Anomalous wave structure in magnetized materials described by non-convex equations of state

We analyze the anomalous wave structure appearing in flow dynamics under the influence of magnetic field in materials described by non-ideal equations of state. We consider the system of magnetohydrodynamics equations closed by a general equation of state (EOS) and propose a complete spectral decomposition of the fluxes that allows us to derive an expression of the nonlinearity factor as the mathematical tool to determine the nature of the wave phenomena. We prove that the possible formation of non-classical wave structure is determined by both the thermodynamic properties of the material and the magnetic field as well as its possible rotation. We demonstrate that phase transitions induced by material properties do not necessarily imply the loss of genuine nonlinearity of the wavefields as is the case in classical hydrodynamics. The analytical expression of the nonlinearity factor allows us to determine the specific amount of magnetic field necessary to prevent formation of complex structure induced by phase transition in the material. We illustrate our analytical approach by considering two non-convex EOS that exhibit phase transitions and anomalous behavior in the evolution. We present numerical experiments validating the analysis performed through a set of one-dimensional Riemann problems. In the examples we show how to determine the appropriate amount of magnetic field in the initial conditions of the Riemann problem to transform a thermodynamic composite wave into a simple nonlinear wave.


I. INTRODUCTION
Simulation codes to solve physical processes in fluid dynamics are based on nonlinear hyperbolic systems of conservation laws as Euler and magnetohydrodynamics (MHD) equations.The fluid equations describe conservation of mass, momentum, energy and magnetic field in the latter case and are closed with the constitutive relations represented by an equation of state (EOS) defining the equilibrium thermodynamic properties of the considered material.Hydrodynamic models are considered to simulate applications involving the response of solids at high pressure where the dynamic properties of the material are characterized by an EOS [1][2][3] .
The ideal EOS framework has been traditionally used in theoretical and numerical studies because of its simplicity and convenience in simulation codes [4][5][6][7][8][9][10] .Nevertheless, the ideal EOS does not represent laboratory environments as well as non-ideal equations of state do.Nonideal equations of state provide more realistic descriptions of the materials and the models representing them are more difficult to analyze and use in simulation codes [11][12][13][14][15][16][17][18][19][20] .
The study of the wave dynamics in magnetohydrodynamics is essential to understand physical processes like the ones performed in laboratory experiments associated to high density physics, material processing and astrophysical phenomena.Magnetic acceleration is the basis of magnetic pinch facilities where large magnetic pressure is used to compress material samples [21][22][23][24][25] .Magnetic field is also commonly used to process materials in industry.
Metallurgical MHD include, among other applications, magnetic stirring, magnetic damping and magnetic destabilization of liquid-liquid interfaces 2,3,26 .Magnetic outflows, accretion disks, the evolution of stellar collapse supernovae are examples of astrophysical scenarios where states of matter at extreme high densities and strong magnetic field are involved in the evolution 18,25,27 .The study of the shock wave phenomena and material response at high pressure and strong magnetic field is commonly addressed by means of fluid models of compressible flows closed with non-ideal EOSs.The understanding of these models is a challenge and a research area of interest with important applications in industry, geophysics and astrophysics among other areas [1][2][3][15][16][17][18][21][22][23][24][25][26][27] .
The analysis of the wave structure in the evolution of classical hydrodynamics described by Euler equations closed with a non-ideal EOS has been widely studied in the literature 1,13,14,[28][29][30][31][32] .It is well known that the structure and dynamics of waves are deter-mined by the properties of the material through a thermodynamic magnitude, the funda- where V is the specific volume (V ≡ 1 ρ , ρ the density), S the specific entropy and P the pressure.G measures the convexity of isentropes in the pressure-volume plane 32,33 .In this context it is common to refer as convex EOS to those characterizing materials with strictly positive fundamental derivative and non-convex EOS to those with negative one at some state in the thermodynamic plane.
The acoustic waves in classical hydrodynamics are genuinely nonlinear when the fundamental derivative is positive 12,30,33 .This implies that the only possible waves allowed in the solution are linear waves (contacts), compression shock waves and expansion fans.In hydrodynamics the convexity of the EOS determines the convex dynamics of the system.
Most materials satisfy a strictly positive fundamental derivative.In other cases, where the material undergoes phase transitions or the fluid is near the critical point region, non-convex dynamics arise.Then isentropes lose convexity implying a loss of genuine nonlinearity of the acoustic waves and a negative value of the fundamental derivative.The acoustic wave family becomes more complex and the solution may generate anomalous structures like shock waves attached to an expansion fan, double shocks separated by a rarefaction fan and double rarefaction fans separated by a shock wave 29,32,34,35 .The emerging of composite wave structures (typical in non-convex dynamics) where shocks break expansion fans might not be a desirable effect in some applications.For example, in material processing, composite waves represent the loss of continuity in the medium and in consequence inhomogeneities, small cavities and porosity surfaces appear.
In the work of Lax 36,37 the concepts of genuine and non-genuine nonlinearity of the acoustic waves are generalized to hyperbolic systems of conservation laws.Decoupling the system of hyperbolic conservation laws into characteristic wavefields these are classified through an expression named nonlinearity factor that measures the monotonicity of the characteristic wave speeds along wave curves.Positive nonlinearity factor implies genuine nonlinearity of the nonlinear wavefields.In Euler dynamics it is proved that the nonlinearity factor is proportional to the fundamental derivative and therefore change of sign of this thermodynamic magnitude determines the same for the nonlinearity factor 12,14 .This relation is not longer valid when magnetic field is involved in the physical system under study.
The MHD system of equations is a system of hyperbolic conservation laws that results from coupling Euler equations of hydrodynamics and Maxwell equations 38,39 .One of the main properties of the MHD system is that the rotation of the magnetic field may induce non-genuine nonlinearity in some characteristic wavefields.It has been analytically proved for the case of the MHD system closed with convex EOSs that the nonlinearity factor vanishes in isolated points depending on the values of the magnetic field 5,8,40 .MHD systems closed with convex EOSs may exhibit anomalous wave structure because of the rotation of the magnetic field.Let us point out that, in this case, the non-convex dynamics of the system arises because of the orientation of the magnetic field and not because of the thermodynamic properties inherited from the EOS.
In this work we focus on the characterization of the convexity of the MHD system closed with non-convex EOSs.We explore the shock wave structure and discern the influence of thermodynamic properties of materials and magnetic field on the formation of complex wave structures.
In our analysis we consider the MHD system of equations governed by a general EOS.
We provide an analytical expression of the nonlinearity factor for the nonlinear wavefields and prove that the loss of genuine nonlinearity of the wavefields depends on both: the thermodynamic properties of the material and the magnetic field.We demonstrate that phase transitions might not imply the loss of genuine nonlinearity.We also show that complex wave structure induced by the loss of thermodynamic convexity can be neutralized by prescribing a specific amount of magnetic field.Indeed, we prove that the non-convex thermodynamic behavior induced by a non-convex EOS in a hydrodynamics system can be reverted into convex dynamics by introducing an appropriate intensity of magnetic field in the system.We illustrate our analytical approach considering the MHD system and two non-convex EOS that exhibit anomalous behavior.We analyze a van der Waals model that exhibits phase transition 28,30 and a Mie-Grüneisen type EOS 1,41,42 which is considered as representative of real liquids and solids for which the fundamental derivative changes sign.In both cases we analyze the structure and behavior of wave curves determined by the nonlinearity factor.We present numerical experiments validating the analytical approach through a set of one-dimensional Riemann problems.Firstly we show examples exhibiting non-convex dynamics in the absence of magnetic field and secondly we display their MHD counterparts where the magnetic field has been chosen to revert the non-convex dynamics into convex and therefore eliminating complex wave structure.
The characterization of the wave structure of the MHD system closed with a general EOS presented in this paper provides a better understanding of the influential role of the magnetic field in the dynamics of these complex flows.Our study might help to improve characteristic-based code capabilities to simulate processes relevant in high energy-density laboratory astrophysics where high power lasers (not well approximated by the ideal gas EOS) are used to study jets and bow shocks.The proposed approach can also be of interest in industrial applications of material processing when the properties of the material are described through a non-convex EOS.Our characterization provides a mathematical tool that might guide to predict the intensity of magnetic drive to increase pressure in a system and therefore avoid shocks and other unwanted wave structures (loss of homogeneity in a sample) intrinsic to non-convex dynamics.
The paper is organized as follows.In Sec.II we review general concepts on nonlinear hyperbolic systems of conservation laws.Sec.III is devoted to the analysis of the wave structure of MHD systems closed by a general equation of state where we derive a simple expression of the nonlinearity factor in terms of the fundamental derivative and the magnetic field.Sec.IV includes numerical examples of anomalous wave structure in magnetized materials described by a van der Waals EOS and a Mie-Grüneisen EOS.In Sec.V we draw our conclusions.

