Thanks the anonymous referees for their suggestions and comments to improve the quality of the paper. We also would like to express our gratitude to Prof. Chensong Zhang (Chinese Academy of Sciences) for the discussion on construction of preconditioners and Dr. Sheng Gui (Chinese Academy of Sciences) for constructing the finite element meshes. This work was supported by the National Key Research and Development Program of Ministry of Science and Technology ( 2016YFB0201304 ), the National Natural Science Foundation of China (Grant No. 11701576 , 11501053 and 11771435 , 22073110 ), the Natural Science Foundation of Hunan Province (Grant No. 2019JJ50786 ), and the Education Department of Hunan Province (Grant No. 15C0026 ).
Thanks the anonymous referees for their suggestions and comments to improve the quality of the paper. We also would like to express our gratitude to Prof. Chensong Zhang (Chinese Academy of Sciences) for the discussion on construction of preconditioners and Dr. Sheng Gui (Chinese Academy of Sciences) for constructing the finite element meshes. This work was supported by the National Key Research and Development Program of Ministry of Science and Technology (2016YFB0201304), the National Natural Science Foundation of China (Grant No. 11701576, 11501053 and 11771435, 22073110), the Natural Science Foundation of Hunan Province (Grant No. 2019JJ50786), and the Education Department of Hunan Province (Grant No. 15C0026).