Experimental and Numerical Study of Steady and Modulated Transverse Jets at Low Blowing Ratios

Abstract

The effects of jet flow pulsation were investigated in the case of a fully modulated vertical jet in crossflow over a flat plate using Mie scattering visualizations, hotwire anemometry and Large Eddy Simulations (LES). A preliminary steady state study was conducted over a wide range of blowing ratios to provide a baseline comparison to pulsed results in terms of vortical structures and performance. Based on observation of the vortical structures and hotwire signatures, two distinct regimes were identified separated by a transitional region. LES using the dynamic Smagorinsky sub-grid model were carried out to provide additional insights on vortical structures formation mechanisms and their effect on heat transfer results, showing good agreement with the experimental observations and measurements. The influence of the pulsing parameters such as mean, high and low blowing ratios, forcing frequency and duty cycle was explored in terms of jet coverage metrics and spanwise adiabatic effectiveness. Average blowing ratios of 0.250, 0.350 and 0.450, duty cycles of 25, 50 and 70% and forcing frequencies of St∞ = 0.008, 0.016, 0.079, and 0.159 were investigated. While in most of the cases, forced jets show a decrease in average performance compared to steady state configurations, some improvement was found in part of the forcing cycle. Vortical structures formed at the jet onset are consistent with the classification given by previous studies in terms of stroke-ratio and high blowing ratio over an overlapping range of conditions. The dynamics of the vortical structures formed at the pulse rise are highly responsible for the decrease in performance observed under forced conditions.