Study On The Motion Performance Of Underwater Gliders Under The Effects Of Depth

As a new class of buoyancy-driven underwater vehicles,underwater gliders(UGs)are being applied in various fields for their features of low energy consumption,low cost,and long range.Considering the highly complex and dangerous ocean environment,the motion performance of UGs,including their motion characteristics,dynamic stability and maneuverability,is important basis for their preliminary design.With the increasing application demands for UGs in the deep-sea and large-scale region,improving the prediction accuracy of their motion performance when running in the large-depth-difference environment becomes the key to advancing their applications.On the motion characteristics of UGs,common dynamic models generally neglect the effects of seawater depth variation on the UGs,which,however,produces large prediction errors on their motion in the deep-sea region,as the variation of buoyancy caused by seawater depth variation is of the same order of magnitude as driving force(net buoyancy)of UGs.On the dynamic stability and maneuverability,because of their unique buoyancy-driven mode,the UGs differ greatly in dynamic model and working principle from the propeller-driven underwater vehicles,such as submarines,torpedoes and autonomous underwater vehicles.Therefore,it’s not suitable to use the evaluation models and indexes of dynamic stability and maneuverability for the latters,which are established on the basis of their dynamic models,to evaluate the performance of UGs.There is a lack of applicable evaluation models and indexes to UGs.The effects of seawater depth variation and hydrodynamic shape on their motion performance still needs to be investigated further.In addition,as the uncertainties of key model parameters,including seawater density and the distance between the center of gravity and the center of buoyancy,are neglected in the common certainty methods,it’s difficult to precisely predict the boundaries of motion performance of UGs in the real ocean environment by those methods.To improve the prediction accuracy of common dynamic models,in the present work,a full dynamic model was proposed by considering the variation of seawater density and hull deformation with seawater depth,which was used to study the effects of seawater depth variation on the motion characteristics of the UG and precisely predict its motion characteristics in the deep-sea and large-scale region.The results showed that the buoyancy variation caused by seawater depth change resulted in the decreasing of the magnitude of velocity and pitch angle,the asymmetry of motion parameters during the ascent and descent gliding motions,and the decreasing of radius of spiral motion.To solve the lack of applicable evaluation models and indexes of dynamic stability and maneuverability to UGs,based on the established applicable dynamic model and evaluation models(the free perturbation and maneuverability equations)in the vertical plane,the applicable evaluation indexes were derived and the effects of seawater depth variation on the dynamic stability and maneuverability of UGs were investigated.The results showed that,because of their unique buoyancy-driven mode,the evaluation models and indexes of UGs had significant difference with those of the propeller-driven underwater vehicles.As velocity of the UG was affected by seawater depth variation,its dynamic stability was also affected obviously.With the increasing seawater depth,its dynamic stability became stronger and its change rate of surfacing speed increased obviously.To clarify the relationship between the hydrodynamic shape and the motion performance,and improve the motion performance of UGs,a novel experiment setup with the ability to measure the angle of attack directly was developed based on the computer vision technology.The setup was used to investigate the effects of airfoil of the wings(one of the most important structures of hydrodynamic shape)on the motion performance of UGs,which enriched the motion performance theory of UGs and provided guidance for the design of UGs with deformable wings.The results indicated that with increased trailing-edge angle of the wings,the UGs could keep high gliding economy in the large-pitch motion and also showed stronger dynamic stability and larger change rate of surfacing speed,and that the UGs with wings of symmetrical airfoil showed better performance in the depth-keeping motion.To improve the prediction accuracy of motion performance under the uncertainty of parameter estimation,the models of uncertainty seawater density and distance between the center of gravity and the center of buoyancy were established by probability modeling and parameter identification based on the sea trials.Effects of these two uncertainty factors on the dynamic stability and maneuverability of UGs were investigated,which improved the prediction accuracy of motion performance.The results showed that,the uncertainty of motion performance of UGs mainly depends on that of seawater density.With the increasing seawater depth,the dynamic stability uncertainty of UGs became weaker,and the change rate uncertainty of their surfacing speed showed less difference.The present work enriches the dynamics and motion performance theory of UGs,and provides a precise dynamic model,evaluation indexes of dynamic stability and maneuverability,motion strategies,and valuable guidance for their applications in the deep-sea and large-scale region.