Sounds and letters - two different things in English

When you say the letter of the alphabet 'U', it is pronounced with a long vowel sound /uː/ like in many words.  

e.g. fuel, pollution, issue, student , module

But the letter 'U' is also pronounced as the short vowel sounds /ʌ/  

  e.g. cut, consumption, study, subject → /ʌ/

or /ʊ/

  e.g put → /ʊ/

▪ If you add 'r' after the 'u', the pronunciation is different again, as you pronounce it /ɜː/   

  e.g turn, burn → /ɜː/       

or even /ʊə /   

   manure → /ʊə /