C. Wong

Phase-averaged velocity in a fluidic precessing jet nozzle and in its near external field

Abstract Phase averaged laser-Doppler measurements of the axial velocity components made within and in the near exit field of a precessing-jet nozzle have verified a number of flow features reported in the research literature. The nozzle is a short cylindrical tube with an axisymmetric inlet at one end, and with a centrebody and a small exit lip at the other end. The diameter ratio of the abrupt expansion at the inlet is 1:5. The measurements of the internal flow field reveal a radially deflected internal jet which reattaches asymmetrically and precesses around the wall of the chamber. The phase-averaged flow inside the chamber can be divided into regions of forward flow and regions of reverse flow. The distribution of these regions inside the chamber implies the presence of large-scale recirculation. Representative reverse mean flow speeds of recirculation are about 30% of the forward flow speed. Measurements inside the chamber suggest that the effect of reversed flow on the velocity decay of the inlet-jet flow is similar to that of an ambient counter flow. Measurements in the external jet suggest that the initial entrainment rate of the external precessing jet is between six and seven times that of an equivalent free turbulent jet. The phase-averaged deflection angle of the present emerging jet is 50° but this decreases to about 30° within 0.4 chamber diameters of the exit plane.

The naturally oscillating flow emerging from a fluidic precessing jet nozzle

Phase-averaged and directionally triggered digital particle image velocimetry measurements were taken in longitudinal and transverse planes in the near field of the flow emerging from a fluidic precessing jet nozzle. Measurements were performed at nozzle inlet Reynolds and Strouhal numbers of 59000 and 0.0017, respectively. Results indicate that the jet emerging from the nozzle departs with an azimuthal component in a direction opposite to that of the jet precession. In addition, the structure of the ‘flow convergence’ region, reported in an earlier study, is better resolved here. At least three unique vortex-pair regions containing smaller vortical ‘blobs’ are identified for the first time. These include a vortex-pair region originating from the foci on the downstream face of the nozzle centrebody, a vortex-pair region shed from the edge of the centrebody and a vortex-pair region originating from the downstream surface of the nozzle exit lip.

PIV as a Complement to LDA in the Study of an Unsteady Oscillating Turbulent Flow

In the last decade, particle image velocimetry (PIV) has become a standard laser diagnostic tool in numerous fluid mechanics laboratories worldwide. At first glance, it appears straight-forward to set up an off-the-shelf system for a quick investigation of the flow of interest. However, unless additional effort is taken to understand the flow, results from ‘quick investigations’ may lead to limited and sometimes spurious interpretation of the physical flow phenomena. This is especially so for highly three-dimensional and turbulent flows. This chapter examines the efficacy of phase-averaged particle image velocimetry results in assessing the physical phenomena occurring in highly periodic flows and how they complement results from phase-averaged laser Doppler anemometry (LDA) and surface flow visualisation techniques. A specific case study will be presented to demonstrate the complementary nature of these techniques.