Fig. 1. Two-dimensional interpolation stencil of the ghost cell method: (a) No ghost cells; (b) one ghost cell; (c) two ghost cells.

Fig. 2. Flowchart of numerical algorithm for stationary or moving boundary flows.

Fig. 3. Comparison of force coefficients under five oscillation frequencies: (a) Average drag coefficient; (b) root mean square lift coefficient.

Fig. 4. Variation of drag and lift coefficients with the dimensionless time under three oscillation frequencies: (a) Drag coefficient; (b) lift coefficient.

Fig. 5. Local instantaneous vortex contours for = 0.15 at two moments: (a) ; (b) .

Fig. 6. Local instantaneous vortex contours for = 0.45 at two moments: (a) ; (b) .

Fig. 7. Local instantaneous vortex contours for = 0.75 at two moments: (a) ; (b) .

Fig. 8. Geometrical model of the undulating hydrofoil in array arrangement.

Fig. 9. Comparison of force coefficients between the central hydrofoil in array arrangement for three gap distances and the single hydrofoil under five frequencies: (a) Average drag coefficient; (b) root mean square lift coefficient.

Fig. 10. Variation of drag and lift coefficients on the central hydrofoil in array arrangement with the dimensionless time under three oscillation frequencies: (a) Drag coefficient; (b) lift coefficient.

Fig. 11. Local instantaneous vortex contours of undulating hydrofoils in array arrangement for = 0.15 at two moments: (a) ; (b) .

Fig. 12. Local instantaneous vortex contours of undulating hydrofoils in array arrangement for = 0.45 at two moments: (a) ; (b) .

Fig. 13. Local instantaneous vortex contours of undulating hydrofoils in array arrangement for = 0.75 at two moments: (a) ; (b) .