On the Connections Between Discontinuous Galerkin and Flux Reconstruction Schemes: Extension to Curvilinear Meshes
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Abstract
This paper investigates the connections between many popular variants of the wellestablished discontinuous Galerkin method and the recently developed highorder flux reconstruction approach on irregular tensorproduct grids. We explore these connections by analysing three nodal versions of tensorproduct discontinuous Galerkin spectral element approximations and three types of flux reconstruction schemes for solving systems of conservation laws on irregular tensorproduct meshes. We demonstrate that the existing connections established on regular grids are also valid on deformed and curved meshes for both linear and nonlinear problems, provided that the metric terms are accounted for appropriately. We also find that the aliasing issues arising from nonlinearities either due to a deformed/curved elements or due to the nonlinearity of the equations are equivalent and can be addressed using the same strategies both in the discontinuous Galerkin method and in the flux reconstruction approach. In particular, we show that the discontinuous Galerkin and the flux reconstruction approach are equivalent also when using higherorder quadrature rules that are commonly employed in the context of over or consistentintegrationbased dealiasing methods. The connections found in this work help to complete the picture regarding the relations between these two numerical approaches and show the possibility of using over or consistentintegration in an equivalent manner for both the approaches.
Keywords
Spectral/hp methods Discontinuous Galerkin Flux reconstruction Aliasing Irregular grids1 Introduction
Popularity of highorder compact discretisations using spectral/hp element spatial approximations is rising rapidly and their deployment to industrialtype problems is becoming a real possibility. Spectral/hp methods can achieve an arbitrary order of accuracy and they are significantly less dissipative than the more traditional loworder methods making them a key tool in some areas of computational fluid dynamics, such as in the numerical simulation of unsteady flows. On the other hand, spectral/hp element discretisations are still affected by a lack of numerical stability which makes them not highly reliable unlike loworder methods. One of the most popular approaches to discretise hyperbolic conservation laws is based on the discontinuous Galerkin (DG) method firstly introduced by Reed and Hill [1]. Usually, the DG method is formulated in a weak form where the domain is divided into nonoverlapping elements and on each of these elements a basis of polynomials and a set of quadrature points are chosen to calculate the integrals arising from the weak formulation of the problem. Some of the most efficient and ubiquitous forms of the DG method are the socalled nodal DG schemes, whereby Lagrange interpolants are combined with a set of nodal solution points on a given element [2]. We note that in a general nodal DG scheme, given an expansion on each element in terms of P Lagrange interpolants, we may perform quadrature on a separate set of Q quadrature points. If \(P=Q\) and the distribution of solution and quadrature points is identical, then we recover the socalled discontinuous Galerkin spectral element method (DG\(_{\text {SEM}}\)) which diagonalises the mass matrix, allowing for further computational optimisation [3, 4].
In contrast to the DG method which makes use of a weak representation of the equations, a set of schemes based upon the differential formulation have been recently introduced. They offer another route which avoids the need for calculating integrals and therefore defining quadrature rules, making these schemes attractive from an implementation point of view. The first of these, namely the spectraldifference scheme, was introduced by Kopriva and Kolias [5] in 1996 and was extended to quadrilateral and triangular elements by Liu et al. [6] in 2006. Most recently, the flux reconstruction (FR) method was presented by Huynh [7].
The FR approach encapsulates various energy stable discretisations where the particular scheme recovered is dictated by a single realvalued parameter and is referred as Vincent–Castonguay–Jameson–Huynh (VCJH) scheme [8]. The FR approach allows one to recover not only differentialtype schemes such as a particular SD method, but also integraltype schemes such as nodal DG schemes.
Connections between different types of DG and FR schemes derived in this work for elements with a spatially varying Jacobian
DG\(_{\text {SEM}}\)GLL  DG\(_{\text {SEM}}\)  DG\(_{(\text {Q}\,>\,\text {P})}\)  

Lumped mass matrix  Exact mass matrix  Exact mass matrix  
Flux type  Linear  Nonlinear  Linear  Nonlinear  Linear  Nonlinear 
FR\(_{\text {HU}}\)  \(\checkmark ^{*}\)  \(\checkmark ^{*}\)  
FR\(_{\text {DG}}\)  \(\checkmark \)  \(\checkmark \)  
FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\)  \(\checkmark \)  \(\checkmark \) 
The relationships found in this paper are summarized in Table 1. In particular, we demonstrate that the equivalences found in [10] between the DG\(_{\text {SEM}}\) scheme with lumped mass matrix (LMM) and the FR\(_{\text {HU}}\) scheme introduced in [7] (also referred to as FR\(_{\text {g2}}\)) as well as the equivalences between the DG\(_{\text {SEM}}\) scheme with exact mass matrix (EMM)^{2} and the FR\(_{\text {DG}}\) scheme (also presented in [7]) hold true for irregular tensorproduct meshes (i.e. deformed/curved tensorproduct grids). These equivalences are complemented with some numerical experiments on regular (used as a baseline) and irregular grids for linear and nonlinear problems.
We also show numerically that, using \(Q>P\), where Q is the number of quadrature points and P are the Lagrange interpolants inside each element, further extends the equivalences between DG and FR\(_{\text {DG}}\). Specifically, we found that DG\(_{(\text {Q}\,>\,\text {P})}\) and FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\) are identical when using \(Q>P\) for linear and nonlinear flux functions as well as for regular and irregular tensorproduct meshes^{3}. This indicates that polynomial aliasing sources for these two schemes are identical and can therefore be addressed using equivalent strategies, such as the consistent integration (through additional quadrature points) of the nonlinearities arising either from the equations themselves or from the geometry [11, 12].
We finally present some results related to the computational time required by the FR and DG schemes.
This paper is organised as follows. In Sect. 2 we prove theoretically the connections in the first four columns of Table 1, in Sect. 3 we further assess the theoretical work with some numerical experiments for both linear and nonlinear problems and we show numerically how DG\(_{(\text {Q}\,>\,\text {P})}\) and FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\) are identical and, finally, in Sect. 4 we draw the conclusions.
2 Theory
We describe the DG and FR schemes in the context of a 2D scalar conservation law. We assume that the quadrilateral elements are deformed/curved (i.e. possess spatially varying Jacobians) and we prove that the FR\(_{\text {DG}}\) scheme and DG\(_{\text {SEM}}\) method with an exact mass matrix evaluation are equivalent on irregular grids (i.e. spatially varying Jacobians) when the same polynomial approximation of the geometry is employed. We also demonstrate that the FR\(_{\text {HU}}\) scheme and the DG\(_{\text {SEM}}\) method with lumped mass matrix are equivalent on irregular quadrilateral grids.
2.1 2D Scalar Conservation Law on Irregular Quadrilateral Grids
2.2 FR\(_{\text {DG}}\) Scheme as DG\(_{\text {SEM}}\) Method with EMM
Remark
The equivalence above holds true for any point distribution used for defining the Lagrange basis.
2.3 FR\(_{\text {HU}}\) Scheme as DG\(_{\text {SEM}}\) Method with LMM
Remark
Note that this equivalence holds true only for GLL points.
3 Numerical Experiments
For both mesh configurations we tested the linear advection equation and the nonlinear compressible Euler equations. The test cases considered span all the possible combinations in Table 1.
We tested two different set of points, Gauss–Lobatto–Legendre (GLL) and Gauss–Legendre (GL) where the interpolations required at the boundaries for GL points were performed in a consistent manner for the various DG and FR schemes considered. Note that we show the results for the regular and most deformed grids on GLL points only for the sake of brevity. The conclusions we can draw from the results obtained on GL points and on the grids in the Figs. 1b and 2b are identical to those which can be drawn from the results presented here. Some additional results concerning GL points and the two grids in the Figs. 1b and 2b are reported in the “Appendix”.
3.1 Linear Problem
3.1.1 Mesh A
Figure 3 represents the L\(^{2}\) error vs the polynomial order obtained using GLL points for the various schemes tested where Fig. 3a refers to the regular mesh represented in Figs. 1a and 3b refers to the mesh depicted in Fig. 1c. Table 4 in the “Appendix” quantifies up to sixteen digits the results obtained for \(P=10\), for both GLL and GL points and all the three grids in Fig. 1. The results for the other polynomial orders are not tabulated for the sake of compactness and because they provide the same information as those which can be obtained from Table 4.

FR\(_{\text {HU}}\) and DG\(_{\text {SEM}}\) with lumped mass matrix on GLL points;

FR\(_{\text {DG}}\) and DG\(_{\text {SEM}}\) with exact mass matrix on any point distribution;

FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\) and DG\(_{(\text {Q}\,>\,\text {P})}\) on any point distribution.
3.1.2 Mesh B
Figure 4 represents the L\(^{2}\) error vs the polynomial order obtained using GLL points for the various schemes tested, where Fig. 4a refers to the regular mesh represented in Figs. 2a and 4b refers to the mesh in Fig. 2c. Table 5 in the “Appendix” quantifies up to sixteen digits the results obtained for both GLL and GL points for \(P=5\) and all the three grids in Fig. 2. The connections presented for the singleelement mesh configuration in the previous subsection hold true also for the multielement mesh configuration.
3.2 Nonlinear Problem
\({\beta }\)  R  \({\gamma }\)  \(({x_{0}}, {y_{0}})\)  \(({u_{0}}, {v_{0}})\)  T  P  TI  

Mesh A  5  1  1.4  (0, 0)  (0.5, 0.5)  2s  10, 11, 12, 13  RK4 
Mesh B  5  1  1.4  (5, 0)  (0, 5)  2s  5, 6, 7, 8  RK4 
3.2.1 Mesh A
Figure 5 represents the L\(^{2}\) error associated to the density (\(\rho \)) vs the polynomial order obtained using GLL points for the various schemes tested where Fig. 5a refers to the regular mesh represented in Figs. 1a and 5b refers to the mesh in Fig. 1c. Table 6 in the “Appendix” quantifies up to sixteen digits the results obtained for both GLL and GL points for \(P=10\) and all the three grids in Fig. 1. Also in the case of a nonlinear problem the same equivalences presented before maintain their validity up to machine precision as shown in Table 6.
However, considerations on polynomial aliasing are different because in this case the aliasing sources arise both from the equations themselves, which are nonlinear, and from the geometry. Significant, in particular, is the gap between the pair FR\(_{\text {DG}}\), DG\(_{\text {SEM}}\) and the pair FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\), DG\(_{(\text {Q}\,>\,\text {P})}\).
3.2.2 Mesh B
3.3 Comparison of Computational Time
In this subsection we present the comparison between the various schemes considered in terms of computational time. In particular, we show the results for the linear problem on the multielement mesh (mesh B) for both the regular case (A\(_{2}\) = 0.0) and for the deformed case (A\(_{2}\) = 0.2). Note also that we show the results for both GLL and GL points. The results on the single element mesh (mesh A) as well as the results for the nonlinear case are not presented since the conclusions we can draw are identical to the one we can obtain from the results presented.
3.4 Summary
In this section we presented several numerical experiments for both a linear and a nonlinear problem on two mesh configurations. For each mesh configuration we applied three different deformation levels, the first was a regular mesh, which was used as a baseline, while the other two were meshes with incremental deformation levels. In particular we showed how the identities shown in Table 1 and proved in Sect. 2 are further assessed (up to machine precision) across all the tests performed. An additional result shown in this section is the equivalence of FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\) and DG\(_{(\text {Q}\,>\,\text {P})}\) schemes which implies that the aliasing issues arising in FR\(_{\text {DG}}\) and DG\(_{\text {SEM}}\) are numerically the same and can be alleviated using a higher quadrature \(Q > P\), i.e. consistent integration of the nonlinearities arising in the problem (although the concept of integration could be seen as out of context for an FR scheme because there are no integrals present in the formulation of this approach). Note that the use of a better point distribution with a more powerful quadrature, such as the GL points, can also alleviate aliasing issues. However, when the nonlinearity cannot be fully described by the GL quadrature, then also in this case one could use additional quadrature points for consistently integrate the nonlinearity sources. In addition, we showed the computational costs associated to the FR and DG schemes for both the GLL and GL point distributions on the linear advection problem. Although no attempt was made to optimise the underlying algorithms of the two numerical approaches, it can be seen that both provide similar computational costs as expected. The differences observed can be imputed to the different data structures between the FR and the DG approach. Specifically, the first was implemented in the standard element space (\(\xi \), \(\eta \)), while the second on the local element space (x, y).
4 Conclusions
In this paper we established that the connections between various discontinuous Galerkin methods and highorder FR schemes are also valid on quadrilateral irregular/curved meshes. In addition, we found that the polynomial aliasing sources for the FR and DG schemes considered are identical and can be addressed using the same techniques. Specifically, we established that the FR and DG schemes taken into account are identical when a higherorder quadrature rule, usually adopted when applying overintegrationbased (also referred to as consistentintegration) dealiasing strategies, is employed.
The schemes considered for the DG approach are the DG\(_{\text {SEM}}\) with a lumped mass matrix (LMM) and a collocation projection of the solution and of the inner product, the DG\(_{\text {SEM}}\) with an exact mass matrix (EMM) and a collocation projection of the solution and of the inner product and the DG\(_{(\text {Q}\,>\,\text {P})}\) with exact mass matrix (EMM) and an additional quadrature point for representing the solution and performing the inner product (i.e. using a Galerkin projection of the solution and of the inner product). For what concerns the FR schemes we took into account the FR\(_{\text {HU}}\) scheme, the FR\(_{\text {DG}}\) scheme and FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\) scheme which used an additional quadrature point to represent the solution (i.e. using a Galerkin projection of the solution).
We found that the connections between discontinuous Galerkin methods and highorder flux reconstruction schemes explored in [10] for regular grids hold true also for irregular/curvilinear meshes. In particular we mathematically proved the equivalences between FR\(_{\text {HU}}\) and DG\(_{\text {SEM}}\) with lumped mass matrix and the connections between FR\(_{\text {DG}}\) and DG\(_{\text {SEM}}\) with exact mass matrix. Both demonstrations were further assessed by numerical experiments on two different mesh configurations at different grid deformation levels and for both linear and nonlinear problems. In addition, we showed numerically how these connections are valid when applying a higher quadrature \(Q>P\) (i.e. using more quadrature points than a collocation projection: FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\) and DG\(_{(\text {Q}\,>\,\text {P})}\)) whose use can be crucial for stabilising problems with significant polynomial aliasing issues such as under or marginallyresolved problems (e.g. high Reynolds number simulations). This result indicates that the aliasing sources for FR\(_{\text {DG}}\) and DG\(_{\text {SEM}}\) are identical and can be addressed using identical strategies.
The latter result is particularly significant because it shows how the machinery for the DG method for tackling aliasing issues and improving the numerical stability of this class of schemes can be directly deployed to the FR schemes.
Finally, we showed that, in our current codebase, the computational costs are similar when considering a pair of identical FR and DG schemes. The differences observed are mainly imputable to the different implementation—related to the data structures—of the two approaches.
Footnotes
 1.
In [3, 4] the DG\(_{\text {SEM}}\) method implies a collocation quadrature rule for the inner product of all the terms of the discretisation and not only of the advection term. This leads to a diagonal mass matrix which, in the case of Gauss–Lobatto–Legendre points, is not an exact mass matrix but a lumped one.
 2.
The terms ‘lumped’ and ‘exact’ refer to the mass matrix when solving a linear problem.
 3.
In this introduction P indicates the Lagrange interpolant. This allows to maintain a compact notation for the DG(Q \(>\) P) and FRDG(Q \(>\) P) schemes. Note however that in the rest of the paper P indicates the order of the Lagrange polynomials.
 4.
In the case of regular mesh the results for the pairs FR\(_{\text {DG}}\), DG\(_{\text {SEM}}\) with exact mass matrix and FR\(_{\text {DG}(\text {Q}\,>\,\text {P})}\), DG\(_{(\text {Q}\,>\,\text {P})}\) are not equivalent because we used a different projection of the initial conditions. Using the same projection of the initial conditions provides identical results.
Notes
Acknowledgments
This work was supported by the Laminar Flow Control Centre funded by Airbus/EADS and EPSRC under grant EP/I037946. PV would like to acknowledge the Engineering and Physical Sciences Research Council for their support via an Early Career Fellowship (EP/K027379/1). SJS additionally acknowledges Royal Academy of Engineering support under their research chair scheme.
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