A demonstration of the direction water swirls in two hemispheres Sync the videos yourself: http://toiletswirl.com For the record Destin and I repeat... 2 different Videos synced. The explanation of why water swirls in a specific direction is at about 4 minutes into the video.
Even after you eliminate initial movement, temperature and so on, there will still be other perturbation factors that would be harder to pin down and get rid of. The way that these guys performed it, I'm just guessing that the experiment would show maybe a 52 - 48 % result in favor of the hemisphere -based direction, over many trials with the same setup.
Imagine that there are some small imperfections very near the outlet of the container. For example, consider some tiny burrs left after cutting the hole in the bottom of the kiddy pool. Or burrs from cutting the hose. Or even just a manufacturer's embossed stamp near the center. Whatever. These imperfections have the effect of vanes that deflect the water as it approaches the outlet. From that tiny swirl, the vorticity would quickly spread and take over the whole system. In that case, of course there would be no chance of seeing the hemisphere effect.
Well, in order to eliminate defect-induced bias, how perfect does the shape have to be? Maybe the scale of the irregularities /roughness / bumpiness would have to be brought down below some scale that is driven by the Reynolds number. But I would guess that it would take some considerable effort to get, say, 70 % statistical consistency with the hemisphere.
