The effects of caudal fin deformation on the hydrodynamics of thunniform swimming under self-propulsion

To investigate the effects of the caudal fin deformation on the hydrodynamic performance of the self-propelled thunniform swimming,we perform fluid-body interaction simulations for a tuna-like swimmer with thunniform kinematics.The 3-D vortices are visualized to reveal the role of the leading-edge vortex(LEV)in the thrust generation.By comparing the swimming velocity of the swimmer with different caudal fin flexure amplitudes fa,it is shown that the acceleration in the starting stage of the swimmer increases with the increase of fa,but its cruising velocity decreases.The results indicate that the caudal fin deformation is beneficial to the fast start but not to the fast cruising of the swimmer.During the entire swimming process,the undulation amplitudes of the lateral velocity and the yawing angular velocity decrease as fa increases.It is found that the formation of an attached LEV on the caudal fin is responsible for generating the low-pressure region on the surface of the caudal fin,which contributes to the thrust.Furthermore,the caudal fin deformation can delay the LEV shedding from the caudal fin,extending the duration of the low pressure on the caudal fin,which will cause the caudal fin to generate a drag-type force over a time period in one swimming cycle and reduce the cruising speed of the swimmer.

Numerical and experimental studies of hydrodynamic performance of bionic leading-edge tubercle airfoil

This paper studies the bionic tubercle airfoils based on the NACA 0020 prototype airfoil,with 7 models derived from different designs of the span-chord ratios and different tubercle types,along with the option of the wave airfoil surface matching the leading-edge tubercle.Open water experiments are conducted to test the airfoil profile at a Reynolds number of 3×10^5 and an angle of attack of 0°-30°.Data are recorded by a four-component force balance and the resulted lift,drag,and force moment are analyzed.From the analysis of the hydrodynamic characteristics and the flow field characteristics,it is shown that the stall of the tubercle airfoils is milder without an evident stalling angle as compared with that of the normal airfoil profile.The lift coefficient of the optimized airfoil is significantly elevated at the angle where the NACA 0020 airfoil is in the stall condition.The lift-drag ratio is also markedly improved at the same angle.The optimal airfoil profile is finally identified and a further optimization plan is proposed based on the comparison results against the CFD numerical simulation.