Dynamics of round turbulent offset jets in quiescent and coflowing environments

This study experimentally investigates the dynamics of turbulent round offset jets introduced into both a quiescent environment and a coflowing stream. The acoustic Doppler velocimeter was employed to precisely measured the velocity field of the jet in a water flume. The results pertaining to the offset jets released into a quiescent background was compared against those of the previous studies and the measurement technique was validated. Coflow confines the offset jets, leading to narrower mean and root mean square (rms) velocity profiles. Furthermore, the spreading and decay rates decrease, indicating that the external flow constrains the growth and development of the offset jets. The effect of coflow becomes more pronounced and the rates further decrease when the offset ratio reduces. The rms velocities of coflowing offset jets increase compared to those of jets in a quiescent background. Moreover, by decreasing the offset ratio, the rms velocities decrease, as the wall proximity further restricts the jet's meandering and intermittency. The velocity spectra of coflowing offset jets have higher values in the inertial subrange compared to those of the offset jets in a quiescent environment. However, this trend reverses within the energy-containing range. This coincides with a decrease in the integral length scales, suggesting that the coflow may suppress the large-scale jet structures mainly responsible for engulfing the ambient fluid into the jet. The observed reduction in spreading and decay rates, as well as integral length scales of the coflowing offset jets, suggest a decrease in jet entrainment, resulting in a reduction in overall jet mixing.