Effects of velocity shear layer on detonation propagation in a supersonic expanding combustor

Abstract This paper investigates open-loop control of the mixing between an inner stream and an outer stream in a coaxial combustor geometry. The inner and outer air streams enter the combustor geometry at different temperatures and mimic a gaseous fuel and combustion air stream respectively. Of specific interest in this study is the behavior of the coherent structures in the coaxial jet streams, and the manipulation of these coherent structures with controlled perturbation to enhance jet-spreading and scalar-mixing. The spectra of the unforced flow reveal the presence of a dominant coherent mode at 100 Hz (identified as the fundamental mode fo), as well as 150 Hz (3 fo/2) and 300 Hz (3 fo). Single-mode forcing of both the inner-jet and the outer-jet at 100 Hz, 150 Hz, 300 Hz, and 1 kHz is explored, and attention is focused on the spreading of the inner-jet shear layer and the outer-jet shear layer. It is observed that the 300 Hz mode shows the greatest enhancement in the spreading rate of the velocity shear-layer in the near field (x/D < 1), while downstream of x/D = 1, the 100 Hz appears to show the most significant effect. Scalar mixing is also significantly enhanced, with the 300 Hz forcing showing the largest enhancement. Dual-mode forcing is also investigated with 100 Hz inner-jet, 300 Hz outer-jet forcing and 300 Hz inner-jet, 100 Hz outer-jet forcing. The 100 Hz inner-jet, 300 Hz outer-jet forcing is shown to lead to greater enhancements in scalar mixing man all other cases considered.

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Mean field solar surface dynamo in the presence of partially ionized plasmas and sub-surface shear layer

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2008 ◽

2019 ◽

Vol 488 (3) ◽

pp. 4329-4337

Author(s):

B S Paradkar ◽

S M Chitre ◽

V Krishan

Keyword(s):

Magnetic Field ◽

Shear Layer ◽

Solar Surface ◽

Mean Field ◽

Ambipolar Diffusion ◽

Velocity Shear ◽

Surface Velocity ◽

Dynamo Model ◽

Surface Shear ◽

Partially Ionized

Abstract A non-linear α − Ω dynamo in the partially ionized turbulent plasma in the presence of sub-surface velocity shear is studied with mean-field electrodynamics. Such a dynamo is probably operational in the near-surface region of the Sun, where the presence of both neutrals and the velocity shear (due to sub-surface shear layer in the rotation profile) is observationally well established. In particular, we show that the inclusion of ambipolar diffusion leads to a saturation of magnetic field amplitudes in the α − Ω dynamo. We also demonstrate that the temporal evolution of large-scale global magnetic fields follows the well-known pattern similar to the ‘butterfly’ diagram displayed by sunspots. As usual the velocity shear converts part of the poloidal into the toroidal magnetic field which in turn is regenerated largely by the combined kinetic plus Hall helicity, thus closing the dynamo loop. In addition, by allowing temporal variation in the helicity and ambipolar diffusion coefficient we are able to reproduce the grand-minimum type behaviour of the solar dynamo. Details of theoretical model along with numerical computations of dynamo equations in the partially ionized plasma are outlined. The solar surface dynamo model envisaged in this work could operate in conjunction with the global dynamo present in the bulk of the convection zone.

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