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

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

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Fluid Mechanics
A stream line is

Always the path of particle
A line connecting the mid points of flow cross-section
Fixed in space in steady flow
Drawn normal to the velocity vector at every point

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