Turbulent mixing in a two-dimensional jet

An ensemble of instantaneous temperature profiles across a two-dimensional heated jet was obtained by shooting a fine platinum resistance thermometer across the jet at speeds much higher than the local velocity of the jet. Each recording of the ensemble was shifted to have a common center, and ensemble averages were obtained counting only the profiles for which the spatial location in question was inside the jet. This conditional averaging produced mean and mean-square fluctuations which are quite flat across the jet, indicating that the fluid inside the jet is well-mixed. Visual inspection of instantaneous records showed no cold spots in the jet. Ensemble averages were also measured by aligning the edge or beginning of the instantaneous profiles. This led to a sharper rise in the mean temperature profile and an overshoot in the mean-square temperature fluctuations. This indicates that mixing across the turbulent-nonturbulent interface is stronger than in the main body of the jet.