The acceleration of air flow into a duct is inherently a highly efficient process and the difference between even the crudest radius inlet, and the most aerodynamic shape possible is slight, amounting to no more than a few percent. The flow coefficient of a perfect entry would be 1.0 while the coefficient for a sharp edged entry would be 0.6 and a re-entrant plain pipe 0.5.
The acceleration of air flow into a duct is highly efficient, and the difference between even the simplest inlet shape and the most aerodynamic design is minimal, amounting to no more than a few percentage points. The flow coefficient of an ideal entry would be 1.0 while that for a sharp-edged entry would be 0.6, and for a re-entrant plain pipe it would be 0.5.