Yes, that’s exactly what I think my problem is, too!
Thanks for the perfect explanation. Next time I will make sure to take this to heart :-)

For me the biggest challenge to changing the rotation rate for this kind of experiment is to have the self-control to not make the change in rotation rate too big. It is very tempting to think “oh that’s hardly a noticeable difference” and increase more. But if obstacle is distance r from center of tank and tank rotation rate is changed by dw to new value w, then the induced water speed near the obstacle will be
V = r dw,
and Coriolis parameter is
f = 2w
and Rossby number of object with length scale L is
Ro = V/FL = dw r /(2wL) = .5(dw/w)(r/L).

Since in the apparatus r/L is about 5 to 10, you need to have dw < 10% of w to have any hope of Ro < 1. If you think you can change rotation period from say 15 s to less than 1 s longer, that might be good enough.

Hi Barry,
Thanks for your comment! Having a white obstacle is very high up on my list of things to try next time I have access to a rotating table.
What advice do you have on rotation rate and especially on changing it? I had a lot of issues changing it by hand (as in I never felt I got it right) and am wondering if a micro controller is the way to go…
Best, Mirjam

Another thing I noticed, last time I did this experiment, was that having a very black obstacle on a very white background makes it hard to visualize the taylor column, since whatever tracer shows up well over one color doesn’t over the other.