An experimental investigation of mixing mechanisms in a shock-induced instability flow is described. We obtain quantitative two-dimensional maps of the heavy-gas (SF 6 ) concentration using planar laser-induced fluorescence for the case of a shockaccelerated cylinder of heavy gas in air. The instantaneous scalar dissipation rate, or mixing rate, χ, is estimated experimentally for the first time in this type of flow, and used to identify the regions of most intense post-shock mixing and examine the underlying mechanisms. We observe instability growth in certain regions of the flow beginning at intermediate times. The mixing rate results show that while these unstable regions play a significant role in the mixing process, a large amount of mixing also occurs by mechanisms directly associated with the primary instability, including gradient intensification via the large-scale strain field in a particular non-turbulent region of the flow.

Measurements have been made of the growth by the Richtmyer-Meshkov instability of nominally single-scale perturbations on an air/sulfur hexafluoride ͑SF 6 ) interface in a large shock tube. An approximately sinusoidal shape is given to the interface by a wire mesh which supports a polymeric membrane separating the air from the SF 6 . A single shock wave incident on the interface induces motion by the baroclinic mechanism of vorticity generation. The visual thickness ␦ of the interface is measured from schlieren photographs obtained singly in each run and in high-speed motion pictures. Data are presented for ␦ at times considerably larger than previously reported, and they are tested for self-similarity including independence of initial conditions. Four different initial amplitude/wavelength combinations at one incident shock strength are used to determine the scaling of the data. It is found that the growth rate decreases rapidly with time, d␦/dtϰt Ϫp ͑i.e., ␦ ϰt 1Ϫp ), where 0.67ՇpՇ0.74 and that a small dependence on the initial wavelength 0 persists to large time. The larger value of the power law exponent agrees with the result of the late-time-decay similarity law of Huang and Leonard ͓Phys. Fluids 6, 3765-3775 ͑1994͔͒. The influence of the wire mesh and membrane on the mixing process is assessed.

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Magnetohydrodynamics of the solar corona is simulated numerically. The simulation is initialized with an extrapolated non-force-free magnetic field using the vector magnetogram of the active region (AR) NOAA 12192 obtained on the solar photosphere. Particularly, we focus on the magnetic reconnections occurring close to a magnetic null-point that resulted in appearance of circular chromospheric flare ribbons on October 24, 2014 around 21:21 UT, after peak of an X3.1 flare. The extrapolated field lines show the presence of the threedimensional (3D) null near one of the polarity inversion lines-where the flare was observed. In the subsequent numerical simulation, we find magnetic reconnections occurring near the null point, where the magnetic field lines from the fan-plane of the 3D null form a X-type configuration with underlying arcade field lines. The footpoints of the dome-shaped field lines, inherent to the 3D null, show high gradients of the squashing factor. We find slipping reconnections at these quasi-separatrix layers, which are co-located with the post-flare circular brightening observed at the chromospheric heights. This demonstrates the viability of arXiv:1805.00635v1 [astro-ph.SR] 2 May 2018 the initial non-force-free field along with the dynamics it initiates. Moreover, the initial field and its simulated evolution is found to be devoid of any flux rope, which is in congruence with the confined nature of the flare.

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A Mach 1.2 planar shock wave impulsively accelerates one of five different configurations of heavy-gas ͑SF 6 ͒ cylinders surrounded by lighter gas ͑air͒, producing one or more pairs of interacting vortex columns. The interaction of the columns is investigated with planar laser-induced fluorescence in the plane normal to the axes of the cylinders. For the first time, we experimentally measure the early time stretching rate ͑in the first 220 s after shock interaction before the development of secondary instabilities͒ of material lines in shock-accelerated gaseous flows resulting from the Richtmyer-Meshkov instability at Reynolds number ϳ25 000 and Schmidt number ϳ1. The early time specific stretching rate exponent associated with the stretching of material lines is measured in these five configurations and compared with the numerical computations of Yang et al. ͓AIAA J. 31, 854 ͑1993͔͒ in some similar configurations and time range. The stretching rate is found to depend on the configuration and orientation of the gaseous cylinders, as these affect the refraction of the shock and thus vorticity deposition. Integral scale measurements fail to discriminate between the various configurations over the same time range, however, suggesting that integral measures are insufficient to characterize early time mixing in these flows.

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Parker's magnetostatic theorem extended to astrophysical magnetofluids with large magnetic Reynolds number supports ceaseless regeneration of current sheets and hence, spontaneous magnetic reconnections recurring in time. Consequently, a scenario is possible where the repeated reconnections provide an autonomous mechanism governing emergence of coherent structures in astrophysical magnetofluids. In this work, such a scenario is explored by performing numerical computations commensurate with the magnetostatic theorem. In particular, the computations explore the evolution of a flux-rope governed by repeated reconnections in a magnetic geometry resembling bipolar loops of solar corona. The revealed morphology of the evolution process including onset and ascent of the rope, reconnection locations and the associated topology of the magnetic field lines agrees with observations, and thus substantiates physical realisability of the advocated mechanism.