How would the tides on earth be different if the moon revolved around earth in 15 days instead of 30 days?

In case that this happened, and nothing else changed like the rotational speed or anything similar, everything would remain the same, just the time passed between two tides would change. Instead of the regular spring and neap cycle, you would have it reduced by half, proportional to the reduction of time. If other things regarding this changed too then there would be more changes.

If the moon revolved around the earth in 15 days instead of 30 days then there would be less time between each tide. They would come much faster. 

Explanation:

Neap Tides are those in which the difference between high and low tide is the least. They occur twice a month when the Sun and Moon are at right angles to the Earth. When this is the case, their total gravitational pull on the Earth's water is weakened because it comes from two different directions. On the other hand, spring tide are those in which the difference between high and low tide is the greatest. Spring tides occur when the Moon is either new or full, and the Sun, the Moon, and the Earth are aligned.
But if the moon moved around the earth faster, while the earth maintained the same rotational speed, the period between spring and neap tides would be less.

Pewnej nocy zakradł się pod pobliski dwór i, gdy nikogo nie było w kredensie, sięgnął po wiszące na ścianie skrzypki. zrobił to zbyt nieostrożnie i zbudził lokaja, który oskarżył janka o próbę kradzieży. sąd, nie chcąc posłać go do więzienia, kazał stójce stachowi zbić go rózgą. chłopak, zbity bardzo mocno, zmarł trzeciego dnia.