| View previous topic :: View next topic |
| Author |
Message |
Hari
Guest
|
 Posted: Sat Dec 06, 2003 7:07 pm Post subject: Reynold nos |
 |
|
while calculating reynolds no. why is the characteristic length the
'diameter' for a pipe and the 'length of plate' for a plate. |
|
| |
|
 Back to top |
Ivar S. Ertesvåg
Guest
|
| Posted: Sat Dec 06, 2003 10:19 pm Post subject: Re: Reynold nos |
|
|
Hari wrote:
[quote]
while calculating reynolds no. why is the characteristic length the
'diameter' for a pipe and the 'length of plate' for a plate.
[/quote]
Simply because these are the characteristic lengths of the flows.
For the flow over a plate, you can also define a boundary-layer
Reynolds No. where the characteristic length is the thickness of the
layer (or, actually, one of the thicknesses that can be definied for
a boundary layer). |
|
| |
|
| Back to top |
Ed Ruf
Guest
|
| Posted: Sun Dec 07, 2003 12:23 am Post subject: Re: Reynold nos |
|
|
On 6 Dec 2003 05:07:54 -0800, in sci.mech.fluids harihgk@yahoo.com (Hari)
wrote:
[quote]while calculating reynolds no. why is the characteristic length the
'diameter' for a pipe and the 'length of plate' for a plate.
[/quote]
Remember that Reynolds No. is the ratio of viscous forces to inertial
forces and the viscous forces are in the boundary layers. Most pipe flow is
considered fully developed, meaning the pipe is of sufficient length that
the boundary layers have met at the pipe centerline and can no longer grow
are further. For short lengths of pipe where the flow never becomes fully
developed one can still use the length from the pipe entrance as a
characteristic length. This is indeed how one determines if the pipe is
long enough to be fully developed. After this length however, it no longer
has any real significance., and the meaningful characteristic length is the
pipe diameter in determining the extent of the boundary layer.
A boundary layer on a flat plate, unlike that in the pipe, can continue to
grow "forever." Hence the use of the distance from the leading edge of the
plate as the characteristic distance in determining the extent of the
boundary layer. |
|
| |
|
| Back to top |
|
