Mccrayfitzsimmons5938

We study the effects of dimensional confinement on the evolution of incompressible Rayleigh-Taylor mixing both in a bulk flow and in porous media by means of numerical simulations of the transport equations. In both cases, the confinement to two-dimensional flow accelerates the mixing process and increases the speed of the mixing layer. Dimensional confinement also produces stronger correlations between the density and the velocity fields affecting the efficiency of the mass transfer, quantified by the dependence of the Nusselt number on the Rayleigh number. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.Cheskidov et al. (2016 Commun. Math. Phys. 348, 129-143. (doi10.1007/s00220-016-2730-8)) proved that physically realizable weak solutions of the incompressible two-dimensional Euler equations on a torus conserve kinetic energy. Physically realizable weak solutions are those that can be obtained as limits of vanishing viscosity. The key hypothesis was boundedness of the initial vorticity in [Formula see text], [Formula see text]. In this work, we extend their result, by adding forcing to the flow. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.We develop a theory of strong anisotropy of the energy spectra in the thermally driven turbulent counterflow of superfluid 4He. The key ingredients of the theory are the three-dimensional differential closure for the vector of the energy flux and the anisotropy of the mutual friction force. We suggest an approximate analytic solution of the resulting energy-rate equation, which is fully supported by our numerical solution. The two-dimensional energy spectrum is strongly confined in the direction of the counterflow velocity. In agreement with the experiments, the energy spectra in the direction orthogonal to the counterflow exhibit two scaling ranges a near-classical non-universal cascade dominated range and a universal critical regime at large wavenumbers. The theory predicts the dependence of various details of the spectra and the transition to the universal critical regime on the flow parameters. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.We provide a numerical validation of a recently proposed phenomenological theory to characterize the space-time statistical properties of a turbulent puff, both in terms of bulk properties, such as the mean velocity, temperature and size, and scaling laws for velocity and temperature differences both in the viscous and in the inertial range of scales. In particular, apart from the more classical shear-dominated puff turbulence, our main focus is on the recently discovered new regime where turbulent fluctuations are dominated by buoyancy. The theory is based on an adiabaticity hypothesis which assumes that small-scale turbulent fluctuations rapidly relax to the slower large-scale dynamics, leading to a generalization of the classical Kolmogorov and Kolmogorov-Obukhov-Corrsin theories for a turbulent puff hosting a scalar field. We validate our theory by means of massive direct numerical simulations finding excellent agreement. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.In this paper, we study several problems related to the theory of randomly forced Burgers equation. Our numerical analysis indicates that despite the localization effects the quenched variance of the endpoint distribution for directed polymers in the strong disorder regime grows as the polymer length [Formula see text]. We also present numerical results in support of the 'one force-one solution' principle. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.The one-dimensional Galerkin-truncated Burgers equation, with both dissipation and noise terms included, is studied using spectral methods. When the truncation-scale Reynolds number [Formula see text] is varied, from very small values to order 1 values, the scale-dependent correlation time [Formula see text] is shown to follow the expected crossover from the short-distance [Formula see text] Edwards-Wilkinson scaling to the universal long-distance Kardar-Parisi-Zhang scaling [Formula see text]. In the inviscid limit, [Formula see text], we show that the system displays another crossover to the Galerkin-truncated inviscid-Burgers regime that admits thermalized solutions with [Formula see text]. The scaling forms of the time-correlation functions are shown to follow the known analytical laws and the skewness and excess kurtosis of the interface increments distributions are characterized. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.This is the second part of a two-part special issue of the Philosophical Transactions of the Royal Society A, which recognizes, and hopefully encourages, the growing convergence of interests amongst mathematicians and physicists to scale the turbulence edifice. This convergence is explained in more detail in the editorial which accompanies the first part (Bec et al. selleck products 2022 Phil. Trans. R. Soc. A 380, 20210101. (doi10.1098/rsta.2021.0101)) and includes a tribute to our friend, collaborator and mentor Uriel Frisch, to whom these special issues are dedicated. Uriel, the principal architect of the Nice School of Turbulence, remains the finest example of this synthesis of mathematics and physics in tackling the outstanding problem of turbulence. