Physics Help Forum Reynold Number
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 Jan 21st 2016, 02:16 AM #1 Senior Member   Join Date: Jun 2014 Posts: 306 Reynold Number Why the notes gave that the entry length in turbulent flow less dependent on the Reynold number in the second photo ? What does it mean ? Attached Thumbnails
 Jan 21st 2016, 04:45 AM #2 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Reynolds number can be defined as the ratio of the viscous forces to the inertial forces within the fluid. In other words it is a measure of the ratio of the forces trying to keep the fluid moving in whatever direction it is currently moving relative to the forces trying to make the fluid move in the same manner as whatever it is next to. There is a (not at all straight-forward) relationship between this ratio and the tendency of a laminar flow to become turbulent. So on to your question, The final flow character within the pipe will be turbulent The distance taken for the flow character within the tube to settle to its stable condition will be dependent on the Reynolds number. If the entry flow is already turbulent, it will take less time for it to reach the stable condition than if the entry flow is laminar. The character of the fully developed turbulent flow within the pipe will be different from the turbulent flow in free-air because the maximum size of the whorls and eddies will be determined by the pipe diameter. However the processes discarding large eddies from the pipe flow are largely mechanical happening immediately at the mouth of the pipe and have hardly any relationship to the Reynolds number. __________________ You have GOT to Laugh !
Jan 21st 2016, 08:50 AM   #3
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 Originally Posted by MBW Reynolds number can be defined as the ratio of the viscous forces to the inertial forces within the fluid. In other words it is a measure of the ratio of the forces trying to keep the fluid moving in whatever direction it is currently moving relative to the forces trying to make the fluid move in the same manner as whatever it is next to. There is a (not at all straight-forward) relationship between this ratio and the tendency of a laminar flow to become turbulent. So on to your question, The final flow character within the pipe will be turbulent The distance taken for the flow character within the tube to settle to its stable condition will be dependent on the Reynolds number. If the entry flow is already turbulent, it will take less time for it to reach the stable condition than if the entry flow is laminar. The character of the fully developed turbulent flow within the pipe will be different from the turbulent flow in free-air because the maximum size of the whorls and eddies will be determined by the pipe diameter. However the processes discarding large eddies from the pipe flow are largely mechanical happening immediately at the mouth of the pipe and have hardly any relationship to the Reynolds number.
what do you mean ? can you explain in more simple sentence ? what is eddy ?

 Jan 21st 2016, 12:22 PM #4 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 An eddy is a term for the fluid moving in a circular path. When different parts of a fluid are moving at different speeds (zero speed at the wall of the pipe, full speed at the center) it will tend to behave as if it was made of many small regions, each moving at a different speed Since one side of each region of fluid is moving faster than the other side, this will cause that region of fluid to rotate. This is the basic nature of turbulent flow. __________________ You have GOT to Laugh ! Last edited by MBW; Jan 21st 2016 at 12:25 PM.
 Jan 23rd 2016, 04:39 AM #5 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 I have been thinking about a way that might help visualize this. Imagine a large number of rubber balls all of different sizes moving through a pipe. Now focus your imagination down onto just a small group of the balls, each ball will be rubbing against its neighbors, which will cause it to roll. Now consider the entrance to the pipe, only balls small enough to enter the pipe will get in. In the same way only the smaller eddys will enter the pipe from a turbulent fluid flow. __________________ You have GOT to Laugh !
Jan 23rd 2016, 05:08 PM   #6
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 Originally Posted by MBW An eddy is a term for the fluid moving in a circular path. When different parts of a fluid are moving at different speeds (zero speed at the wall of the pipe, full speed at the center) it will tend to behave as if it was made of many small regions, each moving at a different speed Since one side of each region of fluid is moving faster than the other side, this will cause that region of fluid to rotate. This is the basic nature of turbulent flow.
rotate in which direction ?

 Jan 24th 2016, 03:57 AM #7 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Depends on the direction of rotation of neighboring regions. Next to the wall the rotation will be exactly as you would expect for a ball rolling along the wall. but for the next layer out the roll will be reversed, etc. However remember that these are NOT balls, the analogy must not be pushed too far. Regions of fluid can join, or break apart, form or dissipate, etc... one of the key features of turbulence is that it is chaotic. A good image of turbulence can be found here: http://www.nasa.gov/jupiter.gif Note how features stay identifiable, even though they are constantly changing. __________________ You have GOT to Laugh !

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