arXiv subjects

John D. Gibbon

Publications and source records attributed to John D. Gibbon.

At least 19 recordsLinked to original sources

arXiv2026

Forcing regimes in the two-dimensional Navier-Stokes equations

In the standard theoretical setting of body-forced turbulence, the forcing that sustains the flow is concentrated in a narrow range of length scales. However, in experiments of fractal-grid turbulence and in numerical simulations inspired by the renormalization group approach, more general forcing functions have been considered. These studies have shown that the phenomenology of turbulence is sensitive to the regularity of the forcing, which raises the wider question of the sensitivity of all Navier--Stokes mathematical estimates to the regularity of body forces. To answer this question, it is necessary to convert the traditional estimates based on the Grashof number, a dimensionless measure of the magnitude of the forcing, to a form dependent on the Reynolds number, the usual dimensionless number in experimental measurements and statistical theories of turbulence. To investigate these issues we consider the two-dimensional case and employ the full range of forcing regularity allowed by the theory of weak solutions to extend available estimates not only for the energy and enstrophy dissipation rates, but also for the dimension of the global attractor. What emerges is the existence of three distinct regimes as a function of the regularity of the forcing.

physics.flu-dyn↗

arXiv2026

Is it true that no mathematical relation exists between the Navier-Stokes equations and the multifractal model?

Contrary to accepted turbulence folklore, which holds that no mathematical relation exists between the Navier-Stokes equations (NSEs) and the multifractal model (MFM) of Parisi and Frisch, we develop a theory that reconciles the MFM with Leray's weak solutions of Navier-Stokes analysis. From a combination of Euler invariant scaling and the NSEs set in a three-dimensional box of size $L$, we also derive the Paladin-Vulpiani inverse scale $η_{h,pav}$, which is related to the Reynolds number $\mathit{Re}$ by $Lη_{h,pav}^{-1} = \mathit{Re}^{1/(1+h)}$, and which acts as a mediator between the two theories. This is achieved by considering $L^{2m}$-norms of the velocity gradient to find a correspondence between $m$ and the local scaling exponent $h$ in the multifractal model. The parameter $m$ acts as if it were the sliding focus control on a telescope which allows us to zoom in and out on different structures. The range $1 \leqslant m \leqslant \infty$ is equivalent to $-2/3 \leqslant h_{min} \leqslant 1/3$, which lies precisely in the region where Bandak et al. (2022, 2024) have suggested that thermal noise makes the NSEs inadequate and generates spontaneous stochasticity. The implications of this are discussed.

physics.flu-dyn↗

arXiv2026

The global attractor of the Toner-Tu-Swift-Hohenberg equations of active turbulence and its properties

The Toner-Tu-Swift-Hohenberg (TTSH) equations are one of the basic equations that are used to model turbulent behaviour in active matter, specifically the swarming of bacteria in suspension. They combine features of the incompressible Navier-Stokes, the Toner-Tu and Swift-Hohenberg equations, together with the important properties that they are linearly driven, and that the Laplacian diffusion is taken to be negative in combination with hyper-dissipation. We prove that the TTSH equations possess a finite-dimensional compact global attractor on the periodic domain $\mathbb{T}^d$ ($d=2,3$) and we establish explicit estimates for its Lyapunov dimension which agree with the heuristic prediction based on the Swift-Hohenberg length scale. The predominance of this length scale (as a vortex length scale) has been observed in both numerical and experimental studies of bacterial turbulence, so our methods and results provide a rigorous theoretical foundation for this phenomenon. We also carry out pseudospectral direct numerical simulations of these PDEs in dimension $d=2$ through which we obtain Lyapunov spectra for representative parameter values. We show that our numerical results are consistent with the analytically derived rigorous bounds.

physics.flu-dyn↗

arXiv2025

On the removal of the barotropic condition in helicity studies of the compressible Euler and ideal compressible MHD equations

