B Banos

V N Roubtsov

I Roulstone
email: i.roulstone@surrey.ac.uk

Monge-Ampère Structures and the Geometry of Incompressible Flows

We show how a symmetry reduction of the equations for incompressible hydrodynamics in three dimensions leads naturally to Monge-Ampère structure, and Burgers'-type vortices are a canonical class of solutions associated with this structure. The mapping of such solutions, which are characterised by a linear dependence of the third component of the velocity on the coordinate defining the axis of rotation, to solutions of the incompressible equations in two dimensions is also shown to be an example of a symmetry reduction. The Monge-Ampère structure for incompressible flow in two dimensions is shown to be hyper-symplectic.

Introduction

The incompressible Euler and Navier-Stokes equations pose an enduring challenge for mathematical analysis. Previous works by Roubtsov andRoulstone (1997, 2001), Delahaies and Roulstone (2010), and by [START_REF] Mcintyre | Are there higher-accuracy analogues of semi-geostrophic theory?[END_REF], have shown how quite sophisticated ideas from modern differential geometry offer a framework for understanding how coherent structures in large-scale atmosphere-ocean flows, such as cyclones and fronts, may be modelled using a hierarchy of approximations to the Navier-Stokes equations. These studies focussed on semi-geostrophic (SG) theory 2 The geometry of Monge-Ampère equations 2.1 Monge-Ampère structures [START_REF] Lychagin | Nonlinear differential equations and contact geometry[END_REF] proposed a geometric approach to to study of MA equations using differential forms. The idea is to associate with a form ω ∈ Λ n (T * R n ), where Λ n denotes the space of differential n-forms on T * R n , the MA equation ∆ ω = 0, where ∆ ω :

C ∞ (R n ) → Ω n (R n ) ∼ = C ∞ (R n ) is the differential operator defined by ∆ ω (φ) = (dφ) * ω ,
and (dφ) * ω denotes the restriction of ω to the graph of dφ (dφ : R n → T * R n is the differential of φ).

Following [START_REF] Lychagin | A classification of Monge-Ampère equations[END_REF], a MA equation in n variables can be described by a pair (Ω, ω) ∈ Λ 2 (T * R n ) × Λ n (T * R n ) such that i) Ω is symplectic that is nondegenerate and closed ii) ω is effective, that is ω ∧ Ω = 0. Such a pair is called a Monge-Ampère structure.

A generalized solution of a MA equation is a lagrangian submanifold L, on which ω vanishes; that is, L is an n-dimensional submanifold such that Ω| L = 0 and ω| L = 0. Note that if φ(x) is a regular solution then its graph L φ = {(x, φ x )} in the phase space is a generalized solution.

In four dimensions (that is n = 2), a geometry defined by this structure can be either complex or real and this distinction coincides with the usual distinction between elliptic and hyperbolic differential equations in two variables. Indeed, when ω ∈ Λ 2 (T * R 2 ) is a nondegenerate 2-form (ω ∧ ω = 0), one can associate with the Monge-Ampère structure (Ω, ω) ∈ Λ 2 (T * R 2 ) × Λ 2 (T * R 2 ) the tensor I ω defined by

1 | pf(ω)| ω(•, •) = Ω(I ω •, •) (1)
where pf(ω) is the pfaffian of ω: ω ∧ ω = pf(ω)(Ω ∧ Ω). This tensor is either an almost complex structure or an almost product structure: [START_REF] Lychagin | Nonlinear differential equations and contact geometry[END_REF]Rubtsov (1983a,b, 1993) (see also [START_REF] Kushner | Contact geometry and nonlinear differential equations[END_REF] for a comprehensive account of this theory) have explained the link there is between the problem of local equivalence of MA equations in two variables and the integrability problem of this tensor I ω : PROPOSITION 1. (Lychagin-Roubtsov theorem) The three following assertions are equivalent:

a) ∆ ω = is elliptic ⇔ pf(ω) > 0 ⇔ I 2 ω = -Id b) ∆ ω = is hyperbolic ⇔ pf(ω) < 0 ⇔ I 2 ω = Id
1. ∆ ω = 0 is locally equivalent to one of the two equations

∆φ = 0 φ = 0 2. the almost complex (or product) structure I ω is integrable 3. the form ω √ | pf(ω)| is closed.
3 The geometry of the incompressible Euler equations in two dimensions

The Poisson equation for the pressure

The incompressible Euler equations (in two or three dimensions) are

∂u i ∂t + u j u ij + p i = 0, u ii = 0, (2)
with p i = ∂p/∂x i , u ij = ∂u i /∂x j , and the summation convention is used. By applying the incompressibility condition, we find that the Laplacian of the pressure and the velocity gradients are related by ∆p = -u ij u ji .

