ReadFamily wrote: > > I am trying to build mental model so that I can explore some possibilies > about micro hydro (maybe nano hydro) in my head. .... > > It suprises me that water doesnt run faster. Is there really that much > friction between the water at the edges of a trough and the water running in > the center? > > Where is the water running fastest, in the center of the trough probably, > but at the surface, or below the surface. > > Would the water run at the same speed on an infinitely long incline as it > would with an incline ending in a 'waterfall'. > > With no incline the water surface would be parallel to the bottom of the > trough. As the trough is inclined, does the surface stay parallel to the > bottom, or is there an angle? > > Does anyone have any useful mental models to share to help me understand > intuitively how water behaves in an open trough?
1. There are a few gifted individuals for whom hydraulics and in particular the answers to these questions are intuitively obvious, but for most people this is not so. Indeed I think it rare for anyone's intuitive conception of water flow to be able to explain things like cavitation and hydraulic jumps for example. So for the vast majority of us, including me, understanding will come only if we are prepared to put in some effort. In my case, what little understanding I have has come through hard work and my intuition has failed me over and over again. For example, a point frequently noted in this goup, is that surface friction is proportional to the square of the velocity. Now let's be honest - for how many of us, is this an intitive fact? Let me give one more example. Recently we did some maintenance on a system where three identical turbines are supplied from a single penstock. Each is fed by its own short section of pipe that comes off at right angles from the main penstock feed. Now my intuition tells me, that the turbine furthest away from the intake will have lowest inlet pressure - WRONG! We measured and found that lowest pressure is experienced by the turbine closest to the intake. By coincidence we had on the site an air-conditioning mechanic whose experience with air flow enabled him to adequately explain what was going on. I mention this in particular, because as a check of the archives will show, over the years, I as much as anybody, regularly answer the hydraulics questions that find their way to this discussion group. 2. When water runs steadily through an inclined uniform pipe or channel, the water surface is indeed parallel to the bottom. Under these conditions the flow is said to be "critical". When slope, dimensions or surface roughness change, the flow goes through a stage of "sub-critical" or "super-critical" flow, until eventually a new critical flow state is established. I commend to you a program called Flo Pro 2.0. It is available for free download at: http://www.prosoftapps.com/flowpro.htm. Even if you do not want to do any calculations it is worth it just for the help files, where all these concepts and more are explained. 3. As a practitioner in the area of micro-hydro design, the mental models for flow I use are essentially Bernoulli's equation (over 9,000 hits on Google) and the widely used Manning's equation (refer to Flo Pro 2). I repeat, unless you are gifted, nothing of value will be intuitive. Regards, Max Enfield Planetary Power ------------------------ Yahoo! Groups Sponsor --------------------~--> Make a clean sweep of pop-up ads. Yahoo! Companion Toolbar. Now with Pop-Up Blocker. Get it for free! http://us.click.yahoo.com/L5YrjA/eSIIAA/yQLSAA/FGYolB/TM --------------------------------------------------------------------~-> Does your company feature in the microhydro business directory at http://microhydropower.net/directory ? If not, please register free of charge and be exposed to the microhydro community world wide! NOTE: The advertisements in this email are added by Yahoogroups who provides us with free email group services. The microhydro-group does not endorse products or support the advertisements in any way. More information on micro hydropower at http://microhydropower.net To unsubscribe: send empty message to [EMAIL PROTECTED] Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/microhydro/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