The helicity is a topological conserved quantity of the Euler equations which imposes significant constraints on the dynamics of vortex lines. In the compressible setting the conservation law only holds under the assumption that the pressure is barotropic. We show that by introducing a new definition of helicity density $h_ρ=(ρ\textbf{u})\cdot\mbox{curl}\,(ρ\textbf{u})$ this assumption on the pressure can be removed, although $\int_V h_ρdV$ is no longer conserved. However, we show for the non-barotropic compressible Euler equations that the new helicity density $h_ρ$ obeys an entropy-type relation (in the sense of hyperbolic conservation laws) whose flux $\textbf{J}_ρ$ contains all the pressure terms and whose source involves the potential vorticity $q = ω\cdot \nabla ρ$. Therefore the rate of change of $\int_V h_ρdV$ no longer depends on the pressure and is easier to analyse, as it only depends on the potential vorticity and kinetic energy as well as $\mbox{div}\,\textbf{u}$. This result also carries over to the inhomogeneous incompressible Euler equations for which the potential vorticity $q$ is a material constant. Therefore $q$ is bounded by its initial value $q_{0}=q(\textbf{x},\,0)$, which enables us to define an inverse resolution length scale $λ_{H}^{-1}$ whose upper bound is found to be proportional to $\|q_{0}\|_{\infty}^{2/7}$. In a similar manner, we also introduce a new cross-helicity density for the ideal non-barotropic magnetohydrodynamic (MHD) equations.

math.AP↗

arXiv2024

Dynamics of vorticity moments in shell models of turbulence: A comparison with the Navier-Stokes equations

Shell models allow much greater scale separations than those presently achievable with direct numerical simulations of the Navier-Stokes equations. Consequently, they are an invaluable tool for testing new concepts and ideas in the theory of fully developed turbulence. They also successfully display energy cascades and intermittency in homogeneous and isotropic turbulent flows. Moreover, they are also of great interest to mathematical analysts because, while retaining some of the key features of the Euler and the Navier-Stokes equations, they are much more tractable. A comparison of the mathematical properties of shell models and of the three-dimensional Navier-Stokes equations is therefore essential in understanding the correspondence between the two systems. Here we focus on the temporal evolution of the moments, or $L^{2m}$-norms, of the vorticity. Specifically, differential inequalities for the moments of the vorticity in shell models are derived. The contribution of the nonlinear term turns out to be much weaker than its equivalent for the three-dimensional Navier-Stokes equations. Consequently, pointwise-in-time estimates are shown to exist for the vorticity moments for shell models of any order. This result is also recovered via a high-low frequency slaving argument that highlights the scaling relations between vorticity moments of different orders. Finally, it is shown that the estimates for shell models formally correspond to those for the Navier-Stokes equations 'on a point'.

physics.flu-dyn↗

arXiv2024

Phase transitions in the fractional three-dimensional Navier-Stokes equations

The fractional Navier-Stokes equations on a periodic domain $[0,\,L]^{3}$ differ from their conventional counterpart by the replacement of the $-νΔ\mathbf{u}$ Laplacian term by $ν_{s}A^{s}\mathbf{u}$, where $A= - Δ$ is the Stokes operator and $ν_{s} = νL^{2(s-1)}$ is the viscosity parameter. Four critical values of the exponent $s\geq 0$ have been identified where functional properties of solutions of the fractional Navier-Stokes equations change. These values are: $s=\frac{1}{3}$; $s=\frac{3}{4}$; $s=\frac{5}{6}$ and $s=\frac{5}{4}$. In particular: i) for $s > \frac{1}{3}$ we prove an analogue of one of the Prodi-Serrin regularity criteria; ii) for $s \geq \frac{3}{4}$ we find an equation of local energy balance and; iii) for $s > \frac{5}{6}$ we find an infinite hierarchy of weak solution time averages. The existence of our analogue of the Prodi-Serrin criterion for $s > \frac{1}{3}$ suggests the sharpness of the construction using convex integration of Hölder continuous solutions with epochs of regularity in the range $0 < s < \frac{1}{3}$.

math.AP↗

arXiv2023

Identifying the multifractal set on which energy dissipates in a turbulent Navier-Stokes fluid

