The reason for it being after dark was so the speed of the flash, rather than the speed of the shutter, froze the action.
Inevitably, a lot of water sprayed onto the camera lens and that has taken away some of the quality I was hoping for, but the pictures do show nicely how:
- the cups cut into the jet and chop it up
- the jet is carved in two by the splitter ridge
- the notch in each cup ensures the splitter ridge is the first part to enter the jet
- water then passes down and around the floor of the cup
- water is thrown up and away by the side walls
- pulses of water are created exiting to the side
The photos were made possible only by choosing carefully how to capture them. I use an SMA Windy Boy inverter to connect to the grid, and it takes 4 minutes from the time it first receives power before connecting to the grid. During this time, the pelton overspeeds and causes exhaust water to exit in a different way to when it's at its operational speed. A line of approach relatively free of spray is created which gives a good view of the jet.
The rotational speed in the photos was 1270 rpm, giving a linear velocity for the runner at the pcd (pitch circle diameter) of about 15 m/s.
At this speed, the time taken for a cup to move to the position of the cup ahead of it is about one 400th of a second.
The nozzle orifice was 6.48 mm diameter, the flow 0.91 l/s and the jet velocity about 31 m/s.
The camera was a Sony Cybershot DSG TX10. It is water proof, - and it needed to be !
