Form Drag

enlargeStonewall Bank
Fluid flowing over bumps can undergo transitions that lead to a pressure drop across the bump, which in turn results in momentum lost to the flow. These transitions include a deformation of the sea surface plus an asymmetric structure in surfaces of constant density (or isopycnals; the white lines in this figure - flow is from right to left). This is a hydraulic jump developing over a small bank on Oregon's continental shelf (Stonewall Bank). We have been able to observe this several times but do not yet understand the means by which it is forced.

High turbulence (lower panel) is typically seen in the accelerated near-bottom flow, at the sheared mid-water column interface and in the jump region.



enlargeThree Tree Point
We have measured a similar flow over Three Tree Point in Puget Sound (just south of downtown Seattle). Here, flows like this are repeated tidally. Numerical simulations by Parker MacCready at the University of Washington show the complete evolution of flows over the point. They are in reasonable agreement with many aspects of our measurements.

Note the similarities in the structure of both isopycnals and turbulence in the two images (flow in the latter case is from left to right).

While form drag is generally ignored in large scale simulations of ocean circulation, it is included in simulations of atmospheric flows, where the form drag caused by flow over mountain ranges represents a critical loss of momentum. It is not known under what conditions oceanic form drag is of similar importance.


More on our work on form drag in oceanic flows:

Form drag and mixing due to tidal flow past a sharp point, J. Phys. Oceanogr., 34, 1297-1312, 2004 (K. A. Edwards, P. MacCready, J.N. Moum, G. Pawlak, J. M. Klymak and A. Perlin) [pdf]

Internal hydraulic flows on the continental shelf: high drag states over a small bank, J. Geophys. Res., 106, 4593-4611, 2001 (J.D. Nash and J.N. Moum) [pdf]

Topographically-induced drag and mixing at a small bank on the continental shelf, J. Phys. Oceanogr., 30, 2049-2054, 2000 (J.N. Moum and J.D. Nash) [pdf]