Investigation On The Characteristics For Large-scale Vortex Structures Generated By Submerged Momentum Soursces

The mechanical effects induced by the body and the propeller thrust, as well as thecooling water discharge for an underwater vessel can be equivalent to some momentumsources under the action of the background fluid, where the body produces a drag momentum,while the propeller thrust and cooling water discharge produce jet momentums, this meansthat the mechanical effects of the underwater vessel wakes can be simulated by somesubmerged momentum sources. Some large-scale vortex structures will occur during theevolution procedures of such momentum wakes in the background fluid. In particular, somespecial flow patterns on the free surface due to such large-scale vortex structures can easilyrecongnited by the SAR. Therefore, the investigation of the dissertation is of the academicimportance and the potential military application for the SAR detection of the underwatervessels.On the basis of reviewing and summarizing recent progress on the subject, thedissertation presents a combined experimental, theoretical and numerical investigation for theformation mechanisim and characterization of the large-scale vortex structures generated bythe submerged momentum sources in the density homogeneous and stratified fluids.In charpter II, a constant-pressure spillover system is designed to produce a submergedLaminar round jet with the constant outlet velocity, the evolution mechanism andcharacteristics due to such a jet momentum in a viscous uniform fluid are conductedexperimentally, and analyzed theoretically by use of the Stokes theory and empirical modelfor different combinations of the injection time and Reynolds number, where three non-dimensional parameters for the mushroom-like vortex structure generated by such asubmerged laminar round jet, including the length of the jet L*, the radius of themushroom-like vortex R*and the length of vortex circulation d*, are defined, and the variationcharacteristics of these parameters with non-dimensional time t*are quantitatively analyzed.Serial experimental results show that the formation and evolution procedures of such amushroom-like vortex can be described as three stages: starting, developing and decayingstages. In starting stage, L*and d*increases linearly with t*, while R*approximately remains aconstant. In developing stage, the mushroom-like vortex structure shows a significantself-similarity, where L*, R*and d*are approximately proportional to t*1/2for differentcombinations of the injection time and Reynolds number. In decaying stage, two kind ofdecay cases happen for the mushroom-like vortex structure. In the first case, the decayhappens after the end of the submerged round jet, where L*and R*are related to t*1/5, and d*approximately remains a constant. In the second case, the decay happens before the end of thesubmerged round jet, where some broken phenomenon occurs for the mushroom-like vortexstructure after the jet momentum reaches some critical value.In charpter III, a numerical investigation on the evolution mechanism and characteristicsof the submerged laminar round jet in a viscous homogenous fluid is conducted by use of thecomputational fluid dynamics method based on the incompressible Navier-Stokes equation.The variation characteristics of three non-dimensional parameters L*, R*and d*for themushroom-like vortex structure with the non-dimensional time t*are quantitatively analyzedby use of the Stokes theory and empirical model, results agree well with the experimental andtheoretical ones. Moreover, velocity characteristics at the secondary backflow point, themomentum and geometry centers, the distribution features of the vertical vorticity, as well asthe vorticity-stream function relationship are analyzed for the mushroom-like vortex structure.In charpter IV, the formation mechanism and evolution characteristics of a submergedround jet in a viscous uniform fluid are investigated experimentally by use of theconstant-pressure spillover system with a L-shape jet tube. The experiments are conductedunder different combinations of Reynolds number Re, confinement number C and nondimensional draft d/H, where d is the vertical distance from the submerged jet to the freesurface, and H is the depth of the ambient fluid. Four flow patterns are identified for variousconfinement number C. When C<1, the submerged jet shows the deep-water pattern, while for1≤C<2, it shows the transitional pattern, where the submerged jet does not develop astructured flow for such two jet patterns. When2≤C<10, the submerged jet shows theshallow-water pattern, while for C≥10, the submerged jet shows the extreme-shallow-waterpattern. In both these two patterns, the submerged jet generates vortex dipole structures forthe two jet patterns. In the extreme-shallow water pattern, the nondimensional vortexformation time tf*for the vortex dipole structure is proportional to the nondimensionalinjection time Tinj*for various drafts d/H. In the shallow-water pattern, tf*linearly dependson Tinj*Re1/2when the draft d/H=0.5, however there is no observable relationships between tf*and Tinj*for other drafts d/H.In charpter V, experiments are conducted in a stratified fluid with a momentum sourcemodeled by a horizontally moving jet. The generation mechanism is investigated, as well asthe evolution characteristics are analyzed for the quasi-2D dipolar vortex streets generated bysuch a moving momentum source in the stratified fluid. The combination conditions underdifferent Reynolds and Froude numbers (Re, Fr) where the dipolar vortex streets can begenerated by moving momentum sources are determined by use of data based on the presentseries experiments. Moreover, the dependence of the dimensionless formation time andinverse dimensionless average wavelength of the dipolar vortex street on the Froude numberFr for different values of Re is obtained, results show that these parameters are independent ofRe and approximately follow some power exponent laws on the Froude number Fr.In charpter VI, the model of the towed sphere with a L-shape jet tube was used togenerate both drag and jet momentum wakes due to the body and cooling water let of anunderwater vehicle, the results of experiments are presented for the evolution characteristicsof such combined momentum wakes in a stratified fluid. It is shown that for JD/J>CD, theevolution characteristics for the combined momentum wakes are mainly attributed to the dragmomentum wake generated by the towed sphere,and the far wake becomes quasitwo-dimensional(2D) in the form of a Karman-like vortex street, where JDis the drag momentum flux due to the towed sphere, J is the jet momentum flux due to the L-shape jettube and CDis the drag coefficient of the sphere. For JD/J<CD, the evolution characteristics forthe combined momentum wakes are strongly dependent of the (Rej, Frj) combinations, and thefar wake can become quasi2D in the form of an anti-Karman-like vortex street being mainlyattributed to the jet momentum wake under some (Rej, Frj) combinations, but there may be nooccurrence of large-scale coherent structures, where Rejand Frjare Reynold and Froudenumbers duo to the L-shape jet tube respectively. The combination conditions under differentReynold and Froude numbers (Rej, Frj) where the anti-Karman-like vortex streets can beformed by such combined momentum wakes are determined by use of data based on thepresent series experiments, and the dependence of the dimensionless formation time andinverse dimensionless average wavelength of the quasi2D vortex streets on Frjfor differentvalues of Rejis obtained, results indicate that these parameters are independent of Rejandapproximately follow some power exponent laws on Frj.