David Higgon contributed >Tony mentioned that his hose had a jet attachment that fitted onto the >pipe, and I think this is the crucial bit. If typical of such attachments >it will be of a larger diameter than the pipe, probably twice the size, and >increase in diameter in a series of steps, each perpendicular to the pipe. >This means that fluid flowing along the outside of the pipe is then >presented with a bluff (un-streamlined) body when it meets the back end of >the jet attachment. Drag on bluff bodies can be considerable and I suspect >that this accounts for the observed pull on the end of the pipe once it is >immersed in the flow.
The above all makes sense and the nozzle David describes is the one I'm using BUT as soon as the veriest tip of the nozzle touches the surface of the water it is 'snatched' downwards. Slowly immersing the nozzle right to the bottom of the bucket seems to have no increased effect on the downward pull (or reduced reaction force) I've just tried this several times to confirm! > >If the above is the correct explanation, the effect should only be >particularly noticable once the jet attachment is fully immersed. Indeed >it may need to be a little way into the liquid in order that some sort of >flow is established along the pipe before hitting the jet. Absolutely not I'm afraid! > >I hope I'm not too far off the mark in this explanation - it could have >dire consequences come my exam in October!! > So much for theory! :-) I'm reminded of making toy paddle boats with children of the sort where a rectangular notch is cut at the back of a plywood 'boat' and a cruciform paddle is driven in the gap by twisted elastic. The experiment was to find the most efficient paddle shape. Plane square blades worked OK but made a lot of splash so curved tinplate 'scoop' blades were substituted to give an action like the crawl stroke in swimming. Much to everyone's surprise the boat hardly moved but lots of water was thrown upwards and the stern pushed down. AND THEN *by accident* the same paddle was put in backwards resulting in the opposite of 'the crawl'. Wow! did that boat move! and with no splashing! Everyone was so fixated on the expected efficiency of the 'crawl' blades they would never have given the successful idea a thought - unless they'd first looked at a centrifugal impeller perhaps! Tony Moss
