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