II. NONLINEAR HYPERBOLIC SYSTEMS OF CONSERVATION LAWS
In this Section we review general concepts on the theory of nonlinear hyperbolic systems of conservation laws necessary to develop our study on the MHD wave structure under a general EOS.
Mathematical models in fluid dynamics are represented by hyperbolic systems of conservation laws of the form ∂u ∂t where u is the vector of m conserved variables and F(u) is the corresponding one of m fluxes.
The system is hyperbolic if the Jacobians of the fluxes in any direction are diagonalizable matrices with real eigenvalues and a complete set of eigenvectors in each neighborhood of the solution 36,37,39,43 .
In one dimension the eigenvalues of the Jacobian of the flux f(u), f ′ (u), are denoted as λ 1 (u), • • • , λ m (u) counting each one as many times as its multiplicity.The complete system of right and left eigenvectors are defined as The diagonalization of the Jacobians evaluated at same state decouples the linearized hyperbolic system in m scalar conservation laws defining the so-called local characteristic fields and their corresponding local characteristic fluxes 36,37 .The system is closed by an EOS defining the thermodynamic properties of the material.
The eigenvalues of the Jacobian represent the characteristic speeds of the characteristic fluxes analogous to the first derivative of the flux of a scalar conservation law.The nonlinearity of the corresponding characteristic fields is determined by the scalar quantity called nonlinearity factor where λ k (u) and r k (u) are the kth characteristic field eigenvalue and eigenvector of the Jacobian f ′ (u) respectively.This magnitude corresponds to the second derivative of the scalar flux in a scalar conservation law 36,37,43 .
Convex hyperbolicity occurs when characteristic fields are either linear or genuinely nonlinear.In convex dynamics the wave structure consists only of contacts, shocks and rarefaction waves.
It is well known in classical hydrodynamics represented by Euler equations that the nonlinearity factor of nonlinear wavefields can be written in the equivalent form where G is the fundamental derivative, Eq. ( 1), is the acoustic sound speed, P the pressure, ρ the density and S the specific entropy.
In Euler hydrodynamics the qualitative character of the solution of the conservation laws depends on the thermodynamic properties of the material.From Eq. ( 5) it is clear that in the case of hydrodynamics a change of sign of the fundamental derivative implies a change of sign of the nonlinearity factor and therefore non-convex hyperbolicity and the consequent anomalous wave structure.
In the following Section we analyze the wave structure of the MHD system of hyperbolic conservation laws closed by a general EOS.

