Fish schools are capable of simultaneous linear acceleration. To reveal the underlying hydrodynamic mechanism, we numerically investigate how Reynolds number Re = 1000−2000, Strouhal number St = 0.2−0.7 and wavelength λ = 0.5−2 affect the mean net thrust and net propulsive efficiency of two side-by-side hydrofoils undulating in anti phase. In total, 550 cases are simulated using immersed boundary method. The thrust increases significantly with wavelength and Strouhal number, yet only slightly with the Reynolds number. We apply a symbolic regression algorithm to formulate this relationship. Furthermore, we find that mirror-symmetric schooling can achieve a net thrust more than ten times that of a single swimmer, especially at low Reynolds numbers. The highest efficiency is obtained at St = 0.5 and λ = 1.2, where St is consistent with that observed in the linear-accelerating natural swimmers, e.g. Crevalle jack. Six distinct flow structures are identified. The highest thrust corresponds to an asymmetric flow pattern, whereas the highest efficiency occurs when the flow is symmetric with converging vortex streets.This work was funded by China Postdoctoral Science Foundation (Grant No. 2021M691865) and by Science and Technology Major Project of Fujian Province in China (Grant No. 2021NZ033016). We appreciate the US National Science Foundation award OAC 1931368 (A.P.S.B) for supporting the IBAMR library. This work was also financially supported by the National Natural Science Foundation of China (Grant Nos. 12074323; 42106181), the Natural Science Foundation of Fujian Province of China (No. 2022J02003), the China National Postdoctoral Program for Innovative Talents (Grant No. BX2021168) and the Outstanding Postdoctoral Scholarship, State Key Laboratory of Marine Environmental Science at Xiamen Universit
