Statistics of the temperature fluctuations as a passive scalar in a freejet experiment

The measurement of the fluctuations of a passive scalar in fluids is important for the investigation of the transport features in turbulent flows [1]. A detection of the passive scalar is required with high temporal and spatial resolution in order to gain information over the entire turbulent cascade. We present data of temperature fluctuations taken in a heated free-jet experiment with a newly developed miniaturized thermocouple sensor. This Au/Pt-thermocouple possesses an enhanced temporal resolution extending to the range of microseconds. The measured power spectra show a slight shift of the Reynolds number if the position of the sensor is varied along symmetry axis of the jet. Furthermore, a shift of the inertial range can be observed while approaching the nozzle. The passive scalar spectrum is less steep than anticipated by the theory of ideal turbulence. An -4/3 scaling is observed due to the flow situation at still modest Re numbers [2].