Pressure drop in a sudden expansion Borda calculation

Pressure drop in a sudden expansion (Borda calculation) 

For a flow that is uniform in sections and S2, and denoting the velocities by f/i and U2, mass conservation is written as  

The momentum theorem, of which only the projection onto vector 1 is considered here, is written as  

The pressure force on the sidewall Sl has disappeared as it is perpendicular to 1. Force Rf represents the pressure force on the sohd wall Sp, which is the end wall surrounding throughflow section Si. Borda s model assumes that the pressure also equals Pi on Sp. The pressure force P/ is then given by  

Lastly, the friction, K, exerted by the flow on the sidewall is neglected. Using mass conservation [2.26] and properly [2.28], it is inferred from [2.27] that  

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