III. ANOMALOUS WAVE STRUCTURE IN NON-IDEAL MHD EQUATIONS
The thermodynamic properties of a material have a decisive effect on the nature of the shock wave phenomena that appears in it.In classical hydrodynamics the nature of the EOS determines the behavior of the wave structure.In a region of negative value of the fundamental derivative the dynamics might exhibit non-classical wave phenomena.The case of MHD shock wave dynamics is more involved.Apart from the influence of the specific material in the possible formation of anomalous wave structure, the presence of the magnetic field adds complexity to the nonlinear wave dynamics.It has been proved in ideal MHD that hyperbolic singularities appear when the magnetic field rotates causing the MHD eigensystem to induce non-genuine nonlinearity in some of the nonlinear wavefields 5,6,8,40,44 .In this Section we perform a complete analytical study of the MHD wave structure under a general EOS.We present a full spectral decomposition of the Jacobians of the fluxes and derive an expression of the nonlinearity factor for the nonlinear wavefields.We then analyze the dependency of the non-genuinely nonlinear behavior of the shock wave phenomena in terms of the fundamental derivative and the magnetic field.
The MHD system of equations for real gases can be expressed as where ρ, v = (u, v, w), B = (B x , B y , B z ) and E denote the mass density, the velocity field, the magnetic field and the total energy respectively, being u = (ρ, ρu, ρv, ρw, B x , B y , B z , E) the vector of conserved variables.The energy is expressed as where q 2 and B 2 are the squares of the magnitudes of the velocity field and the magnetic field respectively and ε the specific internal energy.P = P (ρ, ε) is the hydrodynamic pressure defined through a real gas EOS.The system is completed with a condition on the magnetic field, the divergence free constraint where P ρ represents the partial derivative of P with respect to ρ and P ε the partial derivative of P with respect to ε.The specific total enthalpy is defined as where P * = P + 1 2 B 2 is the total pressure.We propose a complete spectral decomposition of the MHD system (7) for the onedimensional case under a general equation of state.In the one-dimensional case the divergence free constraint (9) is reduced to the case where B x is constant.The constancy of B x implies that this quantity is considered an initial condition acting as a parameter.Thus the evolution equation for the x-component of the magnetic field is not solved.The system of equations provides an eigensystem composed of seven waves: one entropy wave, two Alfvén waves and four magnetoacoustic waves.Their corresponding eigenvalues λ, left eigenvectors l and right eigenvectors r are defined as follows.
The spectral decomposition reflects the dependency of the wave structure on the magnetic field B and on the thermodynamics induced by the EOS through magnitudes a 2 and the Grüneisen coefficient 33 , The Alfven wave speed is denoted by c A = |b x | and the fast and slow wave speeds are given by The eight characteristic wave speeds associated to the system (7) are: Depending on the direction and magnitude of the magnetic field, these wave speeds may coincide making the MHD equations a non-strictly hyperbolic system of conservation laws.
Thus, the set of eigenvectors of the system can be singular at the points where the eigenvalues are degenerate.From the original approach by Jeffrey and Taniuti 38 the eigensystem of the MHD equations closed with an ideal EOS has been extensively studied with the purpose of proposing different normalizations avoiding these singularities [5][6][7][8]40 .
In this section we propose a set of eigenvectors which are an extension to the general EOS case of the ones presented for ideal MHD in 8 .The latter approach exhibits an scaled version of the complete system of eigenvectors proposed by Brio and Wu 5,40 and therefore are well defined providing a proper normalization to avoid singularities at the points where the eigenvalues are degenerate.Our scaling also guarantees continuity of the eigenvectors with respect to the conserved variables in the neighborhood of singular points 8 .
We present a unified expression for the eigenvectors of the nonlinear characteristic wavefields.We define sgn(t) = 1 for t ≥ 0 and sgn(t) = −1 otherwise and set β y and β z values from

