Numerical analysis of hydraulic properties of bionic nozzles and structural optimization based on CFD–FEA

The bionic nozzle was designed based on earthworms’ non-smooth surface structure and the properties of auxiliary jet holes. The drag reduction mechanism and hydraulic characteristics of convex structure-auxiliary jet hole inner wall nozzle (SG), convex structure inner wall nozzle (VS), concave structure inner wall nozzle (CS), and smooth inner wall nozzle (S) were simulated and analyzed using computational fluid dynamics (CFD). The simulation results show that the maximum instantaneous velocities of the four nozzles are SG (29.2 m/s) > VS (28.79 m/s) > CS (28.55 m/s) > S (28.35 m/s). The biomimetic nozzle's convex structure generates a reverse velocity field, and this causes the low-speed fluid to cluster along the vertical direction of the jet, generating a low-speed turbulent band and enabling the low-speed field to constantly rise toward the jet's center area, thus reducing fluid resistance. The auxiliary jet holes can further reduce the intensity of turbulent processes and achieve the goal of reducing drag. Therefore, the SG-shape has the most significant drag reduction effect. Based on the coupling of CFD and finite element analysis (FEA), the relationship between input parameters (auxiliary jet hole diameter P1, convex structure diameter P2, and auxiliary jet hole length P3) and output response (maximum principal elastic strain and maximum principal stress) was determined. Response surface methodology was used to optimize P1, P2, and P3 parameters of SG-shape, resulting in the optimal combination of P1 = 1.45 mm, P2 = 2.71 mm, and P3 = 9.27 mm. Through CFD simulation and water jet impact test verification, the maximum velocity of the optimized biomimetic nozzle is 31.2 m/s, which is 6.8% higher than before optimization. The maximum impact force generated by the optimized water jet is 8.61 KG, a significant increase of 7.5% compared to before optimization.