Numerical Simulation On Cavitating Flows For Underwater Vehicle

Cavity is a kind of low-density vacancy filled with vapor and gas that can cover thewhole surface of vehicle, which can be formed in two ways. One is the vaporization causedby a high velocity, which results in the pressure of liquid close to the surface of vehicle fallingbelow the saturation vapor value, the other is ventilation by injecting noncondensable gasartificially through ventilation device. When the length of cavity approaches to or exceedsthat of vehicle, the supercavity is formed. Since most of its surface is surrounded by vapor orgas when the vehicle is in a regime of supercavitating flow, only a portion of the head and tailsurfaces is in contact with water, thus the value of vicious drag will decline to a great extent,and the total drag coefficient of vehicle can be reduced by nearly an order of magnitudecompared to that of full-wet flow regime.Researchers has obtained breakthroughs in the field of scientific research on high speedand drag reduction of underwater weapons by applying supercavity technologies in recentyears, while solutions to a series of technologies related to fluid dynamics during thedevelopment of supercavity weapons must be depended on the acquaintance and grasp of thecomplicated phenomenon and mechanism of supercavitating flow. In this dissertation, thestructure and hydrodynamic characteristics of cavitating flow for underwater vehicle wereinvestigated in detail using a method combining theoretical analysis and numerical simulation,the specific works are as follows:Based on the Mixture multiphase model, the steady cavitating flow around six bodies ofrevolution with different forebodies was calculated and researched numerically using theSinghal’s full cavitation model, while the simulation results of surface pressure coefficientsaccord well with correlative experiment data. The cavitation characteristics of variousforebodies were compared and analyze, and the steady cavitating flow for body of revolutionwith a blunt fore-body under different cavitation numbers was calculated as well toinvestigate the variance law of cavity main dimension with cavitation number, numericalresearch on transient cavitating flow for blunt projectiles with several masses was also carriedout by employing the Dynamic Mesh Technology in FLUENT, it was found that the variancelaw of transient cavity dimension with natural cavitation number for projectile during themotion process consists with the SCAV software, it was also revealed that the main cavity size at the same range corresponding to the projectile with a bigger mass shows to be larger on thewhole when the initial velocity is fixed.Numerical calculation of unsteady natural cavitating flow for the traditionalvariable-drag cavitator was implemented applying the Dynamic Mesh Technology to analysethe ability to cavity shape control for cavitator with different central angles and itscorresponding variance laws of drag coefficient and cavity size with working stokes, theaffect of central element velocity was investigated as well. The results illustrate that throughthe movement of inner central element, the traditional variable-drag cavitator can makeeffective adjustments and control to the supercavity size and drag coefficient withoutchanging the cavitator diameter, the adjustment range broadens with the decreasing of conicalangle for central element, and the transient response of cavity size with working stoke showshysteresis to that of drag coefficient.A new scheme named variable-lateral-force cavitator which was focused on the controlof lift, drag and lateral force for underwater vehicle was proposed on the basis of theory oftraditional variable-drag cavitator, the scheme was declared as a application for inventionpatent and has been authorized recently. The phenomenon of three-dimensional steadycavitating flow around the cavitator at different control statuses was researched using theFLUENT software, the influences of longitudinal displacement of the control element andattack angle on cavity shape and hydrodynamic characteristics were investigated. It was foundthat the cavity size and drag coefficient obtained by the variable-lateral-force cavitator at thesame natural cavitation number will become larger as the longitudinal (lateral) displacementincreases. When the displacement on either side is inconsistent with each other, the cavitatorwill yield a pitch or yaw force whose value is equivalent to30%of drag, and the cavity axiswill be deviated. Through the calculation of cavitating flow for the variable-lateral-forcecavitator under different attack angles, it was found that both the lift and drag coefficients ofcavitator decline to a certain degree as the increasing of attack angle, and the axis deflectionin cavity latter part decreases simultaneously, while the length of cavity presents a trend ofrising on the whole.The mass and energy transfer model between water and water vapor was importedthrough UDF function to simulate the high-temperature unsteady ventilated cavitating flowincluding phases of gas, vapor and water, the results confirm that the model can predict variance of temperature and generation of vapor phase in flow field reasonably, the variancelaw of cavity dimension and hydrodynamic parameters with flow time was studied as well,and the effect of parameters of ventilation, model shape and incoming flow on the shape, flowfield structure and drag-reduction characteristics of high-temperature ventilated cavity wasinvestigated. The calculated results reveal that the size of ventilated cavity with a hightemperature shows to be larger than that of cavity with a normal temperature, and theinterface of cavity will fluctuate under a higher ventilation temperature, a small amount ofwater vapor will be produced simultaneously, and the gas density nearby ventilation orificespresents a directly opposite trend of variance to temperature.The complex mechanism of cavitating flows for natural cavity and multiphase ventilatedcavity was penetratingly understood and mastered in this dissertation by means of numerousnumerical simulation and theoretical analysis, this can provide technological methods tofurther researches on underwater supercavity weapons.