Study on local topology model of low/high streak structures in wall-bounded turbulence by tomographic time-resolved particle image velocimetry

The relationship between the in the logarithmic law （log-law） region of bursting event and the low/high-speed streak a turbulent boundary layer is investigated. A tomographic time-resolved particle image velocimetry （TRPIV） system is used to measure the instantaneous three-dimensional-three-component （3D-3C） velocity field. The momentum thickness based Reynolds number is about 2 460. The topological information in the log-law region is obtained experimentally. It is found that the existence of the quadrupole topological structure implies a three-pair hairpin-like vortex packet, which is in connection with the low/high-speed streak. An idealized 3D topological model is then proposed to characterize the observed hairpin vortex packet and low/high-speed streak.

Coherent structures over riblets in turbulent boundary layer studied by combining time-resolved particle image velocimetry(TRPIV),proper orthogonal decomposition(POD),and finite-time Lyapunov exponent(FTLE)

Time-resolved particle image velocimetry（TRPIV） experiments are performed to investigate the coherent structure＇s performance of riblets in a turbulent boundary layer（TBL） at a friction Reynolds number of 185. To visualize the energetic large-scale coherent structures（CSs） over a smooth surface and riblets, the proper orthogonal decomposition（POD） and finite-time Lyapunov exponent（FTLE） are used to identify the CSs in the TBL. Spatial-temporal correlation is implemented to obtain the characters and transport properties of typical CSs in the FTLE fields. The results demonstrate that the generic flow structures, such as hairpin-like vortices, are also observed in the boundary layer flow over the riblets, consistent with its smooth counterpart. Low-order POD modes are more sensitive to the riblets in comparison with the high-order ones,and the wall-normal movement of the most energy-containing structures are suppressed over riblets. The spatial correlation analysis of the FTLE fields indicates that the evolution process of the hairpin vortex over riblets are inhibited. An apparent decrease of the convection velocity over riblets is noted, which is believed to reduce the ejection/sweep motions associated with high shear stress from the viscous sublayer. These reductions exhibit inhibition of momentum transfer among the structures near the wall in the TBL flows.

Antisymmetric quadrupole mode of coherent structures in wall-bounded turbulence

Abstract Experiments were conducted in a water tunnel by tomographic time-resolved particle image velocimetry （Tomo-TRPIV）. The Reynolds number Reo is 2 460 on the base of momentum thickness. According to the physical mechanism of the stretch and compression of multi-scale vortex structures in the wall-bounded turbulence, the topological characteristics of turbulence statistics in logarithmic layer were illustrated by local-averaged velocity structure function. During coherent structures bursting, results reveal that the topological structures of velocity gradients, velocity strain rates and vorticities behave as antisymmetric quadrupole modes. A three-layer antisymmetric quadrupole vortex packet confirms that there is a tight relationship between the outer layer and the near-wall layer.

Design,Hydrodynamic Analysis,and Testing of a Bio-inspired Movable Bow Mechanism for the Hybrid-driven Underwater Glider

Hybrid-driven Underwater Glider(HUG)is a new type of underwater vehicle which integrates the functions of an Autonomous Underwater Glider(AUG)and an Autonomous Unmanned Vehicle(AUV).Although HUG has the characteristics of long endurance distance,its maneuverability still has room to be improved.This work introduces a new movement form of the neck of the underwater creature into HUG and proposes a parallel mechanism to adjust the attitude angle and displacement of the HUG’s bow,which can improve the steering maneuverability.Firstly,the influence of bow movement and rotation on the hydrodynamic force and flow field of the whole machine is analyzed by using the Computational Fluid Dynamics(CFD)method.The degree of freedom,attitude control range and movement amount of the Movable Bow Mechanism(MBM)are obtained,and then the design of MBM is completed based on these constraints.Secondly,the kinematic and dynamic models of MBM are established based on the closed vector method and the Lagrange equation,respectively,which are fully verified by comparing the results of simulation in Matlab and Adams software,then a Radial Basis Function(RBF)neural network adaptive sliding mode controller is designed to improve the dynamic response effect of the output parameters of MBM.Finally,a prototype of MBM is manufactured and assembled.The kinematic,dynamics model and controller are verified by experiments,which provides a basis for applying MBM in HUGs.