(3)

In two dimensions, the incompressibility condition implies

u 1 = - ∂ψ ∂x 2 = -ξ 2 , u 2 = ∂ψ ∂x 1 = ξ 1 ,
where ψ(x 1 , x 2 , t) is the stream function and (3) becomes

ψ x 1 x 1 ψ x 2 x 2 -ψ 2 x 1 x 2 = ∆p 2 . ( 4
)
Equation ( 3) is normally used as a Poisson equation for the pressure. However, formally, we may consider (4) to be an equation of Monge-Ampère type for the stream function, if the Laplacian of the pressure is given. Roulstone et al. (2009a) discuss the relationship between the sign of the Laplacian of the pressure, the elliptic/hyperbolic type of the associated MA equation, and the balance between rate of strain and the enstrophy of the flow. This is related to the Weiss criterion [START_REF] Weiss | The dynamics of enstrophy transfer in two-dimensional hydrodynamics[END_REF], as discussed by Larcheveque (1993).

Hypersymplectic geometry

Following Section 2.1, (4) can be associated with a Monge-Ampère structure (Ω, ω) on the phase space

M = T * R 2 , where Ω = dx 1 ∧ du 2 + du 1 ∧ dx 2 is a symplectic form (which is written conventionally in the form Ω = dx 1 ∧ dξ 1 + dx 2 ∧ dξ 2 ) and ω ∈ Ω 2 (M ) is the 2-form defined by ω = du 1 ∧ du 2 -a(x 1 , x 2 )dx 1 ∧ dx 2 with a(x 1 , x 2 , t) = ∆p 2 .
(Note that time, t, is a parameter in our examples, and its presence will not be indicated, unless crucial.)

We observe that dω = 0 (5)

and ω ∧ ω = a Ω ∧ Ω. ( 6
)
Moreover, the tensor A ω (cf. ( 1)) defined by ω(•,

•) = Ω(A ω •, •) satisfies A 2 ω = -a.
Following [START_REF] Roubtsov | Holomorphic structures in hydrodynamical models of nearly geostrophic flow[END_REF] (see also [START_REF] Kossowski | Prescribing invariants of lagrangian surfaces[END_REF]), we define a symmetric tensor

g ω on M g ω (X, Y ) = 2 (ι X ω ∧ ι Y Ω + ι Y ω ∧ ι X Ω) ∧ dx 1 ∧ dx 2 Ω 2 .
In coordinates, one has

g ω = dx 1 ⊗ du 2 -dx 2 ⊗ du 1 (7)
and therefore the signature of g ω is (2, 2). Using this split metric we define a dual form ω

∈ Λ 2 (M ) ω(•, •) = g ω (A ω •, •), which in coordinates is ω = -du 1 ∧ du 2 -a(x 1 , x 2 )dx 1 ∧ dx 2 . ( 8
)
This dual form satisfies dω = 0 (9)

and ω ∧ ω = -a Ω ∧ Ω. ( 10
)
Hence, introducing the normalized form Ω = |a|Ω, we obtain a triple Ω, ω, ω , which is hypersymplectic [START_REF] Hitchin | Hypersymplectic quotients[END_REF]) when ∆p = 0:

ω 2 = -ω 2 , ω 2 = ε Ω2 , ω2 = -ε Ω2 , ω ∧ ω = 0, ω ∧ Ω = 0, ω ∧ Ω = 0, with ε = 1 if ∆p > 0 and ε = -1 if ∆p < 0.
Equivalently, we obtain three tensors S, I and T defined by

ω(•, •) = ω(S•, •), ω(•, •) = Ω(I•, •), ω(•, •) = Ω(T •, •), which satisfy S 2 = 1, I 2 = -ε, T 2 = ε, T I = -IT = S, T S = -ST = I, IS = -SI = T.
This structure should be compared with the hyper-Kähler properties of the SG equations (Delahaies and Roulstone 2010), and it extends the results presented in Roulstone et al. (2009a). Moreover, since

dω = dω = 0 and d Ω = 1 2 √ a da ∧ Ω,
we deduce from Lychagin-Roubtsov theorem (see Proposition 1) that i) the product structure S is always integrable ii) the complex structure I and the product structure T (or vice versa) are integrable if and only if ∆p is constant.

Generalized solutions

A generalized solution of ( 4) is a 2d-submanifold L 2 ⊂ M 4 which is bilagrangian with respect to ω and Ω. Since ω = Ω(I, •, •), it is equivalent to saying that L is closed under I: for any non vanishing vector field X on L, {X, IX} is a local frame of L.

PROPOSITION 2. Let L be a generalized solution and h ω the restriction of g ω on L.

