The role of embedded swirl in shear layer on turbulent jet mixing and noise

The concept of thin-layer swirl as a means of turbulent jet mixing and noise control is investigated. Swirl is introduced at the exit of a conical nozzle through 12 short vanes which impart angular momentum primarily to the outer shear layer. In addition to the uniform swirl (case m = 0) design, three azimuthally modulated swirl configurations are created by varying the exit vane angle. To investigate the development of the small disturbances, the spatial linear instability analysis is performed on an inviscid parallel jet with thin-layer swirl to identify the unstable modes and estimate their spatial growth rates. The azimuthal modes, m = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>=</mml:mo> <mml:mo>±</mml:mo> <mml:mn>1</mml:mn> <mml:mo>,</mml:mo> <mml:mo>±</mml:mo> <mml:mn>2</mml:mn> <mml:mo>,</mml:mo> <mml:mi>a</mml:mi> <mml:mi>n</mml:mi> <mml:mi>d</mml:mi> <mml:mo> </mml:mo> <mml:mo>±</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:math> over a range of swirl number S are investigated. For low–moderate swirl, the negative helical modes (m &lt; 0) are more spatially unstable than their positive counterparts; as swirl increases, the modal ordering reverses, with positive modes (m &gt; 0) becoming dominant. High-order LES is used to assess how thin-layer swirl influences jet mixing and noise. Entrainment is quantified using the radial-velocity method and thin-layer swirl cases show a modest enhancement over a plain jet. The simulations show that the uniform swirl case (m = 0) achieves stronger mixing than the modulated designs (m = 1,2,3). Far-field acoustics is estimated by Ffowcs Williams-Hawkings (FWH) method and it shows a net OASPL reduction of 2–4 dB at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>θ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>60</mml:mn> <mml:mo>°</mml:mo> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>90</mml:mn> <mml:mo>°</mml:mo> </mml:mrow> </mml:math> . The gross thrust penalty for introducing low-aspect ratio swirl vanes at the nozzle exit is about 7-11%. Future work targets the optimization of vane geometry and swirl strength to retain acoustic benefits while reducing thrust loss.