Is there a difference between 回转 and 掉头? I have heard them both used to describe a U-turn, so are there any subtle differences between the two words?
回转 = "rotate" e.g. "1000rpm" means "rotate 1000 times per minute" = "每分鐘回转1000 次"
回转 (至相反方向) = "rotate to opposite direction" = "turn 180°/ turn around"
调头 and 掉头 are interchangeable when it means "turn around" But 掉头 can mean "turn one's head" in different context
Don't know about China, but in Taiwan 回轉 mostly explicitly means u-turn performed by a vehicle. As for 掉頭, in addition to the literal "turn your head", you can use it whenever you are heading from A to B, then suddenly requires to return to A.
Example, when you are going to work by taxi, and your just found out you forget to bring something important, then you can tell the driver, "掉頭!". In this case the driver understands you want to return to your house, instead of literally make a u-turn right at the intersection where you are. But if you tell the driver to "回轉", then he might be confused about why you want him to make a u-turn here?