1. if ∆p > 0, the metric h ω has signature (2, 0) or (0, 2)

2. if ∆p < 0, the metric h ω has signature (1, 1).

Proof. Let X a local non vanishing vector field, tangent to L. Then {X, IX} is a local frame of L 2 and {X, IX, SX, T X} a local frame of M 4 . Let α = ω(X, IX). Then we have i) ω(X, X) = 0 by skew-symmetry .

ii) ω(X, IX) = 0 since X and IX are tangent to L.

iii

) ω(X, SX) = -ω(SX, X) = ω(X, X) = 0. iv) ω(X, T X) = ω(SIX, X) = ω(IX, X) = -α.
Therefore, since ω is non degenerate, α cannot be 0. Moreover,

i) g ω (IX, IX) = 1 |a| g ω (A ω X, A ω X) = 1 |a| ω(X, A ω X) = α |a| ii) g ω (IX, X) = 1 |a| g ω (A ω X, X) = 1 |a| ω(X, X) = 0 iii) g ω (X, X) = - 1 a g ω (A 2 ω X, X) = -ε 1 |a| ω(A ω X, X) = αε |a|
We therefore obtain the matrix of h ω in the representation {X, IX}:

h ω = α |a| ε 0 0 1 REMARK 1. If L = L ψ
is a regular solution, then the induced metric h ω is affine. Indeed its tangent space is generated by

X 1 = ∂ x 1 -ψ x 1 x 2 ∂ u 1 + ψ x 1 x 1 ∂ u 2 and X 2 = ∂ x 2 -ψ x 2 x 2 ∂ u 1 + ψ x 1 x 2 ∂ u 2 . The induced metric on L ψ is therefore h ω = 2 ψ xx ψ xy ψ xy ψ yy (11)
and consequently the invariants of this tensor are

det(h ω ) = 2∆p, tr(h ω ) = 2∆ψ.
We note that metric, as written in (11), is related to the velocity gradient tensor (VGT) of a two-dimensional flow. The signature of the induced metric changes when the sign of the Laplacian of the pressure changes, and this is related to the flip between the elliptic/hyperbolic nature of (4) when viewed as a MA equation for ψ.

The VGT is not a symmetric tensor and its trace is the divergence of the flow (which equals zero in the incompressible case). The trace of h ω is proportional to the vorticity of the flow. The determinant of the VGT is the same as det(h ω ), and this is precisely the balance between the rate-of-strain and the enstrophy (vorticity squared). It is possible to construct equations of motion for the invariants of the VGT and for the invariants of h ω : see [START_REF] Roulstone | Geometric invariants of the horizontal velocity gradient tensor and their dynamics in shallow water flow[END_REF] for further details.

4 Monge-Ampère structures in six dimensions

Geometry of 3-forms

In six dimensions (n = 3), there is again a correspondence between real/complex geometry and "nondegenerate" Monge-Ampère structures. [START_REF] Lychagin | A classification of Monge-Ampère equations[END_REF] associated with a Monge-Ampère structure (Ω, ω) an invariant symmetric form

g ω (X, Y ) = ι X ω ∧ ι Y ω ∧ Ω vol
whose signature distinguishes the different orbits of the symplectic group action.

In a seminal paper on the geometry of 3-forms, [START_REF] Hitchin | The geometry of three-forms in six and seven dimensions[END_REF] defined the notion of nondegenerate 3-forms on a 6-dimensional space and constructed a scalar invariant, which we call the Hitchin pfaffian, which is non zero for such nondegenerate 3-forms. Hitchin also defined an invariant tensor A ω on the phase space satisfying

A 2 ω = λ(ω) Id.
Note that in the nondegenerate case,

K ω = A ω |λ(ω)|
is a product structure if λ(ω) > 0 and a complex structure if λ(ω) < 0. Following Hitchin op. cit., we observe 1. λ(ω) > 0 if and only if ω is the sum of two decomposable forms; i.e.

ω = α 1 ∧ α 2 ∧ α 3 + β 1 ∧ β 2 ∧ β 3
2. λ(ω) < 0 if and only if ω is the sum of two decomposable complex forms; i.e.

ω = (α 1 +iβ 1 )∧(α 2 +iβ 2 )∧(α 3 +iβ 3 )+(α 1 -iβ 1 )∧(α 2 -iβ 2 )∧(α 3 -iβ 3 )
Banos ( 2002) noted that on a 6-dimensional symplectic space, nondegenerate 3-forms correspond to nondegenerate Lychagin-Roubtsov symmetric tensors. Moreover the signature of g ω is (3, 3) if λ(ω) > 0 and (6, 0) or (4, 2) if λ(ω) < 0.

