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push vs pull vis a vis rudders
Brian Whatcott wrote in message ... On Thu, 1 Apr 2004 12:24:46 +0100, "JimB" wrote: An experimental rig for visualizing fluid flow over rudders etc., is easy to make and provably representative of 2-D flow. snip At the top of the incline, permanganate crystals trail stream lines down the incline. Nice little touch! This is an easy way to show the "molecules give lift by hitting the proximal surface" enthusiasts how fluid dynamics really works. (about two thirds of the side force from the distal surface, and one third from the proximal surface.) You can work it out from the streamline spacing over both surfaces. OK. This is straightforward foil in a free flow. It confirms the point (among others) that pressure drop and speed change are linked. However, our steering rudder in reverse is a foil in (lets call it) convergent flow, where, if the pivot was actually at the prop origin, the flow lines would always be along the rudder with no deflection. As the rudder moves away, then stream deflections occur, but the speeds (and forces) drop right off, and the flo is funny too, showing a strong s bend. And on top of all of that, my fundamental momentum theory sais that all this input water is starting at zero velocity relative to the boat, but exiting the prop with a new velocity. So up stream action (rudder angle) would only have an effect if it changed the downstream velocity. This is quite feasible, since output velocity is not constrained (as from a hosepipe - Ugh - Feyneman again) and if there's a lateral component at the input end, I'm thinking it would be present at the output end. An extreme model is looking at an elliptical duct on the input side canted at an angle to the prop. So I'll go away and get my brain around that idea to see where it takes me. It does remove the need to think about all the various forces on rudder, prop, hull etc and their interactions and connections in a complex pressure field. JimB |
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