Lagrangian analysis of sweeping jets measured by time-resolved particle image velocimetry

Abstract

The flow dynamics of a sweeping jet generated by a fluidic oscillator is experimentally investigated by time-resolved particle image velocimetry (TR-PIV). Lagrangian transformation is applied to the measured flow fields to better determine the characteristics of the jet flow outside the oscillator. Upon increasing the Reynolds number from Re = 2.5 × 103 to 11.7 × 103, which respectively corresponds to the occurrences of straight and deflected jet columns, the spreading angle of the external jet increases and reaches its saturation value. The overall performance of the flow fields is first examined in Eulerian space. At the higher Reynolds number, the momentum is more directed at the maximum deflected positions of the jet. A clear weak flow is then induced at the middle of the far field region from the jet nozzle. The induced jet column is also bent significantly into a curved shape. To examine the variations of the jet flows with different column shapes, Lagrangian transformation is applied to the measured flow fields by attaching a rotating reference frame on the jet column. It is found that the large bending angle of the jet column at the higher Reynolds number induces higher fluctuations and more uneven oscillation patterns in the jet flow. In the far field region at the higher Reynolds number, the time-averaged jet velocity decreases faster, with higher turbulence intensities than those at the lower Reynolds number. The phase-dependent jet flow fields confirm that the peak velocity and the jet width also have higher fluctuations at the higher Reynolds number. In addition, the jet bending angle in the far field region shows more uneven oscillation patterns compared with those in the near field region. These highly fluctuating and uneven flow behaviors contribute to the uneven distribution of the jet momentum at the higher Reynolds number in Eulerian space. Finally, different fluctuating behaviors of the jet flow due to the different jet shapes are also revealed by Lagrangian dynamic mode decomposition (DMD).