Time-resolved visualization of an impinging jet subjected to bi-modal forcing

High-speed flow visualization is utilized to study the effect of bi-modal forcing on an axisymmetric impinging jet at Reynolds number, based on the jet exit velocity and diameter, of Re D = 4233 . The forcing involves excitation using two frequencies simultaneously: the fundamental and subharmonic frequencies of the initial instability of the jet shear layer. The focus of the study is on the effect of the intermodal phase ϕ while utilizing the same modal amplitude ratio and forcing level. The natural jet and the jet forced using pure harmonic forcing at the fundamental and the subharmonic frequency are also studied as benchmark cases for bi-modal forcing. Results show that all modes of forcing accelerate the development of the jet vortex structure by producing two vortex pairings ahead of the impingement plate. This double-paired structure is rarely seen in the natural jet and is promoted the most under pure subharmonic forcing and bi-modal forcing. The intermodal phase is found to have a strong effect with the double-paired structure exhibiting symmetry and high cycle-to-cycle repeatability at ϕ ≈ 150 ∘ - 165 ∘ , or significant asymmetry and disorganization at ϕ ≈ 90 ∘ - 105 ∘ . The main distinction between bi-modal forcing at ϕ ≈ 150 ∘ - 165 ∘ and pure subharmonic forcing is that the double-paired vortex structure is more persistent and has better repeatability in the former case. With subharmonic forcing alone, the vortex structure exhibits some random switching between the symmetric double-paired structure and the asymmetric structure. Overall, the promotion of double pairing leads to faster narrowing of the jet core and stronger vortex–wall interaction. Graphical Abstract