Abstract
In this article, we consider the Chew, Goldberger and Low (CGL) plasma flow equations, which is a set of nonlinear, non-conservative hyperbolic PDEs modeling anisotropic plasma flows. These equations incorporate the double adiabatic approximation for the evolution of the pressure, making them very valuable for plasma physics, space physics, and astrophysical applications. We first present the entropy analysis for the weak solutions. We then propose entropy-stable finite-difference schemes for the CGL equations. The key idea is to reformulate the CGL equations by rewriting some of the conservative terms in the non-conservation form. The conservative part of the reformulated equations is very similar to the magnetohydrodynamics (MHD) equations which is then symmetrized using Godunov’s symmetrization process for the MHD equations. The resulting equations are in the form where the conservative part combined with non-conservative Godunov’s terms is compatible with the entropy equation and the rest of the non-conservative terms do not contribute to the entropy equations. The final set of reformulated equations is then discretized by designing entropy conservative numerical flux and entropy diffusion operator based on the entropy scaled eigenvectors of the conservative part. We then prove the semi-discrete entropy stability of the schemes for the reformulated CGL equations. The schemes are then tested using several test problems derived from the corresponding MHD test cases.









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Funding
The work of Harish Kumar is supported in parts by VAJRA grant No. VJR/2018/000129 by the Dept. of Science and Technology, Govt. of India. Harish Kumar and Chetan Singh acknowledge the support of FIST Grant Ref No. SR/FST/MS-1/2019/45 by the Dept. of Science and Technology, Govt. of India.
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Appendices
Entropy Scaled Right Eigenvectors for Conservative Part of the Reformulated CGL System
In this section, we present expressions for the right eigenvectors and scaled right eigenvectors for the conservative system
The right eigenvectors are described in Section A.1, while the expressions of scaled eigenvectors have been demonstrated in Section A.2. We will do all the analysis in the term of primitive variable \({{\textbf {W}}} = \{\rho ,u_x,u_y,u_z,p_\parallel ,p_{\perp },B_x,B_y,B_z\}\).
1.1 Eigenvalues and Right Eigenvectors
To derive the eigenvalues and right eigenvectors, it is useful to transform the system (60) in terms of the primitive variables \({{\textbf {W}}}\). The set of eigenvalues \(\Lambda _1\) of the jacobian matrix \({\mathcal {A}}_x\) are given as
where,
The corresponding right eigenvectors are
where,
Above all, eigenvectors are linearly independent. The above-defined right eigenvectors are singular in a variety of cases as listed by Balsara and Roe in [3]. Similarly, in y-direction, the set of eigenvalues \(\Lambda _2\) of the jacobian matrix \(\frac{\partial {{\textbf {f}}}_{y}}{\partial {{\textbf {U}}}}\) are given as
where,
the corresponding right eigenvectors are
where,
1.2 Entropy Scaled Right Eigenvectors
In this section, we have calculated the entropy scaled right eigenvectors for the case of x-direction. Consider the conservative part of the CGL system (60). The eigenvalues and Right eigenvectors in terms of primitive of the Jacobian matrix \({\mathcal {A}}_1\) are described in (A.1). The right eigenvector matrix \(R^x\) for the matrix \({\mathcal {A}}_1\) in terms of conservative variable is given by the relation
where, \(\frac{\partial {{\textbf {U}}}}{\partial {{\textbf {W}}}}\) is the Jacobian matrix for the change of variable, given by
and the matrix \(R_{{{\textbf {W}}}}^{x}\) is given by,
where, \({\mathcal {S}}_{x} = sgn(B_x)\). We need to find a scaling matrix \(T^x\) such that the scaled right eigenvector matrix \(\tilde{R}^x=R^x T^x\) satisfies
where \({{\textbf {V}}}\) is the entropy variable vector as in Eqn. (12). We follow the Barth scaling process [6] to scale the right eigenvectors. The scaling matrix \(T^x\) is the square root of \(Y^x\), where \(Y^x\) has the expression
which results in
Then matrix \(T^x\) is,
Remark 3
For finding the square root of \(3\times 3\) matrix
where a , b , c, d and e are real or complex numbers. we apply the formula,
where, \(\delta = ad-bc\), \(\tau = a+d\) and \(s=\sqrt{\delta }\), \(t=\sqrt{\tau +2s}\). We apply this formula if \(t \ne 0\).
Remark 4
We proceed similarly in the y-direction, the eigenvalues and Right eigenvectors in terms of primitive of the Jacobian matrix \(\frac{\partial {{\textbf {f}}}_{2}}{\partial {{\textbf {U}}}}\) describe in (A.1). The right eigenvector matrix is given by the relation
where, \(R_{{{\textbf {W}}}}^{y}\) is given below
where, \({\mathcal {S}}_{y} = sgn(B_y)\). Accordingly, we obtain the scaling matrix \(T^y\) as,
1.3 Entropy Scaled Right Eigenvectors in x and y Direction
In x direction, the entropy-scaled right eigenvectors corresponding to the above eigenvalues in the term of primitive variables are the following,
In y-direction, the entropy-scaled right eigenvectors corresponding to the above eigenvalues in the term of primitive variables are the following,
Proof of \({{\textbf {V}}}^\top \cdot {{\textbf {C}}}_x({{\textbf {U}}}) = {{\textbf {V}}}^\top \cdot {{\textbf {C}}}_y({{\textbf {U}}}) = 0\)
The entropy variable \({{\textbf {V}}}:=\frac{\partial {\mathcal {E}}}{\partial {{\textbf {U}}}}\) is given by
where \(\beta _\perp =\frac{\rho }{p_{\perp }},\) and \(\beta _\parallel =\frac{\rho }{p_\parallel }\). Now the dot product of \({{\textbf {V}}}^\top \) and first coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Now the dot product of \({{\textbf {V}}}^\top \) and second coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Now the dot product of \({{\textbf {V}}}^\top \) and third coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Now the dot product of \({{\textbf {V}}}^\top \) and fourth coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Now the dot product of \({{\textbf {V}}}^\top \) and fifth coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Now the dot product of \({{\textbf {V}}}^\top \) and sixth coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
The seventh coloum, 8th coloum and ninth coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) denoted by \(C_7,~C_8\) and \(C_9\) is given below,
Now the dot product of \({{\textbf {V}}}^\top \) and seventh coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Now the dot product of \({{\textbf {V}}}^\top \) and 8th coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Now the dot product of \({{\textbf {V}}}^\top \) and ninth coloum of \({{\textbf {C}}}_x({{\textbf {U}}})\) is:
Similarly we can proof that \({{\textbf {V}}}^\top \cdot {{\textbf {C}}}_y({{\textbf {U}}}) = 0\) for y-direction.
Non-Symmetrizability of CGL System
In this section, we have discussed the symmetrizability of the following CGL equations. Consider CGL equations in one dimension as follows:
where \({{\textbf {U}}},~{{\textbf {f}}}_{x}\) and \({{\textbf {C}}}_{x}({{\textbf {U}}})\) are defined in (3). Follow [68], the CGL equations are said to be symmetrizable if the change of variable \({{\textbf {U}}}\rightarrow {{\textbf {V}}}\) applied to (62) and can be written as:
the matrix \(\frac{\partial {{\textbf {U}}}}{\partial {{\textbf {V}}}}\) is a symmetric, positive definite matrix and \(\aleph ({{\textbf {U}}}) = \left( \frac{\partial {{\textbf {f}}}_x}{\partial {{\textbf {U}}}} +{{\textbf {C}}}_{x}({{\textbf {U}}}) \right) \frac{\partial {{\textbf {U}}}}{\partial {{\textbf {V}}}}\) is symmetric matrix. For the CGL equation (62), we calculate matrix \(\aleph ({{\textbf {U}}})\) to examine its symmetry and conclude that CGL equations are non-symmetrizable. The resultant matrix \(\aleph ({{\textbf {U}}}) -\aleph ({{\textbf {U}}})^\top \) is given below:
where, \(\Delta P= (p_\parallel -p_{\perp })\) and \(\alpha _{lmn}= 2 p_{\perp }B_x B_l + \Delta Pb_x\Big (2 \rho u_l (b_m u_m + b_n u_n) - b_l \) \((3p_\parallel +2p_{\perp }+ \rho (u_m^2 +u_n^2 - u_l^2))\Big );~~\forall ~l,m,n \in \{x,y,z\}\)
\(\beta _1 = B_y u_y + B_z u_z + \frac{\Delta P\left( -b_y u_y - b_z u_z + 2 b_x(({\varvec{b}}\cdot {{\textbf {u}}})b_x-u_x)\right) }{|\varvec{B}|}\) and \(\beta _l = B_x u_l - \frac{\Delta Pb_x(2 b_l({\varvec{b}}\cdot {{\textbf {u}}})-u_l)}{|\varvec{B}|};~~\forall ~l\in \{y,z\}\)
\(\omega _1 = B_x - \frac{2 \Delta Pb_x (1-b_x^2)}{|\varvec{B}|}\) and \(\omega _l = B_l - \frac{\Delta Pb_l (1 - 2 b_x^2)}{|\varvec{B}|};~~\forall ~l\in \{y,z\}\)
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Singh, C., Yadav, A., Bhoriya, D. et al. Entropy Stable Finite Difference Schemes for Chew, Goldberger and Low Anisotropic Plasma Flow Equations. J Sci Comput 102, 51 (2025). https://doi.org/10.1007/s10915-024-02763-3
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DOI: https://doi.org/10.1007/s10915-024-02763-3