Experimental Study On Turbulent Boundary Layer Drag Reduction Using Bubbles

With the rapid consumption of global energy and the increasingly serious environmental pollution problem,energy conservation and emission reduction have attracted more and more attention from domestic and foreign scholars.Ships,underwater vehicles and other tools,as the main marine vehicles,their surface frictional resistance accounts for more than half of the total resistance.It is of great economic value to reduce the frictional resistance of marine vehicles.Bubble drag reduction technology is an active flow control drag reduction method,which injects gas into the bottom surface of the ship.The raw material required is compressed air,which has low cost and is environmentally friendly.Bubble or air layer formed by ventilation can effectively change the physical parameters of the fluid,adjust the turbulent structure,and reduce the surface frictional resistance.The research on bubble drag reduction has been going on for more than forty years,but the gas injection method used in previous research is single.For real ships,opening holes on the bottom surface is the easiest method of gas injection with the least impact on the surface structure.Considering the real ship application of bubble drag reduction,this work used flat plate as a model,based on the water tunnel experiment,to analyze the bubble flow patterns and drag reduction under different flow parameters and air chamber outlet parameters.Four typical flow patterns were found in the experiments:bubble flow pattern(BFP),air mass flow pattern(AMFP),water pocket air layer(WPAL)and complete air layer(CAL).The pressure difference effect and wave pinch off mechanism were proposed for the growth and shedding of water pocket air layer and complete air layer.Flow pattern distribution is affected by Froude number Fr,ventilation coefficient C_Q and air chamber outlet parameters.In addition,this work quantitatively analyzed the relationship between air layer area coefficient Sc and the ventilation coefficient C_Q,and divided the air layer area growth into two parts:the rapid growth zone and the growth stability zone.The rapid growth zone included bubble flow pattern and air mass flow pattern,and the growth stability zone included water pocket air layer and complete air layer.This work compared bubble drag reduction under different parameters,and found that bubble drag reduction trend was different under different Froude numbers.Low Froude number(Fr=0.81)had critical drag reduction value.There was a stable zone at medium Froude number(Fr=1.14).The drag reduction increased linearly with ventilation coefficient at high Froude number(Fr=1.45).There was a positive correlation between drag reduction and air layer area coefficient under different Froude number.Different gas injection angles(α=45°、90°、135°)had little influence on drag reduction.The smaller the distance between the outlet holes of the air chamber(l/d=2、4、6、8),the better the drag reduction.Drag reduction of the air chamber with small hole diameter(d=1 mm)was better than that of the air chamber with large hole diameter(d=2 mm).