Riadh,
U is short unless followed by another vowel. (uh)
up, cut, pup, mum, lump, etc
cur, turd......the ur sound is also a short sound
If u is followed by a vowel the sound is long. (oo, yoo, or yur)
cruel
suave
dual
unity
cut......cute
mutt ......mute
pub.....pubic
Upanishad
plum.....plume
crud.....crude
cur....cure
If there are two consonants after the u then the sound is short
mutt
pub.....pubic.....public
putt
butt
plum....plume....plummet
crust
If the second vowel is "le" then three consonants are needed to protect the short sound.
bugle ....long u
puzzle....short u
ruble.....long u
rubble......short u
Exceptions: put, butte ......and probably a few more :)
There are also the blended sounds when u follows a vowel. ( au, ou )