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If sharp edged inlet to pipes connecting to a pump is adopted, flow separation will occur. Flow separates from sharp edges and a recirculation zone is formed. Moreover, turbulence shall form at downstream when the flow at vena contracta subsequently expands to fill up the unfilled void. Flow separation leads to significant head loss.
The design of bellmouth entry is to ensure that the flow is uniform over the entire intake section and the head loss induced at inlet section is minimized.