Issue 27

H. Liu et alii, Frattura ed Integrità Strutturale, 27 (2014) 53-65; DOI: 10.3221/IGF-ESIS.27.07 57 Figure 5: Force analysis diagram of the fluidic amplifier. D ETERMINATION OF BOUNDARY AND INITIAL CONDITIONS USING CFD Computational domain and mesh model his study combined the simulation of the fluid and the motion of the impacting body by using the dynamic mesh technique and a user-defined function(UDF) [16-21]. The computational domain of the fluid field in the YSC178 liquid jet hammer includes the fluid field of a fluidic amplifier, two side passages connected to the vents of the fluidic amplifier, the upper and lower chambers of the cylinder, as well as the connecting passages between the fluidic amplifier and the cylinder. The computational domains were meshed using the Altair Hypermesh software. The mesh model was used as the starting volume mesh for the dynamic mesh modeling technique. The mesh of the computational domains was shown in Fig. 6. In this mesh model, 1636 out of 37713 elements were pentahedron, and the rest were hexahedral. Figure 6: Computational domain and boundary conditions. The mesh dependence test was performed for the mesh model of the YSC178 liquid jet hammer. Three sets of meshes were tested. As shown in Tab. 1, the maximum difference between the obtained results is less than 5.2%. The coarse mesh can also provide sufficient mesh independency. Considering the computational time cost versus the model accuracy, computations were performed using the medium mesh. Number of elements Fluid pressure a (MPa) Terminal velocity b (m/s) Coarse mesh 18562 0.78 1.90 Medium mesh 37713 0.81 1.85 Fine mesh 75169 0.82 1.82 % Difference c 5.13 4.39 a The approximate fluid pressure on the sidewalls of the fluidic amplifier. b The terminal velocity of backward stroke of the impacting body. c The percentage difference between the coarse and fine mesh. Table 1: The details of the mesh dependence test for the YSC178 liquid jet hammer. T