Banos op. cit. established the following simple relation between the Hitchin and the Lychagin-Roubsov invariants

g ω (A ω •, •) = Ω(•, •).
There is a correspondence between non-degenerate MA structures (Ω, ω) and a "real Calabi-Yau structure" if the Hitchin pfaffian λ(ω) is positive, and a "complex Calabi-Yau structure" if the Hitchin pfaffian is negative.

For example, the Monge-Ampère structure associated with the "real" MA equation in three variables (x 1 , x 2 , x 3 )

hess(φ) = 1 , is the pair Ω = dx 1 ∧ dξ 1 + dx 2 ∧ dξ 2 + dx 3 ∧ dξ 3 ω = dξ 1 ∧ dξ 2 ∧ dξ 3 -dx 1 ∧ dx 2 ∧ dx 3
and the underlying real analogue of Kähler structure on

T * R 3 in the coor- dinates (x 1 , x 2 , x 3 , ξ 1 , ξ 2 , ξ 3 ) is            g ω = 0 Id Id 0 , K ω = Id 0 0 -Id .
The Monge-Ampère structure associated with the special lagrangian equation ∆φ -hess(φ) = 0 is the pair

Ω = dx 1 ∧ dξ 1 + dx 2 ∧ dξ 2 + dx 3 ∧ dξ 3 ω = Im (dx 1 + idξ 1 ) ∧ (dx 2 + idξ 2 ) ∧ (dx 3 + idξ 3 )
and the underlying Kähler structure is the canonical Kähler structure on

T * R 3 = C 3 :            g ω = Id 0 0 Id , K ω = 0 -Id Id 0 .
As in the 4-dimensional case, there is a clear connection between the problem of local equivalence of MA equation in three variables and the integrability problem of generalized Calabi-Yau structures on R 6 (Banos 2002): PROPOSITION 3. The three following assertions are equivalent: i) the MA equation ∆ ω = 0 is locally equivalent to one of the three equations

     hess(φ) = 1, ∆φ -hess(φ) = 0, φ + hess(φ) = 0,
ii) the underlying generalized Calabi-Yau structure on

T * R 3 is integrable, iii) the form ω 4 √ |λ(ω)|
and its dual form (in the sense of Hitchin) are closed.

Moreover, the metric g ω should be flat.

The generalized Calabi-Yau structure discussed here is defined in Banos (2002). This notion is different from the generalized Calabi-Yau structures in sense of N. Hitchin and M. Gualtieri. These relations are clarified and discussed further in §14.4 of Kosmann-Schwarzbach and [START_REF] Kosmann-Schvarzbach | Compatible structures on Lie algebroids and Monge-Ampère operators[END_REF].

It is important to note that the geometry associated with a MA equation ∆ ω = 0 of real type (λ(ω) > 0) is essentially real but it is very similar to the classic Kähler geometry. In particular, when this geometry is integrable, there exists a potential Φ and a coordinate system (x i , u i ) i=1,2,3 on T * R 6 such that

g ω = i,j ∂ 2 Φ ∂x i ∂ξ j dx i • dξ j and det ∂ 2 Φ ∂x j ∂ξ j = f (x)g(ξ).

Reduction principle in six dimensions

The Marsden-Weinstein reduction process is a classical tool in symplectic geometry to reduce spaces with symmetries (see for example [START_REF] Marsden | Lectures on Mechanics[END_REF].

It is explained in [START_REF] Banos | Hyper-Kähler geometry and semigeostrophic theory[END_REF] how the theory of Monge-Ampère operators gives a geometric formalism in which the symplectic reduction process of Marsden-Weinstein fits naturally to reduce equations with symmetries, and we recall the basic ideas here.

Let us consider a Hamiltonian action λ : G × M → M of a 1-dimensional Lie group G (G = R, R * or S 1 ) on our symplectic manifold (M = T * R 3 , Ω). We denote by X the infinitesimal generator of this action and µ : M → R is the moment map of this action:

ι X Ω = -dµ.
For a regular value c, we know from the Mardsen-Weinstein reduction theorem that the 4-dimensional reduced space M c = µ -1 (c)/G admits a natural symplectic form Ω c such that π * Ω c is the restriction of Ω on µ -1 (c), where π : µ -1 (c) → M c is the natural projection.

Assume moreover that the action λ preserves also the 3-form ω:

∀g ∈ G, λ * g (ω) = ω.
Then there exists a 2-form ω c on M c such that

π * (ω c ) = ι X ω.
We then obtain a Monge-Ampère structure (Ω c , ω c ) on the reduced space M c , and there is a one-to-one correspondence between regular solutions L c on M c (that is Ω c | Lc = 0 and ω c | Lc = 0) and G-invariant generalized solutions L = π -1 (L c ) on M .