The rich multifractal properties of fluid turbulence illustrated by the work of Parisi and Frisch are related explicitly to Leray's weak solutions of the three-dimensional Navier-Stokes equations. Directly from this correspondence it is found that the set on which energy dissipates, $\mathbb{F}_{m}$, has a range of dimensions $\Dim=3/m$ ($1 \leq m \leq \infty$), and a corresponding range of sub-Kolmogorov dissipation inverse length scales $Lη_{m}^{-1} \leq Re^{3/(1+\Dim)}$ spanning $Re^{3/4}$ to $Re^{3}$. Correspondingly, the multifractal model scaling parameter $h$, must obey $h \geq h_{min}$ with $-\twothirds \leq h_{min} \leq \third$.

physics.flu-dyn↗

arXiv2021

How to extract a spectrum from hydrodynamic equations

Practical results gained from statistical theories of turbulence usually appear in the form of an inertial range energy spectrum $\mathcal{E}(k)\sim k^{-q}$ and a cut-off wave-number $k_{c}$. For example, the values $q=5/3$ and $\ell k_{c}\sim \mathit{Re}^{3/4}$ are intimately associated with Kolmogorov's 1941 theory. To extract such spectral information from the Navier-Stokes equations, Doering and Gibbon (2002) introduced the idea of forming a set of dynamic wave-numbers $κ_n(t)$ from ratios of norms of solutions. The time averages of the $κ_n(t)$ can be interpreted as the 2$n$th-moments of the energy spectrum. They found that $1 < q \leqslant 8/3$, thereby confirming the earlier work of Sulem and Frisch (1975) who showed that when spatial intermittency is included, no inertial range can exist in the limit of vanishing viscosity unless $q \leqslant 8/3$. Since the $κ_n(t)$ are based on Navier-Stokes weak solutions, this approach connects empirical predictions of the energy spectrum with the mathematical analysis of the Navier-Stokes equations. This method is developed to show how it can be applied to many hydrodynamic models such as the two dimensional Navier--Stokes equations (in both the direct- and inverse-cascade regimes), the forced Burgers equation and shell models.

physics.flu-dyn↗

arXiv2021

The Variable Density Model for the Rayleigh-Taylor Instability and its transformation to the diffusive, inhomogeneous, incompressible Navier-Stokes equations

It is shown how the variable density model (VDM) that governs the Rayleigh-Taylor instability (RTI) for the miscible mixing of two incompressible fluids can be transformed into a diffusive version of the inhomogeneous, incompressible Navier-Stokes equations forced by gradients of the composition density $ρ$ of the mixing layer. This demonstrates how buoyancy-driven flows drive and enhance Navier-Stokes turbulence. The role of the potential vorticity $q = ω\cdot\nablaρ$ is also discussed.

physics.flu-dyn↗

arXiv2020

Intermittency, cascades and thin sets in three-dimensional Navier-Stokes turbulence

Visual manifestations of intermittency in computations of three dimensional Navier-Stokes fluid turbulence appear as the low-dimensional or `thin' filamentary sets on which vorticity and strain accumulate as energy cascades down to small scales. In order to study this phenomenon, the first task of this paper is to investigate how weak solutions of the Navier-Stokes equations can be associated with a cascade and, as a consequence, with an infinite sequence of inverse length scales. It turns out that this sequence converges to a finite limit. The second task is to show how these results scale with integer dimension $D=1,\,2,\,3$ and, in the light of the occurrence of thin sets, to discuss the mechanism of how the fluid might find the smoothest, most dissipative class of solutions rather than the most singular.

nlin.CD↗

arXiv2020

How close are shell models to the 3D Navier-Stokes equations?

Shell models have found wide application in the study of hydrodynamic turbulence because they are easily solved numerically even at very large Reynolds numbers. Although bereft of spatial variation, they accurately reproduce the main statistical properties of fully-developed homogeneous and isotropic turbulence. Moreover, they enjoy regularity properties which still remain open for the three-dimensional (3D) Navier-Stokes equations (NSEs). The goal of this study is to make a rigorous comparison between shell models and the NSEs. It turns out that only the estimate of the mean energy dissipation rate is the same in both systems. The estimates of the velocity and its higher-order derivatives display a weaker Reynolds number dependence for shell models than for the 3D NSEs. Indeed, the velocity-derivative estimates for shell models are found to be equivalent to those corresponding to a velocity gradient averaged version of the 3D Navier-Stokes equations (VGA-NSEs), while the velocity estimates are even milder. Numerical simulations over a wide range of Reynolds numbers confirm the estimates for shell models.

