There is always some attempt to provide some radius at the entry. In practice these latter types of entry are never used for engine intakes. 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 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. A longer inlet pipe will create a later intake pressure wave that will help keep the air in the chamber until the intake valve closes. but, it is actually that the intake valve is closing too late and the combustion chamber is simply overfilling and blowing back out the intake port, before the intake valve closes.
It is commonly related that "stand off" (air-fuel mix that gets pushed back out of the port, usually at full throttle / low rpm) is somehow captured by installing a longer intake pipe (stack).
Most current aftermarket stacks are designed to be run "in" the airbox and a company that does research well will have some applications that have all the same length and some applications that have differing lengths of stacks on different cylinders. The length of the stack is known to have a direct effect on a particular engine's boosted power range. A properly tuned intake tract produces a rather audible intake noise under high airflow conditions - that is commonly "untuned" to allow the vehicle to pass EPA and DOT noise regulations. OEM automotive manufacturers have many regulations to adhere to.
In amateur and professional racing, aftermarket velocity stacks are often used, as rules allow, and gains in the order of 2% to 4% can be obtained when inlet radii and stack lengths are optimized for that engine. Modern fuel injection systems with a plenum and single air inlet typically incorporate some sort of radiused entrance, designed to improve power, based on air flow increases. Modified engines often have the original air box and associated ducting removed and velocity stacks are installed as accessories. Modern engines universally have tuned intake tract volumes and associated resonance frequencies, designed to provide higher than atmospheric intake air pressure while the intake valves are open - increasing the density of the trapped air in the combustion chamber (higher compression).