Study of axis-switching and stability of laminar rectangular jets using lattice Boltzmann method

Abstract

We study stability characteristics and axis-switching behavior in low aspect-ratio (AR) laminar rectangular jets using lattice Boltzmann method. The objective of the study is to establish how the jet behavior changes with aspect ratio. Computations are performed with the D3Q19 multiple-relaxation-time lattice Boltzmann equation model. Simulations of three rectangular jets with AR = 1 , 1.5 and 2 at four Reynolds numbers, 10, 100, 150, and 200, are performed. The main conclusions of this study are: (1) At low Reynolds numbers ( Re ≤ 100 ), the three jets are stable and no axis switching is observed. (2) At intermediate Reynolds numbers ( Re = 150 ), axis switching is clearly evident. The square jet ( AR = 1 ) exhibits 45 ∘ axis switching and tends to become unstable. The two rectangular jets continue to be stable and exhibit 90 ∘ axis switching. (3) At high Reynolds numbers ( Re ≥ 200 ), all three jets are unstable and the irregular instantaneous cross-section still shows evidence of axis switching. Instability originates near the jet orifice and then propagates down the stream. The physics underlying the 45 ∘ and 90 ∘ axis switching are examined.