nlin.CD↗

arXiv2018

Weak and strong solutions of the $3D$ Navier-Stokes equations and their relation to a chessboard of convergent inverse length scales

Using the scale invariance of the Navier-Stokes equations to define appropriate space-and-time-averaged inverse length scales associated with weak solutions of the $3D$ Navier-Stokes equations, an infinite `chessboard' of estimates for these inverse length scales is displayed in terms of labels $(n,\,m)$ corresponding to $n$ derivatives of the velocity field in $L^{2m}$. The $(1,\,1)$ position corresponds to the inverse Kolmogorov length $Re^{3/4}$. These estimates ultimately converge to a finite limit, $Re^3$, as $n,\,m\to \infty$, although this limit is too large to lie within the physical validity of the equations for realistically large Reynolds numbers. Moreover, all the known time-averaged estimates for weak solutions can be rolled into one single estimate, labelled by $(n,\,m)$. In contrast, those required for strong solutions to exist can be written in another single estimate, also labelled by $(n,\,m)$, the only difference being a factor of 2 in the exponent. This appears to be a generalisation of the Prodi-Serrin conditions for $n\geq 1$.

nlin.CD↗

arXiv2017

The role of BKM-type theorems in $3D$ Euler, Navier-Stokes and Cahn-Hilliard-Navier-Stokes analysis

The Beale-Kato-Majda theorem contains a single criterion that controls the behaviour of solutions of the $3D$ incompressible Euler equations. Versions of this theorem are discussed in terms of the regularity issues surrounding the $3D$ incompressible Euler and Navier-Stokes equations together with a phase-field model for the statistical mechanics of binary mixtures called the $3D$ Cahn-Hilliard-Navier-Stokes (CHNS) equations. A theorem of BKM-type is established for the CHNS equations for the full parameter range. Moreover, for this latter set, it is shown that there exists a Reynolds number and a bound on the energy-dissipation rate that, remarkably, reproduces the $Re^{3/4}$ upper bound on the inverse Kolmogorov length normally associated with the Navier-Stokes equations alone. An alternative length-scale is introduced and discussed, together with a set of pseudo-spectral computations on a $128^{3}$ grid.

nlin.CD↗

arXiv2017

Integrability of the hyperbolic reduced Maxwell-Bloch equations for strongly correlated Bose-Einstein condensates

We derive and study the hyperbolic reduced Maxwell-Bloch equations (HRMB), a simplified model for the dynamics of strongly correlated Bose-Einstein condensates (BECs), and in particular for the interaction between the BEC atoms and its evaporated atoms under the strong interactions. This equation is one among four which are proven to be integrable via the existence of a Lax pair, and thus the method of inverse scattering transform. Another equation is the reduced Maxwell-Bloch equation of quantum optics and the two others do not have physical applications yet. By studying the linear stability of the constant solutions of these four equations we observe various regimes, from stable, to modulational unstable and unstable at all frequencies. The finite dimensional reduction of the RMB equations is also used to give more insight into the constant solutions of these equations. From this study, we find that the HRMB equation arising from strongly correlated BECS is stable under the particular condition that the transition rate of evaporation is not too large compared to the number of evaporated atoms. We then derive explicit soliton solutions of the RMB equations and use numerical simulations to show collisions of solitons and kink solitons.

math-ph↗

arXiv2017

Bounds on solutions of the rotating, stratified, incompressible, non-hydrostatic, three-dimensional Boussinesq equations

