                    The 3D non-linear, non-stationary NSE solution enabled by an ideal fluid element H(-1/2) Hilbert space framework

(WeH) p. 220: "We are not surprised that a concrete chunk of nature, taken in its isolated phenomenal existence, challenges our analysis by its inexhaustibility and incompleteness; it is for the sake of completeness, as we have seen, that physics projects what is given onto the background of the possible".

The Navier-Stokes equations describe the motion of fluids. The classical Navier-Stokes partial differential equations in a Sobolev space framework is about "fluid element" as elements of the Lebesgue L(2) Hilbert space and the related "fluid motion/velocity" as elements of the  Sobolev (sub-) space H(1)=W(1,2). Quantum mechanics requires a Hilbert space framework. The simple proposal of this homepage is to replace the standard Hilbert space L(2)=H(0) by the weaker distributional Hilbert space H(-1/2). The later one is proposed to replace Dirac's concept of the H(-n/2-e) Hilbert space (n denotes the space dimension, and e>0), which contains the "Dirac/Delta "function"", as his proposed model of a charged electron. We note that this kind of EP is twofold attributed, it is a "particle" with a given finite charge, or, in other words, this EP model is characterized by two specifications,

1. a kind of location in the considered space framework (mathematically speaking, this is basically a real number, which is with 100% probability "only" defined as the limit of a sequence of an infinite numbers of rational numbers, or, physically speaking, it is an whole universe by itself)

2. "equipped" with an electric finite physical "potential difference" between the somewhere in the space located particle and its surrounding space framework.

The Navier–Stokes existence and smoothness problem for the three-dimensional NSE, (given some initial conditions) is to prove that smooth solutions always exist, or that if they do exist, they have bounded energy per unit mass. Continuity resp. differentiability of its solutions are ensured by the Sobolev embedding theorem, where the (space dimension n depending) inequality > n/2 occurs.

By formally operating with "div" operator on the NSE the pressure field must satisfy the Neumann problem. It follows that the prescription of the pressure at the bounding walls or at the initial time independently of the velocity u, could be incompatible with the initial boundary values of the NSE, and therefore, could render the problem ill-posed (GaG). Plemelj's alternative normal derivative concept enables initial boundary value "functions" to define a pressure operator with domain H(1/2) overcoming this issue. We further note that both physical concepts, "pressure" and "energy density", do have the same unit of measure (  N / (m*m) = Nm / (m*m*m) ).

The Serrin gap occurs in case of space dimension n=3 as a consequence of the Sobolev embedding theorem with respect to the energy Hilbert space H(1) with the Dirichlet integral as its inner product. The proposed alternatively model of this homepage (dealing with an (energy Hilbert space H(1/2)) enables an appropriate, currently missing, energy norm estimate for the 3-D non-stationary, non-linear NSE (which also takes into account energy values of the non-linear terms).

As a shortcut reference to the underlying mathematical principles of classical fluid mechanics we refer to (SeJ).

A central concept of the proposed solution Hilbert space frame is the alternative normal derivative concept of Plemelj. It is built for the logarithmic potential case based on the Cauchy-Riemann differential equations with its underlying concept of conjugate harmonic functions. Its generalization to several variables is provided in the paper below. It is based on the equivalence to the statement that a vector u is the gradient of a harmonic function H, that is u=gradH. Studying other systems than this, which are also in a natural sense generalizations of the Cauchy-Riemann differential equations, leads to representations of the rotation group (StE).

We provide a global unique (weak, generalized Hopf) NSE solution of the variational H(-1/2)-representation of the generalized 3D Navier-Stokes initial value problem. The global boundedness of a generalized energy inequality with respect to the energy Hilbert space H(1/2) is a consequence of the Sobolevskii estimate of the non-linear term (1959).

Here we are:

The proposed solution concept is about rotation-invariant fluids (circulation) modelled as elements of a Hilbert space with negative (distributional) Hilbert scale defined by the eigenpairs of the Stokes operator.

The solution concept also addresses the D'Alembert "paradox" which is about unrealistic fluids (interaction) dynamics by which no aircraft would be able to fly, anyway.

The mathematical concept is about adequately defined fractional scaled (energy) Hilbert space. This corresponds to J. Plemelj's alternative normal derivative definition (with reduced regularity assumptions to its domain) and to the generalized Green identities valid for same domain. The resulting regularity requirement reduction is in the same size as a reduction from C(1) to C(0) regularity, which leads to "scale reduction" of weak (variation) partial differential equation representations by 1/2.

The Leray-Hopf operator and the linearized Landau collision operator

The Leray-Hopf operator plays a key role in existence and uniqueness proofs of weak solutions of the Navier-Stokes equations, obtaining weak and strong energy inequalities. For a related integral representation of the NSE solution we refer to (PeR).

In a weak H(-1/2) Hilbert space framework in the context of the Landau damping phenomenon the linerarized Landau collision operator can be interpreted as a compactly disturbed Leray-Hopf operator.

Both operators, the "Leray-Hopf (or Helmholtz-Weyl) operator and the linearized Landau collision operator are not classical pseudo-differential operators, but Fourier multipliers with same continuity properties as those of the Riesz operators (LiP1).

For the related Oseen operators Fourier multiplier we refer to (LeN).

The related hypersingular integral equation theory, including the Prandtl operator, is provided in (LiI).

References

(FeP) Federbush P., Navier and Stokes Meet the Wavelet, Commun. Math. Phys. 155, 219-248 (1993)

(FoC) Foias C., Temam R., Some Analytic and Geometric Properties of the Solutions of the Evolution Navier-Stokes Equations, J. Math. Pures et Appl. 58, 339 (1979)

(GaG) Galdi G. P., The Navier-Stokes Equations: A Mathematical Analysis, Encyclopedia of Complexity and System Science, Springer Verlag, 2009

(LeN) Lerner, N., A note on the Oseen kernels, Advances in Phase Space Analysis of Partial Differential Equations, pp. 161-170, 2007

(LiP) Lions P. L., Boltzmann and Landau equations

(LiP1) Lions P. L., Compactness in Boltzmann’s Fourier integral operators and applications

(LiI) Lifanov I. K., Poltavskii L. N., Vainikko G. M., Hypersingular integral equations and their applications, Chapman & Hall, CRC Press Company, Boca Raton, London, New York, Washington, 2004

(PeR) Peralta-Fabi R., A integral representation of the Navier-Stokes equations

(SoH) Sohr H., The Navier-Stokes Equations, An Elementary Functional Analytical Approach, Birkhäuser Verlag, Basel, Boston, Berlin, 2000

(TeR) Teman R., Navier-Stokes Equations and Nonlinear Functional Analysis, SIAM, Philadelphia, 1983

(WeH) Weyl H., Philosophy of Mathematics and Natural Science, Princeton University Press, Princeton, 1949, 2009   