Wp where A is the area of the flow cross section and Wp is the wetted perimeter (i.e., the length of contact between the fluid and the solid boundary in the flow cross section). For a full circular pipe, Dh = D (the pipe diameter). The hydraulic diameter is the key characteristic geometric parameter for a conduit with any cross-sectional shape.

A. Laminar Flows

By either integrating the microscopic momentum equations (see Example 5-9) or applying a momentum balance to a "slug" of fluid in the center

Figure 7-1 Flow in a slit.

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