We study the three-dimensional, incompressible, non-hydrostatic Boussinesq fluid equations, which are applicable to the dynamics of the oceans and atmosphere. These equations describe the interplay between velocity and buoyancy in a rotating frame. A hierarchy of dynamical variables is introduced whose members $Ω_{m}(t)$ ($1 \leq m < \infty$) are made up from the respective sum of the $L^{2m}$-norms of vorticity and the density gradient. Each $Ω_{m}(t)$ has a lower bound in terms of the inverse Rossby number, $Ro^{-1}$, that turns out to be crucial to the argument. For convenience, the $Ω_{m}$ are also scaled into a new set of variables $D_{m}(t)$. By assuming the existence and uniqueness of solutions, conditional upper bounds are found on the $D_{m}(t)$ in terms of $Ro^{-1}$ and the Reynolds number $Re$. These upper bounds vary across bands in the $\{D_{1},\,D_{m}\}$ phase plane. The boundaries of these bands depend subtly upon $Ro^{-1}$, $Re$, and the inverse Froude number $Fr^{-1}$. For example, solutions in the lower band conditionally live in an absorbing ball in which the maximum value of $Ω_{1}$ deviates from $Re^{3/4}$ as a function of $Ro^{-1},\,Re$ and $Fr^{-1}$.

nlin.CD↗

arXiv2016

A regularity criterion for solutions of the three-dimensional Cahn-Hilliard-Navier-Stokes equations and associated computations

We consider the 3D Cahn-Hilliard equations coupled to, and driven by, the forced, incompressible 3D Navier-Stokes equations. The combination, known as the Cahn-Hilliard-Navier-Stokes (CHNS) equations, is used in statistical mechanics to model the motion of a binary fluid. The potential development of singularities (blow-up) in the contours of the order parameter $ϕ$ is an open problem. To address this we have proved a theorem that closely mimics the Beale-Kato-Majda theorem for the $3D$ incompressible Euler equations [Beale et al. Commun. Math. Phys., Commun. Math. Phys., ${\rm 94}$, $ 61-66 ({\rm 1984})$]. By taking an $L^{\infty}$ norm of the energy of the full binary system, designated as $E_{\infty}$, we have shown that $\int_{0}^{t}E_{\infty}(τ)\,dτ$ governs the regularity of solutions of the full 3D system. Our direct numerical simulations (DNSs), of the 3D CHNS equations, for (a) a gravity-driven Rayleigh Taylor instability and (b) a constant-energy-injection forcing, with $128^3$ to $512^3$ collocation points and over the duration of our DNSs, confirm that $E_{\infty}$ remains bounded as far as our computations allow.

physics.flu-dyn↗

arXiv2014

Regimes of nonlinear depletion and regularity in the 3D Navier-Stokes equations

The periodic $3D$ Navier-Stokes equations are analyzed in terms of dimensionless, scaled, $L^{2m}$-norms of vorticity $D_{m}$ ($1 \leq m < \infty$). The first in this hierarchy, $D_{1}$, is the global enstrophy. Three regimes naturally occur in the $D_{1}-D_{m}$ plane. Solutions in the first regime, which lie between two concave curves, are shown to be regular, owing to strong nonlinear depletion. Moreover, numerical experiments have suggested, so far, that all dynamics lie in this heavily depleted regime \cite{DGGKPV13}\,; new numerical evidence for this is presented. Estimates for the dimension of a global attractor and a corresponding inertial range are given for this regime. However, two more regimes can theoretically exist. In the second, which lies between the upper concave curve and a line, the depletion is insufficient to regularize solutions, so no more than Leray's weak solutions exist. In the third, which lies above this line, solutions are regular, but correspond to extreme initial conditions. The paper ends with a discussion on the possibility of transition between these regimes.

nlin.CD↗

arXiv2013

The 3D incompressible Euler equations with a passive scalar: a road to blow-up?

The 3D incompressible Euler equations with a passive scalar $θ$ are considered in a smooth domain $Ω\subset \mathbb{R}^{3}$ with no-normal-flow boundary conditions $\bu\cdot\bhn|_{\partialΩ} = 0$. It is shown that smooth solutions blow up in a finite time if a null (zero) point develops in the vector $\bB = \nabla q\times\nablaθ$, provided $\bB$ has no null points initially\,: $\bom = \mbox{curl}\,\bu$ is the vorticity and $q = \bom\cdot\nablaθ$ is a potential vorticity. The presence of the passive scalar concentration $θ$ is an essential component of this criterion in detecting the formation of a singularity. The problem is discussed in the light of a kinematic result by Graham and Henyey (2000) on the non-existence of Clebsch potentials in the neighbourhood of null points.

nlin.CD↗