Fluid Mechanics
The hydraulic radius for flow in a rectangular duct of cross-sectional dimension H, W is

HW/4(H + W)²
√(HW/π)
2HW/(H + W)²
HW/2(H + W)²

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Fluid Mechanics
The Prandtl mixing length is

Useful for computing laminar flow problems
Independent of the shear stress
Independent of radial distance from the pipe axis
Zero at the pipe wall and is a universal constant

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