Study On The Mechanism Of The Supercavitating Flow Influenced By Turubulent Drag-reducing Additives

Cavitation, a kind of boiling phenomenon, happens when the local pressure is below the saturation pressure. In hydrodynamic or hydraulic engineering application devices, cavitation is usually an undesirable phenomenon for its negative influences on the performance of hydraulic turbomachinery, su ch as load asymmetry, noise, vibration, surface erosion and so forth. However, the cavitating effects can also be used for positive benefits in some particular applications. Supercavitation is such technology used for drag reduction. The research object of the dissertation is this multiphase flow-supercavitation.In supercavitation scheme, a submerged body is totally enveloped in a large cavitation bubble; the skin friction felt by the navigating body can be reduced dramatically. The key point of maintaining a cavity to survive is to stabilize the surrounding liquid-gas or liquid-vapor interface. Some aqueous solutions of surfactant may possess threefold effects, i.e., reduced surface tension as compared with water, turbulent drag-reducing effect and enhancing the occurrence of boiling phenomenon, which may all play positive roles in the supercavitating drag reduction of underwater navigators.Based on the supercavitation scheme, the infunece mechanism of turbulent drag-reducing additives on supercavitation is investigated. A new control strategy for supercavitating multiphase flow influenced by drag-reducing additives is explored. The detailed content is as follows.The Cross viscosity model is employed to represent the shear-thinning characteristic of the drag-reducing solution. The model is consist with the rheology parameters of PAM and CTAC dilute aqueous solutions measured in experiments, which indicates the feasibility and creditability of the Cross viscosity model.Natural supercavitations in water and turbulent drag-reducing solution were numerically simulated using unsteady Reynolds averaged Navier-Stokes(RANS) scheme with mixture-multiphase model. The Cross viscosity equation was adopted to represent the fluid property of aqueous solution of drag-reducing additives. The characteristics of natural supercavity configuration and overall resistance of the navigating body were presented, respectively. The numerical simulation results indicated that, at the same cavitation number, the length and diameter of supercavity in drag-reducing solution are larger than those in water, and the drag coefficient of navigating body in solution is smaller than that in water. Based on the results, the influence of relaxation time, zero shear viscosity and surface tension are investigated respectively.Water entry supercavities in water and turbulent drag-reducing solution were numerically simulated based on unsteady RANS scheme, together with application of VOF multiphase model. The configuration and dynamic characteristics of water entry supercavity, and flow resistance were discussed respectively. It was obtained that the numerical simulation results are in consistence with experimental data. Numerical and experimental results all show that, the length and diameter of supercavity in drag-reducing solution are larger than those in water; the drag coefficient is smaller than that in water. At the same instant, the projectile velocity is faster and penetration distance is longer in drag-reducing solution. Further more high impact velocity case involving natural cavitation effect is numerical simulated.A new control strategy for ventilated cavitation influenced by turbulent drag-reducing solution is proposed. The configurational and hydrodynamic characteristics of ventilated cavities were experimentally studied in water tunnel. Experimental results show that, within the presently tested cases, the lengths of cavity influenced by drag-reducing solution are smaller than normal condition(ventilated cavity) in water, but the asymmetry of the cavity is improved. The drag resisted by the test model is reduced dramatically and the re-entrant jet is more complex after the CTAC solution is injected into the cavity.The infunece mechanism of turbulent drag-reducing additives on supercavitation is investigated. A new control strategy for supercavitati ng flow influenced by drag-reducing additives is explored. The dissertation has impartant academic value, engineering and military application value.