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.Following Arnold's geometric interpretation, the Euler equations of an incompressible fluid moving in a domain [Formula see text] are known to be the optimality equation of the minimizing geodesic problem along the group of orientation and volume preserving diffeomorphisms of D. This problem admits a well-established convex relaxation that generates a set of 'relaxed', 'multi-stream', version of the Euler equations. However, it is unclear that such relaxed equations are appropriate for the initial value problem and the theory of turbulence, due to their lack of well-posedness for most initial data. As an attempt to get a more relevant set of relaxed Euler equations, we address the multi-stream pressure-less gravitational Euler-Poisson system as an approximate model, for which we show that the initial value problem can be stated as a concave maximization problem from which we can at least recover a large class of smooth solutions for short enough times. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.We investigate numerically the model proposed in Sahoo et al. (2017 Phys. Rev. Lett. 118, 164501) where a parameter λ is introduced in the Navier-Stokes equations such that the weight of homochiral to heterochiral interactions is varied while preserving all original scaling symmetries and inviscid invariants. Decreasing the value of λ leads to a change in the direction of the energy cascade at a critical value [Formula see text]. In this work, we perform numerical simulations at varying λ in the forward energy cascade range and at changing the Reynolds number [Formula see text]. We show that for a fixed injection rate, as [Formula see text], the kinetic energy diverges with a scaling law [Formula see text]. The energy spectrum is shown to display a larger bottleneck as λ is decreased. The forward heterochiral flux and the inverse homochiral flux both increase in amplitude as [Formula see text] is approached while keeping their difference fixed and equal to the injection rate. As a result, very close to [Formula see text] a stationary state is reached where the two opposite fluxes are of much higher amplitude than the mean flux and large fluctuations are observed. Furthermore, we show that intermittency as [Formula see text] is approached is reduced. The possibility of obtaining a statistical description of regular Navier-Stokes turbulence as an expansion around this newly found critical point is discussed. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.We present an overview of the current status in the development of a two-point spectral closure model for turbulent flows, known as the local wavenumber (LWN) model. The model is envisioned as a practical option for applications requiring multi-physics simulations in which statistical hydrodynamics quantities such as Reynolds stresses, turbulent kinetic energy, and measures of mixing such as density-correlations and mix-width evolution, need to be captured with relatively high fidelity. In this review, we present the capabilities of the LWN model since it was first formulated in the early 1990s, for computations of increasing levels of complexity ranging from homogeneous isotropic turbulence, inhomogeneous and anisotropic single-fluid turbulence, to two-species mixing driven by buoyancy forces. The review concludes with a discussion of some of the more theoretical considerations that remain in the development of this model. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.Helicity, a measure of the breakage of reflectional symmetry representing the topology of turbulent flows, contributes in a crucial way to their dynamics and to their fundamental statistical properties. We review several of their main features, both new and old, such as the discovery of bi-directional cascades or the role of helical vortices in the enhancement of large-scale magnetic fields in the dynamo problem. The dynamical contribution in magnetohydrodynamic of the cross-correlation between velocity and induction is discussed as well. We consider next how turbulent transport is affected by helical constraints, in particular in the context of magnetic reconnection and fusion plasmas under one- and two-fluid approximations. Central issues on how to construct turbulence models for non-reflectionally symmetric helical flows are reviewed, including in the presence of shear, and we finally briefly mention the possible role of helicity in the development of strongly localized quasi-singular structures at small scale. This article is part of the theme issue 'Scaling the turbulence edifice (part 2)'.A randomly stirred model, akin to the one used by DeDominicis and Martin for homogeneous isotropic turbulence, is introduced to study Bolgiano-Obukhov scaling in fully developed turbulence in a stably stratified fluid. The energy spectrum E(k), where k is a wavevector in the inertial range, is expected to show the Bolgiano-Obukhov scaling at a large Richardson number Ri (a measure of the stratification). We find that the energy spectrum is anisotropic. Averaging over the directions of the wavevector, we find [Formula see text], where εθ is the constant energy transfer rate across wavenumbers with very little contribution coming from the kinetic energy flux. The constant K0 is estimated to be of O(0.1) as opposed to the Kolmogorov constant, which is O(1). Further for a pure Bolgiano-Obukhov scaling, the model requires that the large distance 'stirring' effects dominate in the heat diffusion and be small in the velocity dynamics. These could be reasons why the Bolgiano-Obukhov scaling is difficult to observe both numerically and experimentally.