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gas dynamics (ratio of the molecular mean free path length to a representative physical length scale) Kutateladze number: Ku = / (()) / fluid mechanics (counter-current two-phase flow) [14] Laplace number: La
In physics, a characteristic length is an important dimension that defines the scale of a physical system. Often, such a length is used as an input to a formula in order to predict some characteristics of the system, and it is usually required by the construction of a dimensionless quantity, in the general framework of dimensional analysis and in particular applications such as fluid mechanics.
The head loss Δh (or h f) expresses the pressure loss due to friction in terms of the equivalent height of a column of the working fluid, so the pressure drop is =, where: Δh = The head loss due to pipe friction over the given length of pipe (SI units: m); [b]
The friction loss is customarily given as pressure loss for a given duct length, Δp / L, in units of (US) inches of water for 100 feet or (SI) kg / m 2 / s 2. For specific choices of duct material, and assuming air at standard temperature and pressure (STP), standard charts can be used to calculate the expected friction loss.
where is the density of the fluid, is the average velocity in the pipe, is the friction factor from the Moody chart, is the length of the pipe and is the pipe diameter. The chart plots Darcy–Weisbach friction factor f D {\displaystyle f_{D}} against Reynolds number Re for a variety of relative roughnesses, the ratio of the mean height of ...
For the limiting case of a very wide duct, i.e. a slot of width b, where b ≫ a, and a is the water depth, then D H = 4a. For a fully filled duct or pipe whose cross-section is a convex regular polygon , the hydraulic diameter is equivalent to the diameter D {\displaystyle D} of a circle inscribed within the wetted perimeter .