; otherwise
We define τ = Γ a 2 to express the eigenvector associated to linear wavefield λ 4 as The eigenvectors associated to the Alfven wavefields with eigenvalues λ 2 and λ 6 are expressed as proposed in Ref. 38 as The proposed complete system of eigenvectors is continuous with respect to the conserved variables similarly as defended in Ref. 8 for ideal MHD.
The extension of this seven wave eigensystem to the multidimensional case of the MHD system under a general equation of state such that it satisfies the eight wave approach by Powell et.al 7 is obtained in a simple manner.Similarly as explained in 6 , the extended left eigenvectors l 1,8 , l 2,7 , l 3,6 and l 4 are obtained, respectively, by inserting into l 1,7 , l 2,6 , l 3,5 and l 4 of the seven wave eigensystem and a 5th component whose value is −ΓαB x .Likewise the right eigenvectors r 1,8 , r 2,7 ,r 3,6 and r 4 are obtained, respectively, by adding a 5th component with a null value to r 1,7 , r 2,6 ,r 3,5 and r 4 of the seven wave eigensystem.In addition, l 5 and r 5 are given by r 5 = (0, 0, 0, 0, 1, 0, 0, B x ) T , l 5 = (0, 0, 0, 0, 1, 0, 0, 0, 0) The system of eigenvectors for the Jacobian of the flux in the y− direction can be obtained by interchanging B x by B y , u by v, the second component by the third and the fifth by the sixth.
From the expression of the nonlinear right eigenvectors (13) we can derive the nonlinearity factor for the nonlinear wavefields and study the wave structure of the MHD system under a general EOS.
Let us first provide a thermodynamic relation that will be useful in the derivation of the nonlinearity factor of the nonlinear wavefields.The following identity links the component of the gradient of the specific internal energy in the direction of any nonlinear right eigenvector with a thermodynamic relation that appears in the calculation of derivatives at constant specific entropy with respect to density of thermodynamic magnitudes.
Lemma 1 Let r c (u) represent the right nonlinear eigenvector of the Jacobian of the MHD system and ε the specific internal energy.The following thermodynamic relation is satisfied Proof.We consider equation ( 8) to obtain an expression for the specific internal energy We calculate the gradient of the specific internal energy with respect to the conserved variables: Using expression (13) and considering to simplify notation, the scalar product becomes Then, from expressions (10) and (8), substituting E and using identities ᾱa sgn(c 2 − a 2 ) = Theorem 1 (Formula of the nonlinearity factor for general MHD) The nonlinearity factor for the nonlinear wavefields of the MHD system is expressed as where λ c (u) = u + c with c = ±c f,s is the eigenvalue corresponding to a fast (f ) or slow (s) characteristic field, r c (u) the associated right eigenvector, G is the fundamental derivative and α and ᾱ are defined following the normalization of the nonlinear wavefields.
In the case of no rotation of the magnetic field, expression ( 17) is simplified to where s are the eigenvalues corresponding to a fast or slow characteristic wavefield, r ± c f (u) and r ± cs (u) are the associated right eigenvectors and α f and α s are defined following the normalization of the nonlinear wavefields.
Proof.For the purpose of computing the scalar product ∇λ c (u) • r c (u) we first calculate the expression of the vector ∇λ c (u) where λ c = u + c is an eigenvalue associated to a nonlinear characteristic wavefield and c represents the fast or slow wave speeds, ±c f or ±c s , respectively.The gradient of the eigenvalue is calculated as In order to simplify the calculation of the partial derivatives of c with respect to the conserved variables u j , j = 1, • • • , 7 we use an equivalent expression of ∂c ∂u j .We consider the identity ∂c 2 ∂u j = 2c ∂c ∂u j and manipulate expression (12) as follows.From (12) it is stated that c satisfies Taking partial derivatives of ( 21) with respect to the conserved variables Since c is associated to a nonlinear wavefield, we have that 2c We can then isolate ∂c 2 ∂u j from (22) and we obtain for j = 1, • • • , 7, Taking into account that we compute the gradient of the eigenvalue λ c (u) plugging ( 23) into (20) and applying the chain rule as The nonlinearity factor is obtained through the scalar product between ( 24) and ( 13) by Then, we first substitute the value of ∇ u ε • r c (u) accordingly with Lemma 1 and second we From the equivalent expression of the fundamental derivative and the facts and α 2 + ᾱ2 = 1 we can rewrite (26) as Remark 1 An alternative dimensionless expression of the nonlinearity factor (17) could be derived if the general expression of the right eigenvectors (13) was multiplied by ρ c (the left eigenvectors should be consequently multiplied by c ρ ).However, with this normalization the eigensystem would not be complete since the eigenvectors would present singularities when c = c s degenerates to zero at singular points.
Remark 2 Let us remark that the expressions of the nonlinearity factors, Eqs. ( 18) and (19), differ from the corresponding one for the hydrodynamic case, Eq. ( 5), on terms that depend on the magnetic field.
The following corollaries describe particular cases of special relevance.
Corollary 1 Let B be a magnetic field with B x = 0 and B 2 y + B 2 z = 0 and no rotation and let the fundamental derivative G < 0. The following cases apply 1.If − 3 2 < G < 0 at some point then there exists a δ > 0 such that if ρa 2 − δ ≤ B 2 ≤ ρa 2 + δ then the nonlinearity factors are both positive and therefore the wave structure is classical.
2. If G < − 3 2 at some point then at least one of the nonlinear wavefields induces a negative nonlinearity factor and therefore there might be anomalous wave structure.

Proof:
In order to demonstrate both statements we first shall prove that both nonlinearity factors are non-negative if and only if Indeed, by hypothesis 0 < α 2 s < 1 and 0 < α 2 f < 1.The non-negativity of nonlinearity factors is satisfied if In order to prove the first assertion we use the explicit expressions obtained from the definitions of α 2 s and α 2 f .Then, using the formulas of the fast and slow wave speeds we have that From this expression it is straightforward to see that We set ǫ = 1 − 2 3 |G| > 0. From the definition of limit in (28) we can find a δ > 0 such that if . This implies that Thus, from (27) it follows that both nonlinearity factors are positive.
The other case, ρa 2 − δ ≤ B 2 ≤ ρa 2 , follows using the same argument over the ratio Second assertion of the Corollary follows from the fact that min and therefore if G < − 3 2 at some point then (27) never holds and therefore at least one of the nonlinearity factors is negative.
The following corollary describes the cases where nonlinear wavefields degenerate to linear wavefields depending on the magnitude and direction of the magnetic field.

Corollary 2
The following are possible degenerate cases 1.If B ≡ 0 (i.e.(B x , B y , B z ) = (0, 0, 0)), then α 2 f = 1 and α 2 s = 0 and system (7) reduces to Euler equations and therefore The only nonlinear wavefields are fast and the change of sign of G implies nonconvex hyperbolicity of the fast wavefield.
The only nonlinear wavefields are the slow ones.Fast nonlinear wavefields degenerate to linear Alfven waves.Change of sign of G implies non-convex hyperbolicity of the slow wavefield.
Nonlinear wavefields degenerate to linear.