Example We consider as a trivial example the 3D-Laplace equation

φ x 1 x 1 + φ x 2 x 2 + φ x 3 x 3 = 0.
The corresponding Monge-Ampère structure is (Ω, ω), which in local coordinates x i , ξ i is

Ω = dx 1 ∧ dξ 1 + dx 2 ∧ dξ 2 + dx 3 ∧ dξ 3 and ω = dξ 1 ∧ dx 2 ∧ dx 3 + dx 1 ∧ dξ 2 ∧ dx 3 + dx 1 ∧ dx 2 ∧ dξ 3 .
Let G = R act on T * R 3 by translation on the third coordinate:

λ τ (x 1 , x 2 , x 3 , ξ 1 , ξ 2 , ξ 3 ) = (x 1 , x 2 , x 3 + τ, ξ 1 , ξ 2 , ξ 3 ).
This action is trivially Hamiltonian with moment map µ(x, ξ) = -ξ 3 and infinitesimal generator

X = λ * d dτ = ∂ ∂x 3 .
For c ∈ R, the reduced space M c is canonically R 4 and the reduced Monge-Ampère structure is (Ω c , ω c ) with

Ω c = dx 1 ∧ dξ 1 + dx 2 ∧ dξ 2 and ω c = ι ∂ ∂x 3 ω = dξ 1 ∧ dx 2 + dx 1 ∧ dξ 2 .
Then we obtain the Laplace equation in two dimensions as the reduced equation.

5 The geometry of Burgers'-type vortices in three dimensions

3D-2D reduction in fluid dynamics: introduction

It is well known in fluid dynamics that certain solutions of the incompressible Euler (and Navier-Stokes) equations in three dimensions can be mapped to solutions of the incompressible equations in two dimensions using the so-called Lundgren transformation. The class of solutions for which this mapping is possible is characterised by the linearity of the third component of velocity in the third spatial coordinate, with the other two components of the velocity being independent of the third spatial coordinate. The canonical Burgers' vortex is a member of this family of solutions, and in this section we show how the formulation described in Section 2.2 can be applied to illuminate the geometry of these solutions. [START_REF] Joyce | U (1)-invariant special lagrangian 3-folds: I. Nonsingular solutions[END_REF] used the reduction principle to construct examples of U (1)invariant special lagrangian manifolds in C 3 after reducing the 6-dimensional "complex" Calabi-Yau structure. The 4-dimensional geometry can be seen as a symplectic reduction of a 6-dimensional "real Calabi-Yau" geometry, which we describe now.

In this section we are concerned with solutions of the incompressible Euler equations of Burgers'-type. That is, we consider flows of the form

u i = (-γ(t)x 1 /2 -ψ x 2 , -γ(t)x 2 /2 + ψ x 1 , γ(t)x 3 ) T , ( 12
)
which satisfy incompressibility u ii = 0, and ψ, a stream function, is independent of x 3 . Here γ(t) is the strain, which is a function of time alone. We shall consider the analogue of (4) for this class of flows in three dimensions and, to this end, it is convenient to introduce a new stream function

Ψ(x 1 , x 2 , x 3 , t) as follows Ψ = ψ(x 1 , x 2 , t) - 3 8 γ(t) 2 x 2 3 . ( 13
)

Extended real Calabi-Yau geometry

We now extend the 4-dimensional geometry associated with (4) to a 6dimensional geometry. Using ( 12) and ( 13), (3) becomes

Ψ x 1 x 1 Ψ x 2 x 2 -Ψ 2 x 1 x 2 + Ψ x 3 x 3 = ∆p 2 (14)
where the Laplace operator on the right hand side is in three dimensions, but note (from ( 13)) that ∆p is independent of x 3 (cf. [START_REF] Ohkitani | Numerical study of singularity formation in a class of Euler and Navier-Stokes flows[END_REF].

Denote by (Ω, ̟) the corresponding Monge-Ampère structure on T * R 3 , with Ω the canonical symplectic form

Ω = dx 1 ∧ dξ 1 + dx 2 ∧ dξ 2 + dx 3 ∧ dξ 3 ,
where ξ i = ∇ i Ψ and, following Roulstone et al. (2009), ̟ is the effective 3-form

̟ = dξ 1 ∧ dξ 2 ∧ dx 3 -adx 1 ∧ dx 2 ∧ dx 3 + dx 1 ∧ dx 2 ∧ dξ 3 , ( 15
)
with a(x 1 , x 2 ) = ∆p/2. The corresponding "real Calabi-Yau" structure on

