We propose a tomographic method to reconstruct the optical properties of a highly scattering medium from incoherent acousto-optic measurements. The method is based on the solution to an inverse problem for the diffusion equation and makes use of the principle of interior control of boundary measurements by an external wave field.

In this Letter, we address two issues affecting the use of a variational argument to determine stability of conservative fluid systems. We build on ideas from bifurcation theory, and thereby for families of steady flows, we link turning points in a velocity-impulse diagram to gains or losses of stability. We further introduce concepts from imperfection theory into these problems, enabling us to reveal hidden solution branches. Our approach applies to a wide range of flows. As an illustration involving a well-defined problem, we study a pair of counterrotating vortices. The approach results in stability boundaries in agreement with linear analysis, yet further enables us to discover a new family of steady vortices, which surprisingly do not exhibit any symmetry. All applications of our approach so far, using imperfect-velocity-impulse (IVI) diagrams, lead us to the discovery of lower-symmetry solutions.

A crucial point for the understanding of the von Kármán–sodium (VKS) dynamo experiment is the influence of soft-iron impellers. We present numerical simulations of a VKS-like dynamo with a localized permeability distribution that resembles the shape of the flow driving impellers. It is shown that the presence of soft-iron material essentially determines the dynamo process in the VKS experiment. An axisymmetric magnetic field mode can be explained by the combined action of the soft-iron disk and a rather small $\alpha$ effect parametrizing the induction effects of unresolved small scale flow fluctuations.

We study the stability of cylindrical Taylor-Couette flow in the presence of azimuthal magnetic fields, and show that one obtains nonaxisymmetric magnetorotational instabilities, having azimuthal wave number $m=1$. For ${\Omega }_o/{\Omega }_i$ only slightly greater than the Rayleigh value $(r_i/r_o{)}^2$, the critical Reynolds and Hartmann numbers are ${\mathrm{Re}}_c\sim {10}^3$ and ${\mathrm{Ha}}_c\sim {10}^2$, independent of the magnetic Prandtl number Pm. These values are sufficiently small that it should be possible to obtain these instabilities in the Potsdam Rossendorf Magnetic Instability Experiment.

We predict that thermal emission can be controlled by selective heating of periodic structures. Previous studies on thermal emission modification via photonic crystals have been limited to uniform temperature. We consider frequency-dependent nonuniform absorption in a periodic structure and show that thermal emission peaks can be associated with specific locations in the structure. Consequently, local periodic heating allows control over which peaks appear in the thermal emission spectrum, as we demonstrate with calculations on two-component metallodielectric structures.

In the gravity field, density changes triggered by a kinetic scheme as simple as $A+B\to C$ can induce or affect buoyancy-driven instabilities at a horizontal interface between two solutions containing initially the scalars $A$ and $B$. On the basis of a general reaction-diffusion-convection model, we analyze to what extent the reaction can destabilize otherwise buoyantly stable density stratifications. We furthermore show that, even if the underlying nonreactive system is buoyantly unstable, the reaction breaks the symmetry of the developing patterns. This is demonstrated both numerically and experimentally on the specific example of a simple acid-base neutralization reaction.

The complex evolution of turbulent mixing in Rayleigh-Taylor convection is studied in terms of eddy diffusivity models for the mean temperature profile. It is found that a nonlinear model, derived within the general framework of Prandtl mixing theory, reproduces accurately the evolution of turbulent profiles obtained from numerical simulations. Our model allows us to give very precise predictions for the turbulent heat flux and for the Nusselt number in the ultimate state regime of thermal convection.

We describe a method to create effective gauge potentials for stationary-light polaritons. When stationary light is created in the interaction with a rotating ensemble of coherently driven double-$\Lambda$ type atoms, the equation of motion is that of a massive Schrödinger particle in a magnetic field. Since the effective interaction area for the polaritons can be made large, degenerate Landau levels can be created with degeneracy well above 100. This opens up the possibility to study the bosonic analogue of the fractional quantum Hall effect for interacting stationary-light polaritons.

We characterize experimentally how three dimensionality appears in wall-bounded magnetohydrodynamic flows. Our analysis of the breakdown of a square array of vortices in a cubic container singles out two mechanisms: first, a form of three dimensionality we call weak appears through differential rotation in individual 2D vortices. Second, strong three dimensionality characterized by vortex disruption leads on the one hand to a remarkable vortex array that is both steady and 3D, and, on the other hand, to scale-selective breakdown of two dimensionality in chaotic flows. Most importantly, these phenomena are entirely driven by inertia, so they are relevant to other flows with a tendency to two dimensionality, such as rotating, or stratified flows in geophysics and astrophysics.

Measurements of heat transport, as expressed by the Nusselt number Nu, are reported for turbulent Rayleigh-Bénard convection of water containing up to 120 ppm by weight of poly-[ethylene oxide] with a molecular weight of $4\times {10}^6\mathrm{g}/\mathrm{mole}$. Over the Rayleigh number range $5\times {10}^9\lesssim \mathrm{Ra}\lesssim 7\times {10}^{10}$ Nu is smaller than it is for pure water by up to 10%.

