Dilution and mixing in an unsteady jet

The mixing characteristics of a round, turbulent, unsteady jet were studied experimentally. A gravity-driven flow was created by releasing dyed fluid from a vertical tube into a large water tank. The jet velocity increased from zero to a maximum and then decreased continuously such that each run lasted about 1 s. The jet dilution was examined by an optical absorption technique that measured the line integral of concentration across the jet diameter. These measurements revealed that the portion of the unsteady jet corresponding to the deceleration phase dilutes more than the steady jet. The molecular scale mixing, as deduced from an acid-base neutralization reaction, corroborated the finding that the jet mixes in a shorter distance than the steady jet.