Four times a flow passage's cross-sectional area divided by its wetted perimeter. It provides a characteristic length for many internal-flow correlations, with applicability depending on geometry and flow regime.
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The hydraulic diameter is a measure used in fluid mechanics to characterize the flow cross-section of a non-circular conduit or channel. It provides a way to calculate flow characteristics, such as velocity and pressure drop, in situations where the geometry is not a simple circular pipe.