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