular, the waves that appear behind isolated three-dimensional orography in near-critical, but slightly subcritical, flow are long compared to the obstacle dimensions and stretch laterally across the flow with little change of form. <br>Our results show that these aspects of the wake can be observed experimentally. The experiments have been done in the CNRM-GAME (CNRSToulouse) stratified water flume, using an optical measurement technique developed at IMFT and a drag measurement technique developed at CNRMGAME. This unique facility is indeed very pertinent for such a study, in particular due to its ability to generate density stratified flow at high Reynolds numbers with low confinement effect.
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