Physica A 362 (2006) 151–157 Lattice Boltzmann equation simulation of rectangular jet ðAR ¼ 1:5Þ instability and axis-switching Huidan Yu à , Sharath S. Girimaji Aerospace Engineering Department, Texas A&M University, College Station, TX 77843-3141, USA Available online 7 October 2005 Abstract We investigate the axis-switching and instability onset characteristics of a rectangular jet with aspect ratio (AR) of 1.5. Jet flow simulations of four different Reynolds numbers, 10; 100; 150, and 200, are performed. Half-width of streamwise velocity along with velocity vector and streamwise vorticity on transverse planes at different downstream locations are examined. The simulation results show the following instability characteristics: (i) At relatively low Reynolds numbers (10 and 100), the jet flow is laminar and stable; (ii) At Re ¼ 150, the flow is still laminar and stable but axis-switching occurs with downstream distance; and (iii) At Re ¼ 200, the jet flow becomes unstable but the axis-switching is still seen roughly. Instability first originates near the jet orifice and then propagates down the stream. r 2005 Elsevier B.V. All rights reserved. Keywords: Lattice Boltzmann equation; Rectangular jet; Axis-switching; Instability 1. Introduction Motivated by jet-engine design and flow control needs, extensive investigations of turbulent jet flows have been undertaken experimentally, theoretically, and numerically in the past few decades. Special efforts have been made to study of non-circular jet physics owing to their enhanced entrainment and mixing properties relative to those of comparable circular jets (cf. review [1] and references therein). Rectangular jet (RJ) combines non-unity aspect ratio (AR) feature of elliptic jets with the corner (vertex) feature of square jets. This combination yields features which are of importance in practical applications. Laboratory experiments [2–4] and numerical simulations [5–8] of RJs indicate a peculiar phenomenon called axis-switching. Axis-switching is characterized by the jet cross-section shape axes rotation. As was recognized by many investigators, large- scale coherent structures play a dominant role in the evolution of free mixing layers especially in flows at low Reynolds number (Re) [9–11]. Axis-switching is believed to result from faster growth rate of the jet’s shear layers along minor axis plane than those along the major axis plane [1]. This also leads to destabilization of the jet flow. However, the underlying fluid dynamical mechanisms of axis-switching and instability onset are far from clear. It is crucial to understand the dynamics and topology of the large-scale coherent structures governing the entrainment and mixing. ARTICLE IN PRESS www.elsevier.com/locate/physa 0378-4371/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physa.2005.09.035 à Corresponding author. E-mail addresses: h0y5840@aero.tamu.edu (H. Yu), girimaji@aero.tamu.edu (S.S. Girimaji).