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The current work presents simultaneous, high-speed measurements at 60,000 fields per second of velocity and mole fraction using particle image velocimetry (PIV) and planar laser induced acetone-fluorescence in a Richtmyerâ€"Meshkov instability of an inclined interface (Atwood number, At  = 0.22). Specifically, around 2 ms of temporal evolution of the vortex structures and their associated scalar modes immediately following the interface-reshock interaction is presented. Two initial interface conditions are discussedâ€"(a) a sharp, inclined ‘single mode’ interface and (b) a ‘multi-mode’ interface where small perturbations are imposed on the single mode case. A 2D wavelet decomposition of the scalar flow field shows a highly intermittent distribution of small-scale variance throughout the interface even at late times. These are correlated strongly with the vortex structures and local turbulence intensity, where each small-scale scalar mode is sandwiched between two co-rotating vortex structures. This indicates that the interstitial regions between the vortices are significant hotspots of entrainment, which is then dispersed by the induced, counter-flow velocity fields. The multimode case demonstrates similar organization at large scales, while the scalar field is much more homogeneous at smaller scales. These observations highlight the importance of capturing the early time vortex evolution to accurately estimate any late time intermittency, especially where deposition of intense vorticity on sharp interfaces is present. [ABSTRACT FROM AUTHOR]

The effects of incident shock strength on the mixing transition in the Richtmyer–Meshkov instability (RMI) are experimentally investigated using simultaneous density–velocity measurements. This effort uses a shock with an incident Mach number of 1.9, in concert with previous work at Mach 1.55 (Mohaghar et al. , J. Fluid Mech. , vol. 831, 2017 pp. 779–825) where each case is followed by a reshock wave. Single- and multi-mode interfaces are used to quantify the effect of initial conditions on the evolution of the RMI. The interface between light and heavy gases ($\text{N}_{2}/\text{CO}_{2}$ , Atwood number, $A\approx 0.22$ ; amplitude to wavelength ratio of 0.088) is created in an inclined shock tube at $80^{\circ }$ relative to the horizontal, resulting in a predominantly single-mode perturbation. To investigate the effects of initial perturbations on the mixing transition, a multi-mode inclined interface is also created via shear and buoyancy superposed on the dominant inclined perturbation. The evolution of mixing is investigated via the density fields by computing mixed mass and mixed-mass thickness, along with mixing width, mixedness and the density self-correlation (DSC). It is shown that the amount of mixing is dependent on both initial conditions and incident shock Mach number. Evolution of the density self-correlation is discussed and the relative importance of different DSC terms is shown through fields and spanwise-averaged profiles. The localized distribution of vorticity and the development of roll-up features in the flow are studied through the evolution of interface wrinkling and length of the interface edge, which indicate that the vorticity concentration shows a strong dependence on the Mach number. The contribution of different terms in the Favre-averaged Reynolds stress is shown, and while the mean density-velocity fluctuation correlation term, $\langle \unicode[STIX]{x1D70C}\rangle \langle u_{i}^{\prime }u_{j}^{\prime }\rangle$ , is dominant, a high dependency on the initial condition and reshock is observed for the turbulent mass-flux term. Mixing transition is analysed through two criteria: the Reynolds number (Dimotakis, J. Fluid Mech. , vol. 409, 2000, pp. 69–98) for mixing transition and Zhou (Phys. Plasmas , vol. 14 (8), 2007, 082701 for minimum state) and the time-dependent length scales (Robey et al. , Phys. Plasmas , vol. 10 (3), 2003, 614622; Zhou et al. , Phys. Rev. E, vol. 67 (5), 2003, 056305). The Reynolds number threshold is surpassed in all cases after reshock. In addition, the Reynolds number is around the threshold range for the multi-mode, high Mach number case ($M\sim 1.9$) before reshock. However, the time-dependent length-scale threshold is surpassed by all cases only at the latest time after reshock, while all cases at early times after reshock and the high Mach number case at the latest time before reshock fall around the threshold. The scaling analysis of the turbulent kinetic energy spectra after reshock at the latest time, at which mixing transition analysis suggests that an inertial range has formed, indicates power scaling of $-1.8\pm 0.05$ for the low Mach number case and $-2.1\pm 0.1$ for the higher Mach number case. This could possibly be related to the high anisotropy observed in this flow resulting from strong, large-scale streamwise fluctuations produced by large-scale shear. [ABSTRACT FROM AUTHOR]

The current study reports structural modifications imposed in fully developed, turbulent channel flow by an isolated wall-mounted circular cylinder. The cylinder extended into the logarithmic layer of the flow, perturbing the larger flow scales that embody a significant fraction of the turbulent kinetic energy and Reynolds shear stress. Hotwire measurements were made in the wake of the wall-mounted circular cylinder at multiple wall-normal and streamwise positions. Besides observing the expected mean velocity deficit in the wake of the cylinder, a secondary peak in streamwise Reynolds normal stress away from the wall was observed, coupled with suppression of the near-wall peak native to the incoming unperturbed, turbulent flow. These observations are similar to those made by Ryan et al. ("Effects of Simple Wall-Mounted Cylinder Arrangements on a Turbulent Boundary Layer," AIAA Journal, Vol. 49, No. 10,2011, pp. 2210-2220) for a wall-mounted cylinder protruding into the logarithmic layer of a turbulent boundary layer. Premultiplied velocity spectra elaborated on these energy modifications, specifically the occurrence of two distinct energy peaks at two-thirds of the cylinder height and an attenuation of energy associated with larger flow scales close to the wall. All of these perturbations were found to decay with streamwise distance downstream as the flow relaxed toward the unperturbed state. A clear persistence of the structures at the aforementioned peak at two-thirds cylinder height, similar in scale to the very large-scale motions in canonical wall turbulence, suggests an environment preferring structures of such scale. The influence of the cylinder aspect ratio on the characteristics of the perturbed flow is evaluated, and a distinction in wake structure is identified. [ABSTRACT FROM AUTHOR]