Research On Hydrodynamic Characteristic Of Vehicle Unsteady Cavity Flow

Supercavitation drag reduction is an effective way for realizing high-speedmoving of underwater vehicle, and this technology has a broad applicationprospect. Motion stability of the underwater vehicle is one of the importantpreconditions to insure the vehicle security. The unsteadycharacteristic of cavityflow has significant effect on hydrodynamics and motion stability of the vehicle.Therefore, it is necessary to research the hydrodynamics of the unsteady cavityflow in order that the supercavitation drag reduction can be improved andapplied.There are many influencing factors on the unsteady cavity flow, such as theinstability of ventilation, the change of the vehicle’s motion state, external flowfield disturbance and so on. Domestic and overseas scholars have manypublications on it, but many researches on the unsteady characteristic of thecavity flow are experimental methodology. And the numerical methods about theunsteady characteristic of the cavity flow mainly aim at simple physical models,such as cavitator and hydrofoil. These models are far cry from the realunderwater vehicle in geometrically. In addition, the researches abouttheunsteady cavity flow are generally based on the potential flow assumption.Considering the stability of the vehicle is realized by changing the control forceof control surface and the thrust vector, experimental and numericalmethodologiesare used to study the cavitator rotation, hydrofoil motion, variablemotion, pitching motion and the hydrodynamics of theunsteady cavity flow. Onthis basis, combining the equations of motion of the vehicle and the equations ofcontrol of the cavity flow, a numerical simulation method is presented toforecast the attitude and trajectory of the vehicle with supercavitation. The mainworks of this paper are as follows:Based on homogeneous equilibrium flow theory, the numerical results ofthe unsteady cavitation flow are calculated by using multiphase flow model,turbulence model and cavitation model. The rational mathematical model andnumerical method are selected by comparing numerical results with experiment.Water tunnel experiments are conducted to study characteristics ofventilated supercavitation. The influence of the attack angle of cavitatoronventilated cavity and hydrodynamicsare studied. Combining experimental results,numerical simulations are performed to research the influence of cavitatorrotation of the vehicle on the hydrodynamics of the unsteady cavity flow in thelocal cavity and supercavitation states.Adopting the method of artificial ventilation, the water tunnel experimentsaiming at the hydrofoil of the vehicle with supercavitation are performed,and theinfluence of hydrofoil wedge angle on cavity shape and hydrodynamicsareanalyzed. In addition, using numerical simulations, the influence of hydrofoilposition, hydrofoil eject, hydrofoil attack angle on the hydrodynamicsof theunsteady cavity flow are studied.By adopting the method of relative movement, setting the time function of inflow velocity, numerical simulations are performed to realize the variablemotion of the vehicle. Based on d’Alembert principle, equations of fluid relativemotion are derived. Considering the convected effect of non-inertial coordinatesystem, the equations of fluid relative motion are used to replace the N-Sequations, and the accuracy of numerical simulations is improved. Cavitationmorphology changes and hydrodynamics of the vehicle’s variable motionprocess are studied in the natural cavitation and ventilated cavitation states, andthe relationship between natural cavitation acceleration of the vehicle and thetime lag of cavity form.Numerical simulations are performed to study the supercavitation flow fieldin the pitching motion process of vehicle, and the influence of the pitchingangular velocity and cycle on the hydrodynamics of theunsteady cavity flow isobtained.The equations of motion of the vehicle relative to the ground coordinatesystem are obtained by using theoretical derivation. Coupled solutions of themotion equations of the vehicle and the control equations of cavitation aresolved by using UdF secondary development. Numerical model of thelongitudinal plane attitude and free-flight trajectory of the vehicle is established.