If
The slow wavefield degenerates to a linear one and the only nonlinear wavefields are fast.
We summarize previous results in Table I.
2 is positive and therefore the wave structure is genuinely nonlinear and anomalous wave structure is not given.
Remark 4 Corollary 2 indicates specific scenarios where the wave structure of a fluid is not influenced by the presence of the magnetic field.It also reveals the situation where a specific amount of magnetic field B can counterbalance the possible negative value of the fundamental derivative.Therefore, an appropriate prescription of magnetic field could prevent the formation of composite waves induced by phase transition in the material.
The performed analysis provides a suitable insight on the MHD wave structure.The analysis can also be considered as a mathematical tool to determine the possibilities of transforming non-classical behavior of the wave structure in hydrodynamic simulations into classical regimes under the influence of the magnetic field.In the following Section we illustrate the above analysis considering the MHD system of equations together with two non-convex EOS.For each of them we analyze the wave structure for Riemann problems as the tool to investigate phenomena associated with shocks and expansion waves and their interactions.

IV. NUMERICAL EXAMPLES OF ANOMALOUS WAVE STRUCTURE IN MAGNETIZED MATERIALS
In this Section we consider two analytical models of non-convex EOS namely the van der Waals and the Mie-Grüneisen to illustrate the analysis performed in the previous Section.
Van der Waals and Mie-Grüneisen models are simple models to investigate phenomena related to negative nonlinearity 1,[15][16][17]29,34,35 . We partiularize the study for each non-convex model performing numerical experiments showing the anomalous wave phenomena that appears in the evolution of specific Riemann problems proposed in the literature for hydrodynamic codes.We extend these Riemann problems to planar magnetohydrodynamics scenarios (B z = 0) prescribing specific amounts of magnetic field in the initial conditions that allow to modify the wave structure preventing the formation of thermodynamical composite waves.
In order to determine the amount of magnetic field needed to change the wave structure in each scenario we take into account case 1 of Corollary 1 and case 3 of Corollary 2. Case 1 in Corollary 1 asserts that positive nonlinearities are obtained for the fast and slow wavefields when the fundamental derivative keeps negative values in ] − 3 2 , 0[ and the oblique magnetic field (B x = 0 and B 2 y + B 2 z = 0) does not rotate and is confined to a bounded corona region.On the other hand following case 3 in Corollary 2 we can obtain positive nonlinearity for a specific amount of transversal magnetic field (B x = 0 and B 2 y + B 2 z = 0) and any negative value of the fundamental derivative.
We are going to consider different hydrodynamic problems in which anomalous structure effects due to negative nonlinearities appear in the simulation.In order to revert a nonclassical hydrodynamics wave structure into a classical regime we use either of these results.
The general way we proceed is as follows.
We run the hydrodynamics code for the problem under study at a certain time t * and examine the profile of the fundamental derivative.As the present wave dynamics is anomalous, the thermodynamical variable holds negative values at some points of the flow field inducing negative nonlinearity in the system.We set G min (< 0) the minimum value of the fundamental derivative in our hydrodynamical scene and look up the values of the density ρ and the square of the acoustic sound speed a 2 at the same position and time and label them ρ * and a 2 * respectively.We use two practical criteria to estimate the amount of magnetic field that counterbalances the negative weight of the fundamental derivative in ( 18) and ( 19) so that we obtain positive values of the nonlinearity terms.Consequently the non-classical behavior in the hydrodynamics scenario is reverted into a classical one in the MHD framework.The practical criteria read as follows.
Practical criterion 1: Following case 1 in Corollary 1, if G min ∈] − 3 2 , 0[ we apply an oblique magnetic field to the hydrodynamics initial data such that its magnitude is B 2 ≈ a 2 * ρ * with B x > 0 and B y > 0. Then the appearing wave structure in the MHD scene is entirely classical.In this case where the x−component of the magnetic field is present the slow nonlinear wavefield is activated and a more involved wave structure appears.
Note that for large or very small amounts of oblique magnetic field such that B 2 is not confined into the corona region defined in Corollary 1 the negative weight of the fundamental derivative cannot be counterbalanced and therefore nonlinearity terms remain negative and anomalous wave structure might appear.
Practical criterion 2: Following case 3 in Corollary 2, for G min < 0 we apply a transversal magnetic field (B x = 0) with B y > 2 3 |G min |a 2 * ρ * .Then the non-classical phenomena is turned into a classical regime.Note that in this case there is not lower bound condition for the minimum value of the fundamental derivative.
The evolution of the MHD system with either the oblique or the transversal magnetic field as prescribed in the practical criteria together with the initial data of density, velocity, and pressure as in the purely hydrodynamical original problem will provide a classical wave structure at the considered time t * .Next we present several numerical examples of hydrodynamics problems and their extended magnetohydrodynamics versions where the magnetic field has been determined following the practical criteria described above.In order to perform numerical simulations we extend to general MHD the shock capturing scheme presented in Ref. 8 for ideal MHD.The high order numerical scheme is based on the Marquina Flux Formula (MFF) as Riemann solver 9 .The flux formulation has been proved to behave robust in multiple scenarios 8,10,[18][19][20]25,45,46 . It compues the numerical fluxes at cell interfaces by means of two linearizations at each side of the interface following the Marquina's interface strategy.The procedure satisfies Rankine-Hugoniot relations approximately and avoids the computation of arithmetic averages to define intermediate states as it is common in other Riemann solvers when Roe linearizations are not available 4 .We implement to high order accuracy in space following the Shu-Osher flux formulation 47 by using a third order reconstruction procedure based on hyperbolas 48,49 .High order accuracy in time is achieved by using a third order TVD Runge-Kutta time stepping procedure 47 .Details on the implementation of the high order shock-capturing scheme for general MHD are expanded in the Appendix A.
In our hydrodynamics experiments we compute the approximate solution of the problem using the hydrodynamics version of the MHD code by setting B ≡ 0.

