I followed up yesterday's turbulence post with an experiment in my kitchen. Look at the top picture. The dribble out of the tap is clear and see-through in the top part of the picture. That means laminar flow. Half way down, the stream turns white. That's because it has become turbulent and is reflecting light. The Reynold's number must have passed 2000 half way down. I'm guessing because the water has accelerated by falling and the extra speed has tipped it over 2000. So how fast is it going? I put a measuring jug underneath and collected water for a minute. The double line in the jug is because of the meniscus so we need the lower line. It looks like 105 ml. The scale divisions are 25ml apart but they are wide so it is easy to work out steps between them. I'd say the precision is more like +/- 5ml. So I can say that there was 1.8 ml per second, given to only 2 sig figs because the % uncertainty in the reading is 4%. Volume flow rate = cross-sectional area x speed. Diameter = 2mm so cross-sectional area is about 3 x 10^-6 square metres. This gives speed = 0.6 metres per second. Speed where? I'm not sure since all of the mass has definitely made it regardless of acceleration. It might be speed in the pipe but the pipe is wider than the dribble of water. Anyway, Reynold's number turns out to be about 1 using these numbers, so definitely not turbulent. Working backwards from 2000 would give a speed of over 1000 metres per second for onset of turbulence if my calculations are right. So I've either miscalculated or some of the assumptions about diameter are wrong.
