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The effect of a strong density step on blocked stratified flow over topography

TitleThe effect of a strong density step on blocked stratified flow over topography
Publication TypeJournal Article
Year of Publication2020
AuthorsJagannathan A., Winters KB, Armi L.
Date Published2020/04
Type of ArticleArticle
ISBN Number0022-1120
Accession NumberWOS:000515191100001
Keywordshydraulic control; mechanics; number; physics; severe downslope windstorm; stratified flows; topographic effects; waves

The dynamical connection between topographic control and wave excitation aloft is investigated theoretically and numerically in the idealized setting of two-dimensional stratified flow over an isolated ridge. We consider a constant far upstream inflow with uniform stratification except for a sharp density step located above the height of the ridge crest. Below this step, the stratification is sufficiently strong that the low level flow is blocked upstream and a hydraulically controlled flow spills over the crest. Above the density step, the flow supports upward radiating waves. In the inviscid limit, a bifurcating isopycnal separates the hydraulically controlled overflow from the wave field aloft. We show that, depending on the height of the density step, the sharp interface can either remain approximately flat, above the controlled downslope flow, or plunge in the lee of the obstacle as part of the controlled overflow itself. Whether the interface plunges or not is a direct consequence of hydraulic control at the crest. The flow above the crest responds to the top of the sharp density step as if it were a virtual topography. We find that a plunging interface can excite a wave field aloft that is approximately six times as energetic, with 15 % higher pressure drag, than that in a comparable flow in which the interface remains approximately flat.

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