Numerical Studey Of Tadpole Swimming In Complex Vortex Flow

Complex vortex flow is a situation that aquatic organisms often with,such as school swimming and swimming behind obstructions.Studies have shown that aquatic organisms have the ability of changing their movement with the flow field to use the vortex in the flow field to save their own energy.For non-streamlined tadpoles,however,study about tadpoles swimming in complex vortex flow under relatively high Reynolds number is not common.In this paper,we conducted numerical experiments under relatively high Reynolds number,about tadpoles' school swimming in in-phase and anti-phase patterns,a tadpole swimming behind a D-cylinder at fixed position and a tadpole swimming behind a D-cylinder freely on X-direction with a control method of tail beat frequency.Our study shows that,swimming in paralleled school,non-streamlined tadpoles behavior differently to streamlined fish.When swimming in in-phase pattern,thrust decreases as the distance between tadpoles narrowing,combined with the total power consuming decreasing.Swimming in anti-phase pattern,thrust also becomes lower as tadpoles closing to each other,but it's higher than the thrust of in-phase pattern.The total power consuming increases significantly in anti-phase pattern as the distance between tadpoles is relatively close.In all tadpole swimming cases during our study,there is a pair of vortices shedding from the rear area behind the non-streamlined tadpole head,and these vortices help increasing the thrust generated by tadpole tail.In the study of a tadpole swimming behind a D-cylinder,there is a resonance phenomenon when the tail beat frequency if close to the vortex shedding frequency of D-cylinder.In a small frequency range,the D-cylinder vortex shedding frequency maintains the same with tadpole tail beat frequency.And in this frequency range,drag force and power consumption of the tadpole's swimming appear an abnormal change against the major trend.In our numerical experiment,by controlling the tail beat frequency of tadpole,it is possible to make the tadpole achieve hovering behind the D-cylinder when the movement on X-direction is free.Under this movement,the tail beat frequency of tadpole equals to the vortex shedding frequency of D-cylinder,and the power consumption of tadpole is almost zero.It is coincident with the characteristics of the Kármán Gait in experimental observation of fish swimming behind a D-cylinder.At the resonant frequency mentioned above,by controlling the tadpole's tail beat amplitude,we can also achieve the tadpole hovering after the D-cylinder.However,compared to the controlling of tail beat frequency,tadpole's swimming condition is less sensitive to the change of tail beat amplitude.The range of tadpole's tail beat is close to the diameter of D-cylinder after the swimming condition reaches stable.It is also similar to the phenomenon in Kármán Gait experiment.