I am looking for a basic explanation of the energy loss due to pressure drop through a valve or orifice. I understand the drop in static pressure is accompanied by a increase in velocity head and these can be summed up to determine the total energy lost across the valve. But! most references seem to simply state these losses result in fluid temperature rise and leave it at that. My question is why is there a temp rise? is it simply a matter of fluid turbulence causing molecules to crash into each other? or is there a better theory that I am missing?  
Thanks for any thoughts or referrals on this.
Tom