We simulate the impact of a viscous liquid drop onto a smooth dry solid surface. As in experiments, when ambient air effects are negligible, impact flattens the falling drop without producing a splash. The no-slip boundary condition at the wall produces a boundary layer inside the liquid. Later, the flattening surface of the drop traces out the boundary layer. As a result, the eventual shape of the drop is a “pancake” of uniform thickness except at the rim, where surface tension effects are significant. The thickness of the pancake is simply the height where the drop surface first collides with the boundary layer.

We report on the direct observation of backscattering induced by sidewall roughness in high-index-contrast optical waveguides based on total internal reflection. Our results demonstrate that backscattering is one of the most severe limiting factors in state-of-the art silicon on insulator nanowires employed in densely integrated photonics. We also derive the general relationship between backscattering and geometrical and optical parameters of the waveguide. Further, the role of roughness in polarization rotation and coupling with higher-order modes is pointed out.

The Klein-Gordon model (KG) $\square \varphi =P^{\prime }(|\varphi |)\frac{\varphi }{|\varphi |}$ is Lorenz invariant and has a finite wave speed, yet its localized modes, whether $Q$ balls or vortices, suffer from the same fundamental flaw as all other solitons—they extend indefinitely. Using the KG model as a case study, we demonstrate that appending the site potential, $P_a(|\varphi |)$, with a subquadratic part $P(|\varphi |)=b^2|\varphi {|}^{1+\delta }+P_a(|\varphi |)$, $0\le \delta <1$, induces particlelike modes with strictly compact support. These modes are robust and shorten in the direction of motion. Their interactions, which occur only on contact, are studied in two and three dimensions and are shown to span the whole range from being nearly elastic to plastic.

We numerically demonstrate optical negative refraction in ferrofluids containing isotropic ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles, each having an isotropic Ag shell, in the presence of an external dc magnetic field $H$. The all-angle broadband optical negative refraction with magnetocontrollability arises from $H$-induced chains or columns. They result in hyperbolic equifrequency contour for transverse magnetic waves propagating in the system. The finite element simulations verify the analyses using the effective medium approximation. Experimental demonstration and potential applications are suggested and discussed.

We present an optomechanical displacement transducer that relies on three cavity modes parametrically coupled to a mechanical oscillator and whose frequency spacing matches the mechanical resonance frequency. The additional resonances allow reaching the standard quantum limit at a substantially lower input power (compared to the case of a single cavity mode), as both sensitivity and quantum backaction are enhanced. Furthermore, it is shown that in the case of multiple cavity modes, coupling between the modes is induced via reservoir interaction, e.g., enabling quantum backaction noise cancellation. Experimental implementation of the schemes is discussed in both the optical and microwave domain.

We reexamine the nature of the turbulent magnetic diffusivity tensor of mean field electrodynamics and show that the predicted growth rate of the mean field is, in general, incorrect if the tensor is calculated via consideration of time-independent mean magnetic fields. We describe how the traditional expansion procedure for the mean electromotive force should be extended to include time derivatives of the mean magnetic field, and illustrate the consistency of this approach by means of a perturbation analysis for a mean field varying on long spatial scales. Finally, we examine the magnitude of this new contribution to the magnetic diffusion for a particular flow.

The issue raised in this Letter is classical, not only in the sense of being nonquantum, but also in the sense of being quite ancient: which subset of $4\times 4$ real matrices should be accepted as physical Mueller matrices in polarization optics? Nonquantum entanglement or inseparability between the polarization and spatial degrees of freedom of an electromagnetic beam whose polarization is not homogeneous is shown to provide the physical basis to resolve this issue in a definitive manner.

$X$ waves are spatiotemporal optical waves with intriguing superluminal and subluminal characteristics. Here we theoretically show that for a given initial carrier frequency of the system localized waves with genuine superluminal or subluminal group velocity can emerge from initial $X$ waves in nonlinear optical systems with normal group velocity dispersion. Moreover, we show that this temporal behavior depends on the wave detuning from the carrier frequency of the system and not on the particular $X$-wave biconical form. A spatial counterpart of this behavior is also found when initial $X$ waves are boosted in the plane transverse to the direction of propagation, so a fully spatiotemporal motion of localized waves can be observed.

In this Letter, we explore the possible effects of polymer additives on heat transport in turbulent thermal convective flows. Using both direct numerical simulations and shell-model calculations, we show that polymer additives can significantly enhance the heat transport in homogeneous turbulent thermal convection, which mimics the bulk of turbulent Rayleigh-Bénard convection. We also discuss the implication of our results for turbulent Rayleigh-Bénard convection, in which there are boundary layers in addition to the central bulk.

A solid object impacting on liquid creates a liquid jet due to the collapse of the impact cavity. Using visualization experiments with smoke particles and multiscale simulations, we show that in addition, a high-speed air jet is pushed out of the cavity. Despite an impact velocity of only $1\mathrm{m}/\mathrm{s}$, this air jet attains supersonic speeds already when the cavity is slightly larger than 1 mm in diameter. The structure of the air flow closely resembles that of compressible flow through a nozzle—with the key difference that here the “nozzle” is a liquid cavity shrinking rapidly in time.