Correlations between streamwise velocity and temperature fluctuations in compressible turbulent channel flows from the perspective of coherent structures

In this work, the correlations between streamwise velocity and temperature fluctuations are investigated in compressible turbulent channel flows from the perspective of coherent structures. The intense fluctuation structures and quadrant-event structures of both velocity and temperature have been identified, extracted separately and compared. Analyses show that although their structure sizes are similar in the whole channel, high correlation only exists in the near-wall region with a high overlapping rate of the instantaneous structures. The hierarchy of the temperature structures are passively formed following the dynamic process of the velocity such as ejections, which contributes to the remaining correlation in the outer layer. However, this passive scalar property cannot provide the production mechanism in the outer layer according to the budget analysis after scale decomposition, and the interscale energy transfer progress is also different from the velocity fluctuation field. Therefore, the temperature structures deviate from the velocity structures in the outer layer and cannot be carried by the following dynamic process of the velocity such as sweeps, passively, which can be found from the conditional averaged structures. All of these findings provide a new perspective for understanding the velocity–temperature relationship in compressible channel flows.