Manta rays have high swimming speeds when swimming near the ground.We research the hydrodynamic experiments of bionic ray flapping at different near-ground heights.The shape of the bionic ray was designed based on real manta,and the pectoral fin flapping mechanism was based on the real manta rays fluttering near the substrate.The hydrodynamic results of the bionic ray were measured in a water channel by a force sensor.We have obtained the average thrust coefficient and pitching moment of symmetric and asymmetric flapping at different near-ground heights and the average thrust and pitching moment at different flapping frequencies at a certain near-ground height.The force sensor also obtains the instantaneous thrust and lift coefficient in a period.The results show that the thrust and pitching moments generated by the bionic ray flapping near the ground increase with the decrease of the near-ground height,and the pitching moments increase rapidly when the bionic ray flapping height H<0.5.Studying the hydrodynamic of the bionic ray flapping in a period,it can be seen that the thrust coefficient curves have two peaks and valleys,and the peaks and valleys of the thrust coefficient curves correspond to the maximum and the zero point of the attack angle,respectively.The instantaneous lift coefficient curve only has one peak and valley.In addition,with the increase of the flapping frequency the thrust coefficient and pitching moment of the bionic ray increase,while the increase in thrust coefficient decreases with the increasing flapping frequency.It can be seen that the ground effect has a positive influence on the bionic ray generation of the thrust.
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