Effects of upstream disturbances on headland wakes in coastal waters
Melanie J. O'Byrne1, Ross W. Griffiths1, Graham O. Hughes1, Jason H. Middleton2
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 School of Mathematics, University of New South Wales, Sydney, NSW 2052, Australia
Wakes are important features in the coastal environment. Flows around islands and headlands bring nutrients up from deeper waters, influence the distribution of sediments and provide favourable environments for marine biota. These flows also affect pollutant dispersal and must be considered in offshore disposal strategies. While many wake studies have assumed a smooth steady oncoming flow, Nature provides a more complex flow that often contains eddies and turbulence from other topographic features upstream. These may alter the size and stability of the wake, along with rates of exchange between the wake and the surrounding water.
We have modelled headland wakes, with and without disturbances carried from upstream, using a 3-metre flume and ultrasonic Doppler velocity measurements. A surprising finding is that over a range of conditions, the disturbance to the upstream flow results in a wake that both contains weaker eddies overall and decays more rapidly with distance downstream of the headland (figure 1). Incident disturbances modify the stability of the shear at the edge of the wake and partially suppress the formation of coherent eddy structures in the lee of the headland. The kinetic energy in the region of the wake close to the headland is distributed over a greater spanwise extent and a broader range of (eddy) frequencies, which results in a more rapid dissipation of the eddies with distance downstream. We are currently investigating the interactions of flow structures in the lee of the headland that facilitate this behaviour, and exploring more fully the range of conditions under which such interactions occur.
Figure 1. Cross-stream velocities without and with an incident disturbance, measured across the headland wake at (a) just behind the headland, (b) one headland diameter downstream and (c) two diameters downstream. Note the rapid decay in velocity, and therefore kinetic energy, in the right column as you move downstream. (Velocities are normalised by the freestream velocity; positive velocities indicate motion away from the wall, negative indicates motion towards the wall).