Measurement of fluctuating temperature and POD analysis of eigenmodes in a heated planar jet

In this study, simultaneous measurement of velocity and temperature in a heated planar jet, which has a Reynolds number of 22,000 based on the nozzle height and no co-flow, is performed to investigate the evolution of the scalar field related to the coherent structures in the jet. A combined probe, which consists of a hot-wire and a cold-wire, is developed and used for simultaneous measurement. The cold-wire is placed 2.0 mm upstream of the hot-wire to keep the measurement accuracy of the hot-wire caused by the streamwise velocity fluctuation by the cold-wire. The results of POD analysis show that the shapes of the eigenfunctions of heat flux are different at each downstream location, and they are also different from those of velocity and temperature. They have peak values at a position in-between two peak values of the eigenfunctions of the first mode for velocity and temperature. Further, the position is consistent with that of the peak values in the cross-streamwise profile of turbulent heat flux. From these results, it can be concluded that the turbulent heat flux field can be expressed not only by the dominant modes of the velocity or temperature fields but also as a combination of both.