TR-PIV measurements and POD analysis of the plane wall jet subjected to lateral perturbation

Abstract

An experimental investigation has been carried out using time resolved particle image velocimetry (TR-PIV) to investigate the aerodynamic behavior of a wall jet subjected to external lateral stream (ELS). The experiments are performed on a reduced-scale model representing a generic configuration of a refrigerated display cabinet (RDC) by focusing on the near-field region downstream the nozzle exit (x/e<10) where strong interactions are expected between the jet core, wall boundary and external lateral stream. Comparisons of experimental data obtained with and without external perturbation make it possible to quantify the effect of the perturbation on the time-averaged wall jet characteristics such as airflow patterns, mean velocity and root mean square (RMS) of velocity fluctuations, Reynolds shear stress, space correlations and the development of primary and secondary instabilities. The vortex shedding frequency of the perturbed wall jet has been assessed through a spectral analysis. It was found that the Strouhal number decreases with an increase of lateral-flow velocity, indicating an inhibiting effect exerted by the lateral flow on the formation and the development of Kelvin–Helmholtz (K–H) vortices in the outer shear layer. This paper also gives an insight into the altered behavior of coherent structures related to perturbed jet in terms of topology, alignment, trajectory and vortex shedding frequency. The break-up of vortex filaments in the braid region and its effect on the pairing process and the mutual interaction between inner and outer layer vortices has also been discussed. A proper orthogonal decomposition (POD) analysis has been performed showing the role of the ELS in redistributing the energy content in favor of small length scales and in the emerging of less well-organized structures.