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