Robust fluid–structure interaction analysis for parametric study of flapping motion

Abstract A flapping motion is an important fluid–structure interaction (FSI) phenomenon. Although it has been extensively studied, there are still many unknowns. Because there are numerous parameters in the kinematics and morphology for flapping motions, it is difficult to experimentally determine parameter values that enhance flapping aerodynamics because of the associated time, cost, and space constraints. Therefore, a simulation-based study is a promising approach for investigating flapping motions. In our previous work, we developed an interface-tracking-based three-dimensional (3D) parallel FSI analysis system by the partitioned iterative method. However, this system failed in some cases during parametric calculations for various flapping motions. This seems because of 3D large movements and complex twisting motions of a deformable flapping wing. Because the distortion of a fluid mesh often leads to failure in the FSI analysis, the selection of a mesh control scheme greatly influences the robustness. Improving the robustness of the analysis is essentially important for the parametric analyses with a wide range of parameter sets to be tested. In the present study, we incorporate the solid-extension mesh moving technique (SEMMT) together with a specialized mesh design surrounding the flapping wing into our analysis system to improve the robustness. Furthermore, we quantitatively demonstrate the effectiveness of the above mesh control technique in 3D flapping problems by measuring the degree of mesh distortion. Using the improved FSI analysis method, we have succeeded in conducting wide range parametric studies of flapping motions to compare active and passive pitch motions and investigate lead-lag motions. We found that passive pitch caused due to appropriate Young's modulus in an elastic portion was able to produces a high lift coefficient which was almost equivalent to active pitch cases. We also confirmed that some lead-lag motions enhance the lift force.