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is the Reynolds number with the cylinder diameter as its characteristic length; Pr {\displaystyle \Pr } is the Prandtl number . The Churchill–Bernstein equation is valid for a wide range of Reynolds numbers and Prandtl numbers, as long as the product of the two is greater than or equal to 0.2, as defined above.
The hydraulic diameter, D H, is a commonly used term when handling flow in non-circular tubes and channels. Using this term, one can calculate many things in the same way as for a round tube. When the cross-section is uniform along the tube or channel length, it is defined as [1] [2] =, where
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.
Here l is the turbulence or eddy length scale, given below, and c μ is a k – ε model parameter whose value is typically given as 0.09; =. The turbulent length scale can be estimated as =, with L a characteristic length. For internal flows this may take the value of the inlet duct (or pipe) width (or diameter) or the hydraulic diameter.
A is a reference area, e.g. the cross-sectional area of the body perpendicular to the flow direction, V is volume of the body. For instance for a circular cylinder of diameter D in oscillatory flow, the reference area per unit cylinder length is A = D {\displaystyle A=D} and the cylinder volume per unit cylinder length is V = 1 4 π D 2 ...
The size of the largest scales of fluid motion (sometimes called eddies) are set by the overall geometry of the flow. For instance, in an industrial smoke stack, the largest scales of fluid motion are as big as the diameter of the stack itself. The size of the smallest scales is set by the Reynolds number.
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L = clearance length (metre) As can be seen from the formula, the clearance height c has much more influence on the leakage than the length. The formula clearly hints of pure laminar flow conditions. It is also valid for gases. Contact between the spool and the wall, the value that is generally used for practical calculations: Flow Q e = 2.5 · Qi