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

-- The low tides would be lower, and the high tides would be higher.

-- The time from a low tide to a high tide, or from a high tide to a low tide,
would average 6hrs 24min, instead of the present  6hrs 12.2min.

(By the way ... the moon revolves around the Earth in 27.32 days.
That "30 days" that you mentioned is close to the complete cycle
of the moon's phases ... 29.53 days ... which the tides also follow.)
 

Ctrust me i know its c