A. The van der Waals model for dense fluids under the influence of magnetic field
The van der Waals gas is a model often considered in hydrodynamic scenarios to investigate phenomena related to anomalous shock wave dynamics.It represents a powerful analytical model that allows thermodynamic phase change and therefore complex wave dynamics 1,12,28,30,33,39 .
The expression to define the pressure by the van der Waals EOS model is as follows.Let us define κ = R C V where R is the gas constant, C V is the specific heat at constant volume and η a > 0 and η b > 0 are positive constants accounting for the intermolecular forces and the molecule size respectively.The pressure is obtained from From this expression we can derive the thermodynamic magnitudes relevant for the wave structure analysis: the Grüneisen coefficient, the square of the acoustic sound speed and the fundamental derivative In order to explore the wave dynamics arising in MHD systems under a van der Waals EOS we first examine the behavior of shock waves in the case of hydrodynamics described by Euler equations.
We consider three Riemann problems proposed for Euler equations in references 28,30 .The initial data for the dense gas are defined in the interval [0, 1] as two constant states at both sides of 0.5, u L (left) and u R (right).Initial values for the density, velocity and pressure for the three dense gas problems are shown in Table II   The three problems develop non-classical wave structure.In each case there exist a region where the fundamental derivative G is negative inducing negative nonlinearity.We are going to use these problems to show different ways of inverting the non-classical behavior of the hydrodynamic simulations into a classical wave structure by means of applying a specific amount of magnetic field in the initial conditions of the problem.In order to prescribe an appropriate intensity of magnetic field we follow the practical criteria presented previously.versus the obtained approximate solution using 800 grid points.

DG1 hydrodynamic and MHD Riemann problems
The initial data of the DG1 hydrodynamics Riemann problem in Table II provide positive initial states of the fundamental derivative.The thermodynamical variable becomes negative at some points of the flow evolution inducing formation of non-classical waves.Figure 1 exhibits results for the hydrodynamics and magnetohydrodynamics simulations at time where negative values of the fundamental derivative appear.The nonlinearity term profile also presents negative values in the same region since the nonlinearity and the fundamental derivative share sign in hydrodynamics (29).From the simulation we have that G min = −0.8554and the density and the square of the acoustic sound speed at the same location are ρ * = 0.7781 and a 2 * = 0.2981.
Second and third rows of Figure 1 include MHD simulations with initial values as shown in Table III.The initial values of magnetic field for each simulation have been calculated following practical criteria 1 and 2.
From criterion 1 we obtain r = a 2 * ρ * = 0.4816 from where we choose an oblique magnetic field of 45 • setting B x = B y ≈ r cos( π 4 ).We set B x = B y = 0.34 constant in the whole domain.Results of the MHD Riemann problem are displayed in the second row of Figure 1.
Taking into account the same hydrodynamics scenario, practical criterion 2 determines that applying a transversal magnetic field B = (0, B y , 0) with B y > 0.3637 the wave structure will become classical.We set B x = B z = 0 and B y = 0.45 constant.Results are displayed in the third row of Figure 1.
Both MHD simulations show classical wave structure.In both cases the nonlinearity factors are entirely positive in spite of the phase transition reflected in the profile of the fundamental derivative.The composite wave that appears in the hydrodynamics simulation in the first row of Figure 1 is transformed into a simple expansion fan in the MHD simulations in second and third rows of the same Figure .We also observe the more involved wave structure in the MHD-DG1-Oblique simulation in second row due to the presence of the x-component of the magnetic field.Four nonlinear waves appear, two at each side of the entropy wave.The nonlinearity factor presents a similar profile as the fundamental derivative and becomes negative at the same locations since both share sign in the hydrodynamic case (29).From the  The initial data for the MHD-DG3 are shown in Table V.The initial values of the magnetic field have been estimated satisfying practical criteria 1 and 2.
From practical criterion 1 we have that r = a 2 * ρ * = 0.5488.This value provides an estimation of the radius of the corona region where the total magnitude of magnetic field needs to be defined to counterbalance negative values of the fundamental derivative.In this example we choose a magnetic field in the corona region setting B x = 0.4 and B y = 0.6 which gives B 2 = 0.52.Results are displayed in the second row of Figure 3.
Practical criterion 2 states B y > 0.2738 and we set the magnetic field as B x = B z = 0 and B y = 0.38 constant.Third row of Figure 3 exhibits the results for this MHD test.
We observe positive nonlinearity factors in both, oblique and transversal, MHD computations.In spite of the phase transition that takes place in the interval ]0.3, 0.4[, reflected with a change of sign of the fundamental derivative, there is not thermodynamical composite wave showing up in the MHD quantities because both nonlinearity factors are positive.The composite wave appearing in the hydrodynamics case (represented in the first row of Figure 3) is transformed in second and third rows into simple expansion fans.
In the three examples presented up to this point we have added positive amounts of magnetic field in the initial states of the evolution of the hydrodynamic systems.The magnetic field has dissolved thermodynamical composite waves induced by phase transitions transforming anomalous phenomena in the wave structure in the hydrodynamic simulations into classical regimes in the MHD framework.In each example we have reverted a situation where the nonlinearity was negative in some regions of the domain to the case where the nonlinearity terms associated to nonlinear wavefields in MHD were entirely positive.
As mentioned in Section III it is well known in MHD systems that the rotation of the magnetic field might induce non-genuinely nonlinearity in some of the nonlinear wavefields.
The loss of genuniely nonlinearity entails the consequent anomalous wave structure in the form of composite waves 5,6,8,40 .
In the next example we consider a case where the magnetic field rotates.We add to the DG3 problem initial data a transversal magnetic field with change of sign in the y-direction which will induce a magnetic composite wave.This example shows that the anomalous wave behavior in MHD systems is independent from the EOS model used.