T * R 6 is (g ̟ , K ̟ , Ω, ̟ + ̟, ̟ -̟) with i) g ̟ = 2adx 3 ⊗dx 3 +dx 1 ⊗dξ 1 +dx 2 ⊗dξ 2 -dx 3 ⊗dξ 3 (ε(g ̟ ) = (3, 3)) ii) K ̟ =         -1 0 0 0 0 0 0 -1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 2a 0 0 -1         and thus K 2 ̟ = 1 iii) ̟ + ̟ = 2dξ 1 ∧ dξ 2 ∧ dx 3 , ̟ -̟ = 2dx 1 ∧ dx 2 ∧ (dξ 3 -adx 3 ),
and the Hitchin pfaffian is λ(̟) = 1.

PROPOSITION 4.

1. This "real Calabi-Yau" structure is integrable: locally there exists a potential F and coordinates (x, ξ) in which

g ̟ = 3 j=1 3 k=1 ∂ 2 F ∂x j ∂ξ k dx j • dξ k and Ω = 3 j=1 3 k=1 ∂ 2 F ∂x j ∂ξ k dx j ∧ dξ k 2. The pseudo-metric g ̟ is Ricci-flat. 3. It is flat if and only if ∆p = α 1 x 1 + α 2 x 2 + β.
Proof. Since d̟ = d ̟ = 0, we obtain immediately integrability. Direct computations show that the pseudo metric g ̟ has a curvature tensor which is null if and only if a(x 1 , x 2 ) = ∆p/2 is "affine", whereas Ricci-curvature always vanishes.

REMARK 2. From Proposition 3, we deduce that equation ( 14) is equivalent to hess ψ = 1 if and only if ∆p = α 1 x 1 + α 2 x 2 + β, where α 1 , α 2 and β are constants.

The symplectic reduction

Let G = R acting on T * R 3 by translation on the third coordinate:

λ τ (x 1 , x 2 , x 3 , ξ 1 , ξ 2 , ξ 3 ) = (x 1 , x 2 , x 3 + τ, ξ 1 , ξ 2 , ξ 3 )
This action is trivially hamiltonian with moment map µ(x, ξ) = -ξ 3 and infinitesimal generator

X = λ * d dτ = ∂ ∂x 3 .
For c ∈ R, the reduced space M c is the quotient space

M c = µ -1 (c)/R = (x 1 , x 2 , x 3 , ξ 1 , ξ 2 , -c), (x 1 , x 2 , x 3 , ξ 1 , ξ 2 ) ∈ R 5 /x 3 ∼ x 3 + τ ∼ = (x 1 , x 2 , ξ 1 , ξ 2 , -c), (x 1 , x 2 , ξ 1 , ξ 2 ) ∈ R 4
The vector

Y = K ̟ X = ∂ ∂x 3 + 2a ∂ ∂ξ 3
satisfies Ω(X, Y ) = 2a with a(x 1 , x 2 ) = ∆p/2. Therefore, when ∆p = 0, one can define a 4-dimensional distribution D which is Ω-orthogonal to RX ⊕ RY :

T m R 6 = D m ⊕ ⊥ (RX m ⊕ RY m ) with m = (x, ξ) ∈ R 6 = T * R 3 ,
and Ω and ̟ decompose as follows:

       Ω = Ω c - 1 2a ι X Ω ∧ ι Y Ω ̟ = ̟ 1 ∧ ι X Ω + ̟ 2 ∧ ι Y Ω.
Here, Ω c , ̟ 1 and ̟ 2 in Ω 2 (D) are defined by

                   c = dx 1 ∧ dξ 1 + dx 2 ∧ dξ 2 ̟ 1 = - 1 2a (dξ 1 ∧ dξ 2 -adx 1 ∧ dx 2 ), ̟ 2 = 1 2a (dξ 1 ∧ dξ 2 + adx 1 ∧ dx 2 ).
Using the natural isomorphism T [m] M c ∼ = D m , we get a triple (Ω c , ̟ 1 , ̟ 2 ) on M c which is, up to a re-normalization, the hypersymplectic triple on T * R 4 associated with the 2D-Euler equation (cf. §3.2).

6 Reduction of incompressible 3D-Euler equations 6.1 Introduction: Monge-Ampère geometry of a non-Monge-Ampère equation

We now look at more general solutions to the incompressible equations in three dimensions. The Burgers' vortex is a special solution in that we may define a stream function ( 13) and use the theory described in §5.2 and Banos (2002). However, for general incompressible flows in three dimensions a single scalar stream function is not available and we have to work with a vector potential. Therefore we should stress that there is no underlying MA equation. We now proceed to show how a geometry, very similar to the one described earlier, can be recovered for three-dimensional flows with symmetry. Our results extend those of Roulstone et al. (2009b).

