Investigation of turbulent flows via pseudo flow visualization part II: Lobed mixer

Multipoint, single-component hot-wire anemometer measurements obtained with a rake of probes downstream of a lobed mixer are examined using a pseudo flow visualization (PFV) technique. PFV is a visualization procedure that manipulates data obtained from an array of probes to create a graphical representation of the instantaneous velocity components of a flow field. The indicator function was employed to identify the frequency content of each velocity-time trace, giving insight into the analysis of the visualizations. From this application, shedding frequencies of the large-scale structures were identified to range from 200-700 Hz, averaging around 550 Hz. In general, these results are comparable with streamwise velocity spectra obtained from the same flow field locations. Mean streamwise velocity and turbulence intensity profiles reveal that the turbulence intensity increases substantially at the expense of the mean shear, targeting the region of increased mixing to occur between z/l_a = 3.9 and 5.2 downstream of the trailing edge of the mixer. The ruffle-shaped shear layer is further warped by the secondary velocities in the region where the mean momentum flow emanates from the lobe troughs. This evolution of the shear layer continues until it collapses onto itself, resulting in a burst of turbulent energy that paves the path for turbulence-dominated mixing. The validity of these techniques was successfully verified in the more familiar and fundamental flow field of the near-field axisymmetric jet mixing layer, as described in Part I of this investigation, before they were applied to the flow field downstream of the lobed mixer.