Hydrodynamic Characteristics And Scale Effect Of Undulating Fin

A new type of amphibious robot,equipped with bionic undulating fin as propeller,has the characteristics of stealth,high mobility,environmental adaptability and high efficiency at low speed,which is of significant significance for promoting the construction of national coastal defense and serving the national strategy of "maritime power" and improving the military’s equipment combat and reconnaissance and detection capabilities.Propulsion efficiency is an important index to measure bionic propulsion.With the development of the application of undulating propulsion,the size range of the amphibious propeller with undulating fin as the main propulsion method changes greatly.Due to the difference of characteristic length,the difference of Reynolds number will result in the scale effect of hydrodynamic performance.Based on the long dorsal fin of hairtail propelled by MPF mode,this paper established its kinematic model through motion simplification and approximation,used Fluent software based on finite volume method to conduct numerical simulation of hydrodynamic performance,and used UDF to conduct secondary development of software to solve its propulsion efficiency.The effects of motion posture and fin surface shape are also studied.Finally,the scale effect of hydrodynamic characteristics is studied by using two similarity criteria of Froude number and Strouhal number.The main contents of this paper are as follows:First of all,based on the secondary development of Fluent UDF,the program for calculating the propulsion efficiency of undulating fin is established,and the numerical model for calculating the hydrodynamic performance of undulating fin is established.The experimental verification shows that the numerical prediction results of thrust coefficient have certain accuracy.Secondly,by controlling the single variable method,the wave number,amplitude,wave frequency and other wave parameters of the undulating fin were numerically simulated,and the variation law of the hydrodynamic performance under the single variable was obtained.The thrust mechanism was revealed by in-depth analysis from the perspective of the characteristic contours.And then,by changing the angle of attack of the undulating fin,the motion attitude of the undulating fin is changed,and then the Angle of water push of the inclined plate on the fin surface is affected.The propulsion mechanism of undulating fin is confirmed.The influence of leading-edge suction on hydrodynamic performance of undulating fin was studied by changing the shape of its front and rear edges.Finally,in this paper,the dimensionless number conversion is carried out under the two similar conditions of Froude number and Strouhal number,and the scale effect of the hydrodynamic characteristics of wave fins under three conditions is studied by numerical calculation.It is found that different results are obtained when the thrust coefficient is used to calibrate the critical Reynolds number of fin,and the same results are obtained when the Froude number is used as the similarity criterion or the Strouhal number is used as the equal flow.The conversion error of similarity criterion of Strouhal number at constant frequency is slightly larger.For the calibration of the critical Reynolds number of propulsion efficiency,lateral force coefficient and lift coefficient,relevant studies should be carried out for further correction.