We introduce the following 3-forms ω and θ on the phase space T * R 3 , with local coordinates (x i , u i ) (so here we treat the u i as a velocity, and these coordinates replace the ξ i ; more rigorously, we have identified T R 3 and T * R 3 using the euclidean metric):

ω = a dx 1 ∧dx 2 ∧dx 3 -du 1 ∧du 2 ∧dx 3 -du 1 ∧dx 2 ∧du 3 -dx 1 ∧du 2 ∧du 3 (16) with a = ∆p 2 and θ = du 1 ∧ dx 2 ∧ dx 3 + dx 1 ∧ du 2 ∧ dx 3 + dx 1 ∧ dx 2 ∧ du 3 . ( 17
)
Considering u = (u 1 , u 2 , u 3 ) as a 1-form u = u 1 dx 1 +u 2 dx 2 +u 3 dx 3 on R 3 , it is straightforward to check that u is a solution of -∆p = u ij u ji , u ii = 0 if and only if the graph L u = {(x, u)} ⊂ T * R 3 is bilagrangian with respect to ω and θ; that is

ω| Lu = 0 and θ| Lu = 0. ( 18
)
We observe that a solution here is not necessarily lagrangian with respect to the underlying symplectic form Ω, which is compatible with the fact that our equation is no longer of Monge-Ampère form.

Hitchin tensors and Lychagin-Roubtsov metrics

Note that

ω ∧ θ = 3 vol with vol = dx 1 ∧ dx 2 ∧ dx 3 ∧ du 1 ∧ du 2 ∧ du 3 .
We can therefore define the following analogues of Hitchin tensors K ω and K θ :

< α, K ω (X) >= ι X (ω) ∧ ω ∧ α vol , < α, K θ (X) >= ι X (θ) ∧ θ ∧ α vol
for any 1-form α and any vector field X.

In the 3 × 3 decomposition (x, u), these tensors are

K ω = 2 0 -Id a Id 0 , K θ = 2 0 0 Id 0 . ( 19
)
We have the relations

K 2 ω = -4a Id K 2 θ = 0 K ω K θ + K θ K ω = -4Id [K ω , K θ ] = 4 -Id 0 0 Id (20)
Moreover, using the symplectic form Ω = dx 1 ∧du 1 +dx 2 ∧du 2 +dx 3 ∧du 3 , one can also define the analogues of Lychagin-Roubtsov metrics g ω = Ω(K ω •, •) and g θ = Ω(K θ •, •):

g ω = 2a Id 0 0 2Id , g θ = 2Id 0 0 0 . ( 21
)

The symplectic reduction

Let us consider an hamiltonian action λ of a 1-dimensional Lie group G on our 6-dimensional phase space with moment map µ. Assume moreover that 1. the action λ preserves also ω and θ:

∀g ∈ G, λ * g (ω) = ω and λ * g (θ) = θ 2. the infinitesimal generator X satisfies g ω (X, X) = 0 and g θ (X, X) = 0 NB. We impose that the action preserves the symplectic form Ω to obtain a reduced space, but as we already mentioned, this symplectic form is additional data in this context. As we have seen, there exists a pair (ω c , θ c ) of 2-forms on M c such that

π * (ω c ) = ι X ω , π * (θ c ) = ι X θ (22) REMARK 3. if L c is a 2-dimensional bilagrangian submanifold of M c (that is ω c | Lc = 0 and θ c | Lc = 0) then L = π -1 (L c
) is "bilagrangian" with respect to ω and θ.

Hence, there is a correspondence between bilagrangian surfaces in the reduced space M c and G-invariant solutions of (18). REMARK 4. One can check that the largest group preserving Ω, ω and θ is SO(3, R) acting on M by

A • (x, u) = (A • x, A -1 • u).

Action by translation

Let R acting on T * R 3 by translation on (x 3 , u 3 ):

λ a (x 1 , x 2 , x 3 , u 1 , u 2 , u 3 ) = (x 1 , x 2 , x 3 + τ, u 1 , u 2 , u 3 + γτ ) , γ ∈ R The infinitesimal generator is X = λ * d dτ = ∂ ∂x 3 + γ ∂ ∂u 3
and the moment map is

µ(x, u) = γ x 3 -u 3 .
We observe that µ = constant yields the linearity of u 3 in x 3 , as defined in (12). For c ∈ R, the reduced space M c is trivially T * R 2 :