Planar MHD-DG3 Riemann problem with rotating magnetic field
We consider the hydrodynamics initial data DG3 together with a magnetic field that has a change of sign in the y-component, B L = (0.4,0.6, 0) and B R = (0.4,−0.6, 0).Let us remark that the total amount of magnetic field in this problem is the same as in the previous MDG3-0blique problem.The difference relies on the minus sign of the y-component at the right side of the interface.This change of sign implies rotation of the magnetic field and the subsequent formation of a magnetic composite wave 5,40 .
We compute the approximate solution at time t * = 0.2 and display results in Figure 4.
Similarly as in the previous example the total amount of magnetic field (B 2 ) counterbalances the negative value of the fundamental derivative in the expression of the nonlinearity factor (17) implying the suppression of the thermodynamical composite wave that was originally in the hydrodynamic DG3 example in first row of Figure 3.
In Figure 4 we observe a phase transition in the interval ]0.In this example we consider the DG3 hydrodynamics initial data together with a large amount of magnetic field in an oblique direction.We evolve the initial data DG3 together with a constant magnetic field B = (1, 1, 0) in the interval [−0.5, 1.5].
Let us remark that B y remains positive at both sides of 0.5 and therefore no magnetic composite wave formation is expected.We display in Figure 5  term changes sign once, therefore the composite wave consists only of two components: a compression fan attached to a shock wave.In this example the amount of oblique magnetic field is out of the corona region defined in Corollary 1 and is not the appropriate one to cancel out the negative value induced by the fundamental derivative and avoid the formation of the thermodynamic composite wave.

B. A Mie-Grüneisen model for fluids under the influence of magnetic field
The Mie-Grüneisen type EOS is usually considered to model solid and liquid materials in hydrodynamics and is commonly adopted for reactive problems in which detonations arise in solids.The Mie-Grüneisen EOS is specially interesting for the studies of shock waves with multiple phase transitions and mixtures at high pressures and is used to model condensed phase materials to effectively describe the hydrodynamic shock response of many materials included metals 1,[15][16][17]41,42 . In ths Section we consider a particular case of this formulation representing a fictitious material called "Bizarrium" proposed in Refs.41,42 and extensively studied in Ref. 42 .The Bizarrium EOS is built such that the material describes wave propagation with continuous changes of concavity in the isentropes.Phase transitions are reflected through a change of sign in the fundamental derivative and exhibiting a complex wave structure.
Similarly, from Eq. ( 34) we get As stated in Ref. 42 the domain of validity of this EOS is determined by relevant values of the reference potential and thermodynamic stability.These conditions provide that the density must be restricted between 8113.102 and 16666.666kg/m 3 .
From expression (40) it can be checked that the change of sign of the fundamental derivative happens twice.Indeed, the loss of convexity of the Bizarrium EOS occurs when the density ρ varies in the interval [11428, 13333]   In the following we analyze a test problem proposed in Ref. 42 for Euler equations ruled by the Mie-Grüneisen EOS.We reproduce the hydrodynamic results and extend the data to a MHD test adding specific amounts of magnetic field such that the anomalous wave structure originally present in the hydrodynamic problem is dissolved.

Bizarrium test for hydrodynamics
We consider the so-called Bizarrium test problem for hydrodynamics presented in Ref.   VI.
This test problem has been designed to exhibit composite wave structure at both sides of the entropy wave.The initial state consists of two states where the fundamental derivative is positive and the jump discontinuity in 0.5 contains two points where G changes sign.
The complex wave structure appearing in the evolution of this initial data is considered a benchmark to evaluate the behavior of numerical methods 42 .Before considering an exten- C v 0 (J/kg/K) 1000 q − 42080895 14941154 T 0 (K) 300 sion of this problem to MHD scenarios we approximate the solution of the hydrodynamics problem to test the robustness of the numerical scheme.
We compute the approximate solution at time t * = 8 • 10 −5 using 1000 grid points and CFL= 0.8. Figure 6 displays the profiles of the density, velocity, pressure, fundamental derivative and nonlinearity factor.We observe composite waves at both sides of the entropy wave.The rarefaction fan traveling to the left splits into two branches that are connected through an expansion shock.The shock wave traveling to the right splits into two compression shocks connected through a compression fan.
We calculate the value of the fundamental derivative from the computed values of the conserved variables at final time of the evolution.We observe that the profile of the fundamental derivative presents a numerical artifact near the entropy wave.The "spike" reaches negative values which are non-physical.This numerical effect is the result of the evolution of a startup error which occurs when the initial data includes a phase change as is the case in the Bizarrium test.The formation of the nonlinear wave that propagates away from the entropy wave generates the "spike" due to the presence of numerical diffusion.This numerical artifact cannot generate spurious composite waves because entropy waves are linear waves and the theory states that anomalous wave structure is only developed around nonlinear waves.This effect is also reflected in the profile of the nonlinearity factor.
Our interest focuses on transforming the appearing non-classical wave structure into a classical one.In the next example we add a specific amount of magnetic field in the initial states of the evolution to perturb the complex wave structure induced by phase transitions.We note that the initial constant value of the transversal magnetic field has not only reverted the non-classical waves structure into a classical regime but it has also transformed a blast wave hydrodynamics scenario into a typical shock tube profile.

