Numerical Simulation Of Air-injetting Parameters For Microbubbles Drag Reduction On Transitional Craft

Air Cavity Craft is a new type of energy-efficient ships,which uses micro-bubble drag reduction technology.At home and abroad,a large number of studies show a unified understanding that is the effect of the bubble volume concentration,main flow velocity,air-jet volume,gravity and so on to microbubble drag reduction.But a unified understanding which is about the effect of the scale of the bubbles and the size of air injection orifice and slot to microbubble drag reduction hasn't been formed.This paper,based on Project 863 "high-speed Air Cavity Craft hull form study"(2006AA11Z223),takes the high-speed transition craft as study object and studies the effect of the step-fault bottom jet parameters to drag reduction by numerical method,in order to find out the matching relationship of the height of air injection slot,air-jet volume and main flow velocity,under the ideal drag reduction.In this paper,the physical model used in the numerical calculation is a good transition craft.And then the model used 1:13 scale is made.In order to make the gas inject along the main flow in the bottom wall,dam-break at a suitable location in the bottom is needed.According to the pressure measurement in the towing tank,the location for dam-break is determined near the 7^th section.This paper figures out the government equations(continuity equation,momentum equation and the Reynolds stress equations) of the model without air-injecting by using finite volume method and SIMPLE algorithm,taking standard turbulence model,and neglecting the influence of free-surface.On this basis,and then this paper figures out the two-phase flow's government equations containing the Reynolds-averaged of micro-bubbles (including the mixed-phase continuity equation and momentum equation,group equation,Reynolds stress equations,etc.)by using SIMPLEC algorithm and considering the relative sliding of water and bubbles.After comparison numerical analysis,the difference of viscous flow and the resistance under the model without air-injecting and with air-injecting and the influence of the height of air injection slot,air-jet volume and main flow velocity on the drag reduction is took.Get the following conclusions:â‘ Compared with the non-air jetting,the wall shear stress under air-injecting significantly reduced.â‘¡Under the same R_e,the volume fraction of bubbles and the effective coverage of bubbles near the wall increase along the rise of the relative air-jet volume.And the relative rate of friction resistance reduction is also gradually increasing.â‘¢In addition to the lower air-jet volume,the bubble coverage of the bottom surface increases along the rise of R_e under the same air-jet volume.While R_e is equal to 10.7×10⁶,the maximum relative rate of friction resistance reduction is made.â‘£The escape place of the bubble is moved backwards while R_e increases.The relative air-jet volume and the non-dimensional height of the air injecting slot have less influence on the escape place of the bubble under the same R_e.⑤Under the same relative rate of friction resistance reduction,the relative air-injecting velocity is approach to a fixed value.Considering the drag reduction and power consumption required for the air-jetting,the best non-dimensional height of the air injecting slot is equal to 0.112 while R_e=13.4×10⁶.â‘¥The optimal relative air injecting slot and R_e are non-linear relationship. Along the R_e increasing,the growth rate of B1 drops.