Experimental study of a three-dimensional large-scale structure in a reciprocating oscillatory flow

An experimental investigation has been conducted on the large-scale structure of the reciprocating oscillatory turbulent flow in a rectangular duct. Using hot-wire anemometers and conditional sampling technique, we found that the Reynolds stress generated explosively in the decelerating phase, a characteristic feature of the flow, is caused by the three-dimensional large-scale structure. These structures are revealed to be a pair of counter-rotating fluid motions which change their form gradually as the phase-averaged velocity distribution changes. Phase differences of the turbulence intensity distribution in an oscillatory flow are found to take place in accordance with the generation-growth-decay process of the large-scale structure.