Large Eddy Simulation Of The Interaction Between Ventilated Micro-bubble And Turbulent Boundary Layer

More than eighty percent of the underwater vehicle drag is friction,which prevents speed increase of the vehicle.It has been verified that the friction drag can be reduced significantly by injecting micro-bubble into a turbulent boundary layer,this can help cargo ship to reduce the energy waste and increase the underwater military weapon's attack speed.Therefore,understanding the interaction mechanism between micro-bubble and turbulent boundary layer contributes to a better application prospect of micro-bubble drag reduction technology,which can also improve our country's economic power and military power.A combination of large eddy simulation turbulence model and mixture multiphase flow model was selected by simulating a large-scale cavitation case,and it was as the best choice to simulate the microbubble-water problem.A three dimensional perturbation method was used to generate a turbulent boundary layer with the Reynolds number 1430 based on the momentum thickness ? and free stream velocity U?.Based on these works,a series of turbulent boundary layer containing with micro-bubble cases were simulated and analyzed.Three different average inlet air volume fractions(?v,in = 0.1,0.2,0.3),five different inlet ventilated micro-bubble thicknesses(h+ = 10,50,200,560,850),as well as the different locations of micro-bubble were considered to examine the influence of micro-bubble on turbulent boundary layer.Results show that,if micro-bubble is located above the flat plate,the large scale vortices in the turbulent boundary layer will be broken into small scale vortices due to micro-bubble effect,velocity fluctuation becomes smaller and viscous sublayer is thickened,so that friction drag is reduced.While the inlet micro-bubble thickness equals to the boundary layer thickness(i.e.h+ = 560),the drag reduction rates are respectively 14.5%,35.5%,48.6% for ?v,in = 0.1,0.2,0.3,namely,drag reduction rate will increase as the inlet air volume fraction increases.For the case of constant micro-bubble inlet air volume fraction(i.e.?v,in = 0.1)and changing micro-bubble thicknesses,the effect of drag reduction can be treated as two conditions: if bubble thickness is much less than boundary layer thickness(i.e.h+ ? 560),bubble distribution is affected drastically by the turbulent boundary layer inner region,therefore there is no fixed rule between the drag reduction rate and micro-bubble thinkness,while if bubble thickness is equal to or larger than the turbulent boundary layer thickness(i.e.h+ = 560,850),the outer stable micro-bubble layer will restrain inner bubble instability,which is beneficial to drag reduction,so that the drag reduction rate will increase as the inlet micro-bubble thickness increases.It is found that when the inlet air volume fraction and micro-bubble thickness are both constant,for the case of ?v,in = 0.2,the drag reduction rate of micro-bubble located below the flat plate is about 7.3% higher than the case of micro-bubble located above the flat plate,while the value is 5.0% for the case of ?v,in = 0.3,the reduction mechanism for this case is the significant reduction of near wall fluid dynamic viscosity instead of the change of the turbulent boundary layer.