Numerical Investigation On The Flow Field Of A Shallow Round Jet

The disturbance on the environmental fluid of maneuvering submerged vehicles,such as submarine,is equivalent to the transfer of momentum to the environmental fluid.The effect on the ambient fluid from submerged vehicles can be simplified to a submerged momentum source.In practical ocean environment,a submerged momentum source may induce the formation of a large-scale eddy structure which has a high probability of being captured and tracked by high-precision detection instruments such as synthetic aperture radar(SAR)carried by ocean remote sensing satellites.The research on such a subject has important academic value and potential military significance for the non-acoustic physical detection and perception of submerged vehicles.Therefor,this work is presented to study the the evolution of large-scale coherent vortices and their surface characteristics resulting from a submerged momentum source.In this paper,it is assumed that the momentum source is equivalent to a nozzle jet and the problem is numerical investigated.For the laminar jet,the large-scale coherent vortices formed by a jet in finite depth layer are studied by means of direct numerical simulation.The results show that the structure of the jet appears collapse effect and spiral upward shape due to the existence of upper and lower boundaries,and the distribution of the main dynamic axial velocity is no longer axisymmetric about the center,but only axisymmetric on the horizontal section,which is quite different from that of the unbounded jet.At the same time,the flow fields of z =0 plane and free surface are highly correlated,which indicates that the flow of large-scale eddies shows strong two-dimensional flow characteristics.Further analysis of the vorticity fields in the two planes shows that the average vorticity of the flow field is proportional to the power of time,and the evolution law of the average vorticity in the development and attenuation stages is basically unchanged under different Reynolds number conditions.For the turbulent cases,the RANS method is used to simulate the stage while the jet is acting,and the LES method is used after the jet is stopped.Four different flow pattern appears for different confinement numbers,including deep water pattern with strong 3D characteristics,transition pattern with high 3D characteristics and slight 2D characteristics,shallow pattern characterized by the quasi-2D dipolar vortices with slight 3D feature and extreme shallow pattern characterized by the quasi-2D dipolar vortices with almost no 3D characteristics.According to the Q-criterion,the evolution of the vortex core is investigated on the horizontal and vertical section.The energy and momentum are calculated on the z=0 plane and free surface to discuss their evolution with the vortex core.The results show that after the jet is stopped,the energy on the z=0 plane and free surface declines differently in speed which is larger under a smaller confinement number.The momentum generally declines on the free surface while some sudden jumps can be find in the procedure,which is related to the negative flow induced by the vertical recirculation of the jet.