| Flow problems in the nature field and engineering often involve moving boundaries.The theoretical,computational and experimental research on the flow of moving bound-ary is relatively short of the fruitful results obtained by the static fluid mechanics.In recent years,studies on a slender body experiencing unsteady motion are relatively less than those of steady flows around a body with fixed attatck angle.First,little studies are focused on the unsteady free motion of high attack angle.Second,little studies are focused on stratified flows past a moving marine.A prolate spheroid with the ratio of 6:1 has a simple geometric structure.However,Flows with high attack angle is a complex 3-D separation flow.It is of great military significance to study the moving boundary of a 6:1 prolate spheroid under free motion in a stratified fluid.Firstly,the following studies have been done on numerical method.(1).Based on the Boussinesq approximation,the continuous stratified flow model is realized by a dimensionless ? which is decided by the weighting of the maximum and the minimum density.The pressure boundary conditions are associated with the velocity on inlet and outlet boundary in order to improve the convergence of numerical model.(2).Based on the Finite Volume Method,a reliable numerical discretization method for unstructured tetrahedral mesh is established.A hybrid linear upwind and central dif-ference schemes based on a TVD limiter is better to suppress the non-physical oscillation for the convective term.In order to correct the non-orthogonality of tetrahedral meshes,the over-relax nonlinear correction with a corrected 0.333 is used for the diffusion term.(3).The LES model is established by the Improved Delayed Detachedd Eddy Sim-ulation(IDDES)method.The classical Spalding model is chosen for the wall function,which is not only applicable to the logarithmic layer,also suitable for the viscous sub-layer and transition layer,preventing the local y+ is too small(y+<30)to ensure meetting the requirement of y+ less than 300.(4).Two-way fluid solid coupling based on Aitken acceleration algorithm is realized.The PISO algorithm is improved.The iteration efficiency and adaptability to mesh deformation are increased.(5).Netgen mesh partition provides a good tetrahedral mesh quality and ensures that the maximum non-orthogonal angle is not greater than 55°,and the average non-orthogonal angle is 30°.A hexahedron mesh deformation is used in prolate small-scale pitching motion.A mesh topology deformation and real-time partition method based on tetrahedral dynamic mesh changing is used in prolate large-scale autonomous motion.Secondly,free attenuation pitching oscillation and forced continus pitching oscilla-tion are simulated in a stratified fluid.(1).The numerical simulation of the flow around the 6:1 prolate spheroid under 45° angle of attack is carried out.It is found that the Froude number in transition is around 6.5.In the region of Fr<6.5,the wake is mainly vertical in the near tail area,with a cake structure in the far tail region.(2).The free attenuation pitching oscillation result shows that both sides of the prolate spheroid form four vortex,vertical stratification limits vertical propagation of vortex rings,also accelerated the vortex rings disappear.The limitation contributes to the development of horizontal movement.With the increase of inflow velocity,the drag coefficient does not increase but decrease,which shows that the law of negative damping still appears for the free pitching oscillation.(3).The high frequency forced pitching oscillation result shows that oscillation stirs up the surrounding fluid,expands the vortex upward and downward.The radia-tion point of density vortex consists of the head and tail of prolate,and the spot 2/3L from the head,where density courtour gradient changes violently.The reason for vor-tex break is that vertical stratification restricts the vertical propagation of vortex and accelerates the disappearance of vortex,which encourages the horizontal vortex motion.The flow direction is propagated in the form of inclined upgoing waves and inclined downgoing wave.The spread direction has characteristics of first doublet,then singlet,good persistence and stable waveform.With the increase of flow velocity,the density contour of the vertical downward density wave appears a spiral density vortex.That is,the propagation of density wave does not propagate in a linear uplift mode,but goes downward in the form of spiral density wave.The propagation of density wave is also accompanied by the change of pressure.The increase of the forward pressure and the de-crease of the back pressure mean drag growing.The pressure difference is most obvious under the condition of low Reynolds number.The viscous shear stress of the oscillating prolate has the properties of periodic variation,and the change period is consistent with the oscillation period.Thirdly,six-DOF free motion is simulated in a stratified fluid.(1).It is suggested that different Re numbers cause different surface perssure distri-butions and wake vortex structures for the free-moving prolate spheroid,which in turn influences the moving trail and attitude of the spheroid.When the Reynolds number is 10000 a braid vortex is gradually pulled out at the front end of the spheroid.This braid vortex is accompanied by downward pressure,which makes the spheroid head go down,and its attack angle decreases with the free fall.When the Reynolds number is 4.2 × 106,the annular vortex shedding gradually begins from the end of the prolate.On the surface of the proalte,vortices of upward motion are formed gradually from the tail to its two sides.As freedom falls,attack angle becomes larger,even more than 90°.(2).The falling velocity in stratified flow is obvious slower than in uniform flow.The internal wave formed by the prolate is mainly solitary wave accompanied with non-linear random wave whose wave form varied with the prolate moving. |
PDF Full Text Request