{HEAD ~ A pressure} of the fluid oscillates according to {TAIL ~ a time varying function}, which causes a density of the fluid also to oscillate according to the same time varying function 
{ HEAD ~ A pressure } of the fluid oscillates according to { TAIL ~ a time varying function } , which causes a density of the fluid also to oscillate according to the same time varying function
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH