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Jian-Zhou Zhu

Publications and source records attributed to Jian-Zhou Zhu.

At least 19 recordsLinked to original sources

Real Schur Flow Acoustic Directivity and Helicity Effects within

The statistical description and acoustic diagnostics of anisotropic turbulence remain challenging. Real Schur flows (RSFs), prototype models of component-wise dimensionally reduced flows (CWDRFs), provide effective platforms for studying such problems. We combine absolute statistical equilibrium with the acoustic analogy to investigate helicity effects on the far-field directivity of sound radiated from three types of CWDRFs: two different RSFs, and a further reduction of them (liberated Schur flow). We evaluate the directivity from the vortex-sound source spectrum which is calculated with the Wick theorem from the absolute equilibirum Gaussian ensemble. Helicity amplifies the dimensional reduction induced anisotropy of the directivity functions across all CWDRFs, with distinct patterns.

physics.flu-dyn↗

Component-wise dimensionally reduced flows and helicity conservation

The component-wise dimensionally reduced real Schur flows (RSFs) associated to the classical compressible Euler equation [J.-Z. Zhu, J. Math. Phys. \textbf{62}, 083101 (2021)] is reformulated alternatively in terms of mode-truncation, with the untruncated Fourier modes preserving the original interaction structure and thus other important derivatives. A number of results are set up for the mathematical physics of component-wise dimensionally reduced flows (CWDRFs, including those with further dimensional reductions of RSFs); and, it is particularly shown that previous proofs of the helicity invariance in barotropic ideal flows were overkilling in the sense of using the unnecessary condition of local mass conservation, while our new ``sharper'' proof without invoking the latter carries over to our CWDRFs and the inviscid Burgers equation, verified using recent results [S.~G.~Chefranov \& A.~S.~Chefranov, Phys. Scr. \textbf{94}, 054001 (2019)] for the latter case in the infinite domain.

physics.flu-dyn↗

Staggered dispersions: Part I. Shocliton, quantum revival and fractalization

Alternating the signs of the frequencies for Fourier components with even and odd (normalized) wavenumbers in the nonlinear-wave, such as the Korteweg-de Vries, equations maintains a constantly drifting dispersive shock with similar solitonic oscillations on both sides, like some plasma and quantum shocks. Such \textit{staggered dispersions} support bi-directional waves and can symmetrize and deregularize (to some degree) the nonlinear dynamics, with physical consequences that can also be reflected in the fractalization and quantum revival (Talbot effect).

math-ph↗

Constructing longulence in the Galerkin-regularized nonlinear Schrödinger and complex Ginzburg-Landau systems

(Quasi-)periodic solutions are constructed analytically for Galerkin-regularized or truncated nonlinear Schrödinger (GrNLS) systems preserving finite Fourier freedoms. GrNLS admits travelling-wave or multi-phase solutions, including monochromatic solutions independent of the truncation and quasi-periodic ones with or without additional on-torus invariants. Numerical tests show that instability leads such solutions to nontrivial longulent states with remarkable solitonic structures (called ``longons'') admist disordered weaker components, corresponding to presumably whiskered tori. In the strong-coupling limit (e.g., the self-phase modulation equation in optics), neutral stability holds for the condensates, without the modulational instability, but not generally for other multi-phase (quasi-)periodic solutions from some of which the longulent state developed is also adressed. The possibility of nontrivial Galerkin-regularized complex Ginzburg-Landau longulent states is also discussed for motivation.

nlin.PS↗

Travelling-wave, Quasi-periodic, and Longulent States of the Galerkin-regularized Hydrodynamic-type Systems

Travelling-wave, quasi-periodic and ``longulent'' states of the Galerkin-regularized systems preserving finite Fourier modes are exposed. The longulent states are characterized by solitonic structures, called ``longons'', accompanied by disordered components, which is associated to whiskered tori according to the \textit{a-posteriori} Kolmogorov-Arnold-Moser (KAM) theorem. On-torus invariants are introduced for constructing the KAM tori, towards a potential pseudo-integrability theory. Persistence of the longulent states with respect to certain dispersive and dissipative perturbations are also numerically indicated.

nlin.PS↗

Longons from the nonlinear dispersion of Galerkin regularization

Irregular compactons and peakons from some nonlinear dispersions can be regularized by another type of nonlinear dispersion, defined by a pseudo-differential operator in physical space for the Galerkin truncation preserving finite Fourier modes of wavenumbers no larger than $K$. This resembles yet differs from the Korteweg-de Vries (KdV) regularization of the Burgers-Hopf (BH) equation. The Galerkin-regularized compacton, peakon, KdV, and BH dynamics exhibit novel traveling waves and interacting solitonic structures amidst weaker, less-ordered components (`longons'). Quasi-periodic solutions are also constructed with on-torus invariants, towards a potential Kolmogorov-Arnold-Moser (KAM) argument with presumably whiskered tori. The latter are persistent against the approximation of the truncation by a type of linear dispersion models, resulting in similar longulent states. Time-dependent and stationary behaviors in the large-$K$ limit are addressed with numerical results.

nlin.CD↗

Even-odd alternative dispersions and beyond. Part II. Noninertial and inertial particles, and, astrophysical chirality analogy

Particle transports in carriers with even-odd alternating dispersions (introduced in Part I) are investigated. For the third-order dispersion as in Korteweg-de-Vries (KdV), such alternating dispersion has the effects of not only regularizing the velocity from forming shock singularity (thus the attenuation of particle clustering strength) but also symmetrizing the oscillations (thus the corresponding skewness of the particle densities), among others, as demonstrated numerically. The analogy of such dispersion effects and consequences (on particle transports in particular) with those of helicity in Burgers turbulence, addressed in the context of astrophysics and cosmology, is made for illumination and promoting models. Both dispersion and helicity regularize the respective systems, and both are shown to be transferred by the drag to the flows of the respective inertial particles carried by the latter and to similarly affect the particle clustering. A reward from studying particle transports is the understanding of the (asymptotic) $k^0$-scaling (equipartition among the wavenumbers, $k$s), before large-$k$ exponential decay, of the power spectrum of KdV solitons [resulting in the more general statement (valid beyond the KdV soliton and Burgers shock) that "a (one-dimensional) soliton is the derivative of a classical shock, just like the Dirac delta is the derivative of a step function"], motivated by the explanation of the the same scaling law of the particle densities as the apparent approximation of the Dirac deltas; while, the "shocliton" from the even-odd alternating dispersion in aKdV appears to be, indeed, $shock \oplus soliton$, accordingly the decomposition of the averaged odd-mode spectrum, from sinusoidal initial field, into a $k^{-2}$ part for the shock and a $k^0$-scaling part for the solitonic pulses, only the latter being contained in the averaged even-mode spectrum.

physics.flu-dyn↗

Helical and nonhelical (magneto-)Burgers turbulence: I. Compressibility reduction and beyond

We compare the helical and nonhelical (magneto-)Burgers turbulence for the \textit{helicity fastening effect}. Theoretical arguments and heuristic mathematical analysis are offered for the latter notion in the new system loosing some ``nice'' properties as previously used in addressing the Navier-Stokes and various plasma fluids. Miscellaneous discussions are also offered, including the inferences of several consequences on the transports of passive scalars for both the density and tracer, particularly, the opposite consequences of the helicity fastening effect for the latter two scalars in appropriate situations (with the caveat of the possibility of the inverse cascade of the tracer energy). Basic numerical results of the fractions of the parallel-mode spectra, with maximally-helical random forcing on some small-wavenumber modes, present a benefit of about $0.2$ over those with nonhelical forcing, indicating regularization (to some degree) of the solutions. Such helicity ``fastening'' effect of Burgers turbulence is much more marked than that for low-Mach-number Navier-Stokes turbulence. The magnetic helicity in magneto-Burgers dynamics can present an even stronger benefit, of around $0.5+$.

physics.flu-dyn↗

Transfer Loop and Statistical Equilibrium of Korteweg-de Vries-Burgers Systems Associated to Classical Nonlinear Acoustics and Quantum Shock Waves

We propose and demonstrate, with the one-dimensional Korteweg-de Vries-Burgers model, the scenarios of transfer loop and \textit{all-scale} statistical equilibrium, the former being associated to shock formation and the latter to Gaussian distributions as in a canonical ensemble, but with wavelength-dependent temperatures. The discussions emphasize, among the multi-disciplinary relevance, the classical nonlinear acoustics and quantum shock waves, for the possibility of more favorable experimental tests.

nlin.PS↗

Turbulence compressibility reduction with helicity

Numerical test of isotropic turbulence compressibility reduction with helicity in a cyclic box is performed. The ratios of compressibility-relevant-mode spectra over those of kinetic energy present power laws at large wavenumbers in the dissipation range, indicating a common difference of $11/15$ in the exponents of the algebraic prefactor of the nonhelical power spectra over those of helical ones. Our results being not derived from the shapes of the spectra themselves, the implied information about the helicity effect on the complex singularities of the discretized dynamical system can still be of reasonable value for insights of the Navier-Stokes equation, although the high-order finite difference scheme used for computation may not be as accurate in dissipation range as the state-of-the-art of incompressible turbulence with pseudo-spectral method. Possible applications in controlling flows, for the purposes of, say, decreasing turbulence noise, are also discussed according to the spectral fluctuations.

physics.flu-dyn↗

Real Schur flow computations, helicity fastening effects and Bagua-pattern cyclones

A semi-analytical algorithm is developed for simulating flows with the velocity gradient uniformly of the real Schur form. Computations for both decaying and driven cases are performed, exhibiting basic results for general conception and testing the specific notion of `helicity fastening flows', and, creating the Jiu-Gong/Ba-Gua (ditetragonal/octagonal) pattern of cyclones resembling northern circumpolar cluster of Jupiter.

physics.flu-dyn↗

Structures and Dynamics of Lone Schur Flows with Vorticity but no Swirls

We study the dynamics and indications of the flows with all the eigenvalues of the velocity gradients being real, thus `lone', \textit{i.e.}, without forming the complex conjugate pairs associated to the swirls. A generic prototype is the `lone Schur flow (LSF)' whose velocity gradient tensor is uniformly of Schur form but free of complex eigenvalues. A (partial) integral-differential equation governing such LSF is established, and a semi-analytical algorithm is accordingly designed for computation. Simulated evolutions of example LSFs in 2- and 3-spaces show rich dynamics and vortical structures, but no obvious swirls (nor even the homoclinic loops in whatever distorted forms) could be found. We discovered the flux loop scenario and the anisotropic analogy of the incompressible turbulence at or close to the critical dimension $D_c =4/3$ decimated from 2-space.

physics.flu-dyn↗

Thermodynamic and vortic fine structures of real Schur flows

A two-component-two-dimensional coupled with one-component-three-dimensional (2C2Dcw1C3D) flow may also be called a real Schur flow (RSF), as its velocity gradient is uniformly of real Schur form, the latter being the intrinsic local property of any general flows. The thermodynamic and `vortic' fine structures of RSF are exposed and, in particular, the complete set of equations governing a (viscous and/or driven) 2C2Dcw1C3D flow are derived. The Lie invariances of the decomposed vorticity 2-forms of RSFs in $d$-dimensional Euclidean space $\mathbb{E}^d$ for any interger $d\ge 3$ are also proven, and many Lie-invariant fine results, such as those of the combinations of the entropic and vortic quantities, including the invariances of the decomposed Ertel potential vorticity (and their multiplications by any interger powers of entropy) 3-forms, then follow.

math.GM↗

Compressible helical turbulence: Fastened-structure geometry and statistics

Reduction of flow compressibility with the corresponding ideally invariant helicities, universally for various fluid models of neutral and ionized gases, can be argued statistically and associated with the geometrical scenario in the Taylor-Proudman theorem and its analogues. A `chiral base flow/field', rooted in the generic intrinsic local structure, as well as an `equivalence principle' is explained and used to bridge the single-structure mechanics and the helical statistics. The electric field fluctuations may similarly be depressed by the (self-)helicities of the two-fluid plasma model, with the geometry lying in the relation between the electric and density fields in a Maxwell equation.

physics.flu-dyn↗

Fast rotating flows in high spatial dimensions

The central result about fast rotating-flow structures is the Taylor-Proudman theorem (TPT) which connects various aspects of the dynamics. Taylor's geometrical proof of TPT is reproduced and extended substantially, with Lie's theory for general frozen-in laws and the consequent generalized invariant circulation theorems, to compressible flows and to $d$-dimensional Euclidean space ($\mathbb{E}^{d}$) with $d\ge 3$. The TPT relatives, the reduced models (with particular interests on passive-scalar problems), the inertial (resonant) waves and the higher-order corrections, are discussed coherently for a comprehensive bird view of rotating flows in high spatial dimensions.

math.AP↗

Geometrical and topological description of chirality-relevant flow structures

Issues relevant to the flow chirality and structure are focused, while the new theoretical results, including even a distinctive theory, are introduced. However, it is hope that the presentation, with a low starting point but a steep rise, is appropriate for a broader spectrum of audiences ranging from students to researchers, thus illustrations of differential forms and relevant basic topological concepts are also offered, followed by the demonstration with formulation of differential forms of the classical Navier-Stokes flow theory and the discussions of recent studies in fundamental fluid mechanics and turbulence.

physics.flu-dyn↗

Equilibrium and non-equilibrium time-reversible dynamical ensembles relevant to chiral turbulence

Ideas and theories of turbulence based on modifying the Navier-Stokes equation, to obtain equilibrium and non-equilibrium time-reversible dynamical ensembles relevant to helical turbulence, are presented. Discussions of controlling helicity to control the aerodynamic force, heat and noise are presented, together with the compressible turbulence relevant statistical mechanics analysis. A helical time-reversible system for nonequilibrium dynamical ensemble is constructed. Applications are also remarked.

physics.flu-dyn↗

Statistical mechanics of $d$-dimensional flows and cylindrically reduced passive scalars

Statistical properties of $d$-dimensional incompressible flows with and without cylindrical reduction are studied, leading to several explanations and conjectures about turbulent flows and passive scalars, such as the de-correlation between the flow and scalar, reduction of passive scalar intermittency in the bottleneck regime, et al. The absolute-equilibrium analyses assure the correctness of a recent numerical result. It is implied that passive scalar(s) in two-dimensional (2D) space can be fundamentally different to those in $d>2$, concerning the correlations with the flow, which is not considered in the celebrated Kraichnan model. The possibility of genuine inverse transfer to large scales of 2D passive scalar energy, together with the advection energy, is indicated. The compressible situation is also briefly remarked in the end, in particular the absence of density in a nontrivial Casimir which, without boundary contribution, also vanishes for $d=4$.

physics.flu-dyn↗