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The Reynolds number is the ratio of inertial forces to viscous forces within a fluid that is subjected to relative internal movement due to different fluid velocities. A region where these forces change behavior is known as a boundary layer, such as the bounding surface in the interior of a pipe.
In engineering, the Moody chart or Moody diagram (also Stanton diagram) is a graph in non-dimensional form that relates the Darcy–Weisbach friction factor f D, Reynolds number Re, and surface roughness for fully developed flow in a circular pipe. It can be used to predict pressure drop or flow rate down such a pipe.
For laminar flow in a circular pipe of diameter , the friction factor is inversely proportional to the Reynolds number alone (f D = 64 / Re ) which itself can be expressed in terms of easily measured or published physical quantities (see section below). Making this substitution the Darcy–Weisbach equation is rewritten as
Note that for the case of a circular pipe, D H = 4 π R 2 2 π R = 2 R {\displaystyle D_{\text{H}}={\frac {4\pi R^{2}}{2\pi R}}=2R} The need for the hydraulic diameter arises due to the use of a single dimension in the case of a dimensionless quantity such as the Reynolds number , which prefers a single variable for flow analysis rather than ...
The Reynolds number Re is taken to be Re = V D / ν, where V is the mean velocity of fluid flow, D is the pipe diameter, and where ν is the kinematic viscosity μ / ρ, with μ the fluid's Dynamic viscosity, and ρ the fluid's density. The pipe's relative roughness ε / D, where ε is the pipe's effective roughness height and D the pipe ...
In the rough pipe domain, friction loss is dominated by the relative roughness and is insensitive to Reynolds number. In the transition domain, friction loss is sensitive to both. For Reynolds numbers 2000 < Re < 4000, the flow is unstable, varying with time as vortices within the flow form and vanish randomly.
For example, it is used to calculate flow through circular and non-circular tubes in order to examine flow conditions (i.e., the Reynolds number). In those cases, the characteristic length is the diameter of the pipe or, in case of non-circular tubes, its hydraulic diameter D h {\displaystyle D_{h}} :
For non-circular pipes, such as rectangular ducts, the critical Reynolds number is shifted, but still depending on the aspect ratio. [3] Earlier transition to turbulence, happening at Reynolds number one order of magnitude smaller, i.e. ∼ O ( 10 2 ) {\displaystyle \sim {\mathcal {O}}(10^{2})} , [ 4 ] can happen in channels with special ...