to be directed to the left and vice-versa. An increase in feeder current strength, on the
same side as the vortex shedding, was also observed in some cases.
Limitations of instability analysis. A consequence of having a record length of
only 18 minutes, is that an oscillation width in the longshore at specific cross shore
locations could not be determined with confidence. Therefore, determining a growth rate
for the unstable oscillation could not be completed. An oscillation width in the rip
channel couldn't be found either, therefore the oscillation period of the rip current using
jet instability mechanism could not be determined.
To achieve a finer temporal resolution, than 1 minute, more drifter coverage would
be needed to ensure enough measurements in the desired time step. The author doesn't
think this presents a problem because instabilities in rip current oscillations are associated
with time scales generally larger than 1 minute.
Vorticity
Time-averaged vorticity (co), calculated from Equation 3-3, was determined
throughout the field of view containing the rip current system for the long tests.
dv du
0) = (3-3)
dx dy
The terms dv/dx and du/dy in Equation 3-3 were calculated using the second-order
central difference formula. As stated before, a spatial resolution of 0.5 m was used due to
the available drifter coverage. Figure 3-15 shows an example of time-averaged vorticity
throughout a rip current system for test 16. Remember, the first 5 minutes of tests 15 and
16 are excluded to eliminate the effects of the wave-maker startup on averaged quantities.
Appendix G contains the plots of time-averaged vorticity for every long test.