I called "sir", when I encountered a professor for the very first time in the U.S.I did "namaskar" too,because he was somewhat familiar with Nepalese culture.--sounds funny...? I call my advisor" Dr.+last name"..,but mostly use "prof."while communicating throuh e-mails. Now-a-days,I attend informal meeting regularly with some Americans from different parts of USA.According to them,most Americans don't really care about the way they are called,but this is not always true.some are very strict.