Researches On New Methods Of Drag Reduction For The Surface Ships

Nowadays,the green ships and the economic ships are the main trends in the development and design of medium-sized or large-sized transport ships,which can effectively reduce the environmental pollution,the waste of energy and many other issues.In recent decades,researchers have put forward a lot of drag reduction technologies on the ship,some of which is still in the theoretical research stage,and the other of which has been applied to the ship,achieved better drag reduction effect.According to the previous researches,there are two relatively mature drag reduction technologies.One is the counter-flow drag reduction technology.The pressure on the bow of the ship can be reduced effectively by counter-flow on the stagnation of the blunt body.The other is the drag reduction technology by microbubble or air lubrication.With the injection of the air to the hull surface,microbubble or air lubrication will be generated to change the nature of the medium near the surface of the hull and reduce the hull frictional resistance.In this paper,two kinds of drag reduction technologies are introduced into the20000DWT river-sea bulk carrier,and a specific drag reduction scheme is proposed.Two researches have been done with numerical simulation of the 1:31.98 model of the ship by FLUENT to investigate the discipline of the drag reduction and select the best drag reduction project.Under the numerical simulation of the first method,the results show that:1)the pressure on the bow of the ship decreases considerably by the counter-flow jetting,whereas the pressure distribution on the ship’s parallel middle body and stern is almost unchanged,so that the pressure drag is lowered;2)the effect of drag reduction is much better when the nozzle position is closer to the water surface;3)fix the nozzle area and position,change the speed of counter water.When the speed reaches1m/s,the integrated drag reduction effect is the best,whit the drag reduction rate of4%and the energy saving rate of 2.3%;4)it has a bad effect by the counter air for the air escapes to the surface and cannot cover the hull effectively.Under the numerical simulation of the second method,nozzles are set up along longitudinal direction on the bottom of the ship and air is jetted laterally toward the broadside.The results show that:1)keep the volume of counter air as a constant,the effect of drag reduction is much better when the nozzle position is closer to the bow of the ship;2)fix the nozzle area and position,change the volume of counter air.With the increase of the volume of counter,the friction drag reduction and the total drag reduction increase firstly and then tend to be gentle in the end,and the energy saving rate falls down sharply to a negative value at last.The two methods of drag reduction don’t affect each other.The total drag reduction rate can reach 23.06%and the energy saving rate can reach 3.19%,when the velocity of the incoming flow is 1m/s,the Reynolds number is 4.976×10~6,the velocity of counter water is 1m/s and the total volume of the air jetted on the bottom is 2.5m~3/h.