Path: csiph.com!weretis.net!feeder6.news.weretis.net!feeder.usenetexpress.com!feeder-in1.iad1.usenetexpress.com!border1.nntp.dca1.giganews.com!nntp.giganews.com!buffer1.nntp.dca1.giganews.com!buffer2.nntp.dca1.giganews.com!nntp.earthlink.com!news.earthlink.com.POSTED!not-for-mail NNTP-Posting-Date: Wed, 06 Sep 2017 17:36:10 -0500 Message-ID: <59B07879.338B@mindspring.com> Date: Wed, 06 Sep 2017 18:36:41 -0400 From: Ron Hardin X-Mailer: Mozilla 2.02 (WinNT; I) MIME-Version: 1.0 Newsgroups: sci.physics Subject: Re: Hourglass Question References: <59ADBEA8.5404@mindspring.com> <59B040CE.429@mindspring.com> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 27 X-Usenet-Provider: http://www.giganews.com NNTP-Posting-Host: 71.50.222.0 X-Trace: sv3-UWxpqZRMU661ggbzsdobAGZapEQ1bH8x1S5S1ysoGkP/81VJwujKWYD6IN1G33aSqE9Mz5CBT0QtL5n!2SojWlimeMhs+yHUF7xawOI0+yOcuX7FE1UXs/ttNMy1X7GPxh8sJ23yf+Zs/CQEbY7Ec0e3i341!jkM3Hvmb X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 X-Original-Bytes: 2623 Xref: csiph.com sci.physics:648976 Serg io wrote: > Fluid approximations do not work well at the scale of sand in > hourglasses for most hourglasses. Almost all of the sand is statically > braced against the walls and floor of the hourglass. Instead, you have a > small region of instability above the hole where it is not possible for > the sand to be braced; as that unstable portion falls through the hole, > more falls in from above. There is slow flow of sand downwards, but it's > still mostly statically braced. Thus, to a first approximation, flow > rates are determined by grain size and shape (and material), and the > immediate geometry around the hole (predominantly the size). Thanks! Did they get the sand bridges and instability the same way I did, by thinking about it, or is there a dynamic simulation somewhere? It was wondering how to program a simulation long ago that first started the open question for me. It wasn't and isn't obvious what to program, unlike say fluid particles or stars in a galaxy, or plasmas, where the particles can move independently of each other with the exception of neighboring particles or a global field. Sand motion depends on a configuration over potentially long distances since the particles touch. Incidentally, even with a fluid, the weight is supported by the sides of the container, with the exception of the column over the hole. It's the indifference to pressure at the hole that distinguishes the two. The sand bridges, if unbroken, don't let the pressure through. -- rhhardin@mindspring.com On the internet, nobody knows you're a jerk.