The Power of the Waves

Jospa Blog

December 29, 2008

A test to prepare for the important test - conducted ‘by hand’ to improve the chances of success of a more engineered test - mixed results.

Filed under: Uncategorized — admin @ 6:31 pm

Preliminary report Test (3a) HMRC flume tank UCC

We (Tom and I) ran a very simple trial today in the flume tank. It was crude and preliminary and the method was simply to splash water at regular intervals, synchronizing as well as we could with the passing of the waves, into a sloped pipe connected to the tube. The idea with the sloped pipe was to get air and water into the tube with an initial starting velocity to replicate waves. The ideal result would have been if the water slugs splashed out the opposite end having surfed the whole length of the tank. We did not get such a good result, but we got surfing for a distance of about 7m before the water started to slow down and block the tube. The greater the amount of water we put in the further it traveled. We were on this occasion shorthanded with just Tom to concentrate on splashing the water, using a 10 litre bucket, in time with the waves. I was observing, so I have little good video footage. We tried 1.5 second periods and then 1 second periods. Both had the same amplitude. The tube is only 50mm dia. and 15m long. The water surfed somewhat further with the 1 second period than the 1.5m. Following both of these we stopped the waves completely and tried splashing in the water at the same rate and timing. This was to check how far the water would go under its own initial momentum, unaided by the surfing effect of the waves. In both these cases the water flowed, rather than surfed, which is not surprising. It also moved less quickly, with none of the rapid movement of the bubbles that was evident when the waves were ‘on’. It is difficult to describe at what point the movement started to stall, as the general appearance differed in both cases, with and without waves. However, with the waves on the water moved a few meters further for both periods, before the leading water seemed to break up and stall. The speed of movement seemed significantly higher with the waves than without.

While the trial was crude, and the result not all that we would have liked, I feel reasonably confident that with a regular feed of air & water, and the latter not limited to just 10 litres, or about 5 splashes, we will get surfing all the way and out the end of the 15m long tube. My aim now is to make up such a feed system. It will consist of a driven, transparent winding like the first round of the Revolving demo unit. The aim is to achieve water flowing out the end of the tube, and a pressure difference between the outflow end and a point just after the water & air infeed.

Why introduce this external drive, which will perhaps complicate the proof of concept? Wouldn’t a wave-driven-only device be more convincing and easy to understand? Yes, it would, but my gut feeling is that a single OWC (oscillating water column) will not give enough initial momentum for surfing to happen. Perhaps two OWC’s, one pumping air, the other overtopping water, and both feeding the tube might work, but we could waste a lot of time only to find that we can’t get them to work either. What we need to prove most is that we can get a pressure increase as the air locks travel the length of the tube, and an external drive seems the most certain way of getting to that stage. I see the tube as a pressure amplifier, which will only work if there is a minimum starting energy input for the tube to magnify. I am not sufficiently confident I can get this minimum starting momentum using an OWC.

I have one concern however, and that is the tube diameter of 50mm. With a water speed of 1.5m/sec inside the tube and the tube flexing we might expect a pressure loss of perhaps up to 0.5m head. If the pressure loss in the tube is as much as this, we will not get a positive result. Changing to a larger tube diameter is possible but it would make the length of the HMRC tank an even bigger scaling limitation than it already is. A suitable larger tube may not be easy to source. I will try to get a better estimate of what pressure loss to expect, but I fear intermittent air & water slugs might have a pressure loss almost equivalent to if water alone were flowing.

Both the OWC and today’s trial jig have one problem in common: the transition from the solid part of the jig to the flexible is not very streamlined, so causing unnecessary loss of pressure. I propose fixing both of these problems (more laminar flow of the water and tube diameter) over Christmas, so that if we try either again we will have an improved chance of success.

Joss Fitzsimons

Monday 22nd December 2008

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