V. CONCLUSIONS
We have presented an analytical study of the wave structure of MHD system of equations closed with a general EOS.We have proposed a complete spectral decomposition of the fluxes of the system and derived an expression of the nonlinearity factor for the nonlinear wavefields.We demonstrate that the non-ideal MHD wave structure depends crucially on both: the EOS characterizing the material and the magnetic field and its possible rotation.

)
Let us define (b x , b y , b z ) = (B x , B y , B z )/ √ ρ and b 2 = b 2 x + b 2 y + b 2 z .The general expression of the square of the acoustic sound speed (6) is given by a 2 = P ρ + P ε P ρ 2

α(c 2 −a 2 ) √ b 2 y +b 2 z 2 y +b 2 z
, By √ ρ = b y and β y = by √ b the scalar product simplifies to . The values of the EOS parameters are the same for the three problems: R = 1; C V = 80; η a = 3; η b = 1/3, (η a and η b are chosen such that critical pressure and critical specific volume are equal to 1).

Figures exhibiting results of
Figures exhibiting results ofthe van der Waals dense gas Riemann problems are structured such that the first row displays the hydrodynamic simulation and second and third rows show extensions of the corresponding hydrodynamics Riemann problems to magnetohydrodynamics scenarios where the initial values of the magnetic field have been chosen following the first and second criteria respectively.All pictures in the Figures display a reference solution of the shock tube problem under study computed with 5000 grid points

FIG. 1 .
FIG. 1. Numerical solutions at time t = 0.15 of the Hydrodynamics and MHD van der Waals DG1 Riemann problems.First row, Hydrodynamics-DG1.Second row, MHD-DG1 with Oblique magnetic field.Third row, MHD-DG1 with Transversal magnetic field.Form left to right profiles of: density, fundamental derivative and the corresponding nonlinearity factors of nonlinear wavefields for each case.

FIG. 2 .
FIG. 2. Numerical solutions at time t = 0.45 of the Hydrodynamics and MHD van der Waals DG2 Riemann problems.First row, Hydrodynamics-DG2.Second row, MHD-DG2 with Oblique magnetic field.Third row, MHD-DG2 with Transversal magnetic field.Form left to right profiles of: density, fundamental derivative and the corresponding nonlinearity factors of nonlinear wavefields for each case.
B ≡ 0 DG3 and B = (0.4,0.6, 0) DG3 and B = (0, 0.38, 0) simulation we obtain G min = −0.3733and the values for density and square of the acoustic sound speed at the same location are ρ * = 0.7703 and a 2 * = 0.3910 respectively.With these values we use the practical criteria to determine different amounts of magnetic field to revert non-classical wave phenomena into classical.

FIG. 3 .
FIG. 3. Numerical solutions at time t = 0.2 of the Hydrodynamics and MHD van der Waals DG3 Riemann problems.First row, Hydrodynamics-DG3.Second row, MHD-DG3 with Oblique magnetic field.Third row, MHD-DG3 with Transversal magnetic field.Form left to right profiles of: density, fundamental derivative and the corresponding nonlinearity factors of nonlinear wavefields for each case.

FIG. 4 .
FIG. 4. Numerical solution at time t = 0.2 of the MHD van der Waals DG3 Riemann problem with Rotated Oblique magnetic field.Profiles of density, zoomed region for the compound wave in density, y-component of the magnetic field, fundamental derivative and nonlinearity factors of nonlinear wavefields 3, 0.4[ but not anomalous structure appears in the same region in the density profile.This is because both nonlinearity factors are positive in the interval.However we recognize a magnetic composite wave in the region ]0.48, 0.52[ where B y changes sign.A zoomed region of the anomalous structure in density is displayed at the center of the first row of the Figure.The generation of this composite wave is due to the rotation of the magnetic field and not because of thermodynamic effects (fundamental derivative is positive in ]0.48, 0.52[).Let us remark that this composite wave has the same structure as the one appearing in ideal MHD systems under a rotating magnetic field5,6,8,40,44 .This is an illustrative example of the way the magnetic field influences the wave structure of the MHD system.While the total amount of B has balanced out the negative weight of the fundamental derivative in the nonlinearity term dissolving the thermodynamic composite wave, its rotation caused by a change of sign in the y-component has induced negative values in the nonlinearity term(17).5. Planar MHD-DG3 Riemann problem with large amount of magnetic field: B x = 1 and B y = 1 constant.

FIG. 5 .
FIG. 5. Numerical solution at time t = 0.2 of the MHD van der Waals DG3 Riemann problem with large amount of oblique magnetic field.Profiles of density, fundamental derivative and nonlinearity factors of nonlinear wavefields

FIG. 6 .FIG. 7 .
FIG. 6. Numerical solution at time t = 8 • 10 −5 of the Bizarrium test for hydrodynamics.Top to bottom and left to right profiles of density, velocity, pressure, fundamental derivative and nonlinearity factor of the nonlinear wavefield at time.

Figure 7
Figure7displays the profiles of the density, fundamental derivative and nonlinearity factor of the fast wavefield (the slow wavefield degenerates to linear).We observe a classical wave structure.One expansion fan travels to the left and one compression shock does to the right of the entropy wave.The slow wave degenerates to the linear entropy wave.The composite wave that appeared in the hydrodynamic case has been dissolved because of the effect of the magnetic field.

TABLE I .
Degenerate cases of the nonlinear wavefields described in Corollary 2

TABLE II .
Initial conditions for the Van der Waals hydrodynamics shock tube problems

TABLE V .
Initial data for Hydrodynamics and MHD DG3 Riemann problems

TABLE VI .
Coefficients of the Bizarrium EOS