Besides flow rate, you are also looking for turbulence. Is harder to quantify turbulence though. Turbulence in the combustion chamber creates a fast burn, and combustion pressure develops sooner while the piston is still at the top of its stroke giving more power and fuel economy.
A computer simulation experiment was done on an engine with an extra piston that pumps air through a very small valve creating large turbulence. Even with the extra drag of the extra piston, it still got much better fuel economy.
Another reason why BMW inline 6 engine with variable valve lift get quite good gas mileage and power. The valve opening is matched to engine speed to create the most turbulence in the combustion chamber.
The size of the valve has to be sized to match the engine rpm. For high rpm, larger vavle, and smaller valve for lower rpm.