Prediction of Self-Sustained Oscillations of an Isothermal Impinging Slot Jet

Abstract

The present results are focused on the self-sustained oscillations of a confined impinging slot jet and their role in the flow structure and modeling requirements. Unsteady laminar, large-eddy simulation (LES), and Reynolds-averaged Navier–Stokes (RANS) predictions of an isothermal confined impinging jet were validated for several nozzle-to-plate ratios (H/B=4–15) and for laminar (Re=340 and 480) and turbulent (Re=104–2.7×104) conditions. The impinging flow structure was found to be highly influenced by the H/B ratio. For high ratios (H/B>5), the studied steady RANS turbulence models could not satisfactorily predict the high diffusion reported experimentally in the jet-impinging influence zone. The failure of these models has been attributed to the modeling issues of turbulence closures. However, for H/B=8, unsteady laminar 3D and LES calculations were verified, and a sinuous oscillation mode was developed, revealing self-sustained oscillations and the display of periodic flapping of the impinging jet in good agreement with the experiments. The predicted flapping oscillation is one of the reasons for the higher diffusion near the impingement wall, which was verified in several time-averaged experimental studies. The presence of jet flapping matters for clarifying the already long discussion on the RANS model’s validation in predicting impinging jets with high H/B ratios, adding justification to the failure of these turbulence models. This unsteady behavior is correctly computed through LES.