M c = {(x 1 , x 2 , x 3 , u 1 , u 2 , γx 3 -c} /x 3 ∼ x 3 + t = {(x 1 , x 2 , 0, u 1 , u 2 , -c)} and the pair (ω c , θ c ) = (ι X ω, ι X θ) is ω c = a dx 1 ∧ dx 2 -du 1 ∧ du 2 -γdu 1 ∧ dx 2 -γdx 1 ∧ du 2 θ c = du 1 ∧ dx 2 + dx 1 ∧ du 2 + γdx 1 ∧ dx 2
Considering the following change of variables

X 1 = x 1 , U 1 = γ 2 x 2 + u 2 , X 2 = x 2 , U 2 = - γ 2 x 1 -u 1 , we obtain θ c = dX 1 ∧ dU 1 + dX 2 ∧ dU 2 (23)
and

ω c = ω 0 - γ 2 θ c (24)
with

ω 0 = a + 3 4 γ 2 dX 1 ∧ dX 2 -dU 1 ∧ dU 2 . ( 25
)
Hence we summarize our result as follows:

PROPOSITION 5. If ψ(x 1 , x 2 , t) is solution of the Monge-Ampère equation in two dimensions

ψ x 1 x 1 ψ x 2 x 2 -ψ 2 x 1 x 2 = ∆p(x 1 , x 2 , t) 2 + 3 4 γ 2 (t). ( 26
)
then the velocity u(x 1 , x 2 , x 3 , t) defined by

u 1 = - γ(t) 2 x 1 -ψ x 2 , u 2 = - γ(t) 2 x 2 + ψ x 1 , u 3 = γ(t)x 3 -c(t),
is solution of -∆p = u ij u ji and u ii = 0 in three dimensions.

Proof. Time t is a parameter here, hence c = c(t) and γ = γ(t) are constant.

If ψ(x 1 , x 2 , t) is solution of (26) then

ω 0 | ψ = θ c | L ψ = 0, with L ψ = {(X 1 , X 2 , ψ X 1 , ψ X 2 )}.
From (24), we deduce that ω c | Lψ = θ c | L ψ = 0, and therefore ω| L = θ| L = 0 with L = π -1 (L c ) = (x 1 , x 2 , x 3 , -γ 2 x 1 -ψ x 2 , -γ 2 x 1 + ψ x 1 , γx 3 -c) .

Summary

The Poisson equation for the pressure in the incompressible Euler and Navier-Stokes equations can be written in the form

∆p = ζ 2 2 -TrS 2 , ( 27
)
where ζ is the vorticity and S is the rate-of-strain matrix. Equation ( 27) illuminates the relationship between the sign of the Laplacian of the pressure and the balance between vorticity and strain in the flow. [START_REF] Gibbon | The three-dimensional Euler equations: Where do we stand?[END_REF] has conjectured that much might be learned from studying the geometric properties of this equation, which locally holds the key to the formation of vortical structures -ubiquitous features of both turbulent flows and largescale atmosphere/ocean dynamics.

In this paper we have studied ( 27) through Monge-Ampère structures and their associated geometries. Burgers' vortex, which has been described as the sinew of turbulence, is shown to arise naturally from a symmetry reduction of a Monge-Ampère structure (Section 5). This Monge-Ampère structure is itself derived from a symmetry reduction of the equations for incompressible flow in three dimensions ( §6.3), and mapping to a solution of 2d incompressible flow has been presented (Proposition 5). This is surely associated with the well-known Lundgren transformation.

In contrast to earlier works by the same authors, we have shown explicity how the geometries associated with both signs of the Laplacian of the pressure can be described by almost-complex and almost-product structures. This points to a possible role for generalised geometry (Banos 2007), and this is a topic for future research.

Acknowledgements

The paper was started when I.R and V.N.R. were participants of the Isaac Newton Institute for Mathematical Science Programme "Mathematics for the Fluid Earth" (21st October to 20th December 2013). They are grateful to the organisers and participants of the Programme for the invitation and useful discussions, and to the administration and the staff of the Newton Institute for hospitality, stimulating atmosphere and support during their stay in Cambridge.

The final results of the paper were obtained while all three authors were participants of the Clay Mathematics Institute (CMI) meeting "Geometry and Fluids" at Oxford University (7th-11th April 2014). They are thankful to the organisers and the CMI for the invitation and possibility to present their result during the Meeting. The discussions with M. Gualtieri, N.J. Hitchin, J. McOrist and M. Wolf were very inspiring and useful. The work of V.N.R. were (partly) supported by the grants RFBR-12-01-00525-a and RFBR 15-01-05990.

Banos, B 2002 Nondegenerate Monge-Ampère structures in dimension 6, Letters in Mathematical Physics, 62, 1-15 Banos, B. 2007 Monge-Ampere equations and generalized complex geometry. The two-dimensional case. Journal of Geometry and Physics, 57, 841-853