The apparatus 1 wherein the flow obstruction comprises a plurality of bluff bodies and {HEAD ~ the oscillating flow components} are produced from vortices shed from {TAIL ~ the bluff bodies} 
The apparatus 1 wherein the flow obstruction comprises a plurality of bluff bodies and { HEAD ~ the oscillating flow components } are produced from vortices shed from { TAIL ~ the bluff bodies }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL