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# Theoretical and numerical studies for energy estimates of the shallow water equations with a transmission boundary condition

• * Corresponding author: Md. Masum Murshed
• Energy estimates of the shallow water equations (SWEs) with a transmission boundary condition are studied theoretically and numerically. In the theoretical part, using a suitable energy, we begin with deriving an equality which implies an energy estimate of the SWEs with the Dirichlet and the slip boundary conditions. For the SWEs with a transmission boundary condition, an inequality for the energy estimate is proved under some assumptions to be satisfied in practical computation. In the numerical part, based on the theoretical results, the energy estimate of the SWEs with a transmission boundary condition is confirmed numerically by a finite difference method (FDM). The choice of a positive constant $c_0$ used in the transmission boundary condition is investigated additionally. Furthermore, we present numerical results by a Lagrange-Galerkin scheme, which are similar to those by the FDM. The theoretical results along with the numerical results strongly recommend that the transmission boundary condition is suitable for the boundaries in the open sea.

Mathematics Subject Classification: Primary: 65M06, 76D05, 65L07; Secondary: 65M25, 65M60.

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• Figure 1.  The Bay of Bengal and the coastal region of Bangladesh

Figure 2.  Model domain

Figure 3.  Color contours of $\eta_h^k$ by finite difference scheme (25) for the five cases $(i)$-$(v)$ discussed in Subsection 4.2

Figure 4.  Graphs of $E_h^k$ (left), $\sum_{i = 1}^4 I_{hi}^k \approx \frac{d}{dt} E(t)$ (center) and $I_{hi}^k$, $i = 1, \ldots, 4$, (right) versus $t = t^k\; (\ge 0, k\in \mathbb{Z})$ for the five cases $(i)$-$(v)$

Figure 5.  Color contours of $\eta_h^k$ by Lagrange-Galerkin scheme (26) for the five cases $(i)$-$(v)$ discussed in Section 5

Table 1.  Maximum and minimum values of $I_{hi}^k$, $i = 1,\ldots, 4$, with respect to the number of transmission boundaries

 $\varGamma_T$ $\varGamma_D$ $I_{h1}$ $I_{h2}$ $I_{h3}$ $I_{h4}$ 0 4 Max 0.00 0.00 0.00 0.00 Min 0.00 0.00 0.00 $-8.63 \times 10^{-7}$ 1 3 Max $1.10 \times 10^{-4}$ 0.00 $1.44 \times 10^{-9}$ 0.00 Min $-2.59 \times 10^{-3}$ $-3.37$ $-1.25\times 10^{-9}$ $-3.76\times 10^{-7}$ 2 2 Max $1.86 \times 10^{-4}$ 0.00 $1.72 \times 10^{-9}$ 0.00 Min $-3.38 \times 10^{-3}$ -6.27 $-2.50 \times 10^{-9}$ $-2.31 \times 10^{-7}$ 3 1 Max $1.43 \times 10^{-4}$ 0.00 $2.58 \times 10^{-9}$ 0.00 Min $-5.06 \times 10^{-3}$ $-9.40$ $-3.75 \times 10^{-9}$ $-1.74 \times 10^{-7}$ 4 0 Max $2.87 \times 10^{-4}$ 0.00 $3.47 \times 10^{-9}$ 0.00 Min $-6.75 \times 10^{-3}$ $-12.54$ $-5.01 \times 10^{-9}$ $-1.14 \times 10^{-7}$

Table 2.  $c_0$ and $\mathcal{S}_h(c_0)$

 $c_0$ $\mathcal{S}_h(c_0)$ Case Ⅰ Case Ⅱ Case Ⅲ Case Ⅳ Case Ⅴ Case Ⅵ 0.1 12.17 8.53 8.14 5.47 44.48 44.49 0.2 9.89 6.97 6.35 4.04 34.36 34.37 0.3 8.84 6.24 5.52 3.35 28.74 28.75 0.4 8.27 5.85 5.08 2.98 25.23 25.24 0.5 7.93 5.61 4.84 2.79 22.82 22.83 0.6 7.71 5.46 4.71 2.69 21.05 21.06 0.7 7.58 5.37 4.65 2.66 19.69 19.69 0.8 7.51 5.32 4.63 2.67 18.60 18.61 0.9 7.4805 5.2969 4.64 2.70 17.71 17.72 1.0 7.4807 5.2977 4.68 2.76 16.98 16.98 1.1 7.50 5.32 4.73 2.82 16.36 16.36 1.2 7.55 5.35 4.79 2.89 15.83 15.84 1.5 7.75 5.49 5.02 3.12 14.66 14.66
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