Composite Drag Reduction On Flat Bottom Type Ship Bottom Surface Numerical Simulation Study Of The Characteristics

As the energy problem is getting more and more prominent all over the world,it has become more and more important to develop a green and low-carbon economy.Reducing the resistance in ship operation is also a hot and urgent issue in shipping nowadays.Among the current drag reduction methods,micro-bubble drag reduction is widely used because of environmental protection and strong operability,while microgroove drag reduction is studied by many national researchers because it does not require additional energy consumption and is closely integrated with bionic technology.At present,most of the damping experiments are mostly on the damping of a single damping factor with a continuous optimization process,and the research on the compound damping of two or more damping methods is less.In this paper,we adopt the method of compound drag reduction by micro-groove and micro-bubble,firstly,we take the flat plate as the model,simulate the groove size,and select the optimal size parameter,the width s is 0.1mm,the depth h is 0.07 mm.It is found that the bubble content will reach a saturation air value,and when the saturation air amount is reached,the complete air film is formed at the bottom of the ship,and the drag reduction rate remains unchanged after reaching the maximum value.Further investigation of the influence of bubble scale on the drag reduction rate,analysis of the variation of drag reduction rate at different bubble sizes,found that the increase of bubble size will lead to the decrease of drag reduction rate;increase to 20μm,the drag reduction rate reaches a minimum stable value and then remains unchanged.Based on the above work,a three-dimensional ship physical model is constructed to explore the influence of the ventilation area on the composite drag reduction of the ship,and it is found that when the ventilation volume is low,the drag reduction effect of the ship is improved with the increase of the ventilation area,and when the ventilation volume increases to a certain value,the drag reduction effect of the ship with the original area S is better.To overcome the problem that the microbubble content at the stern of the ship is reduced due to the length of the ship,a ventilation port is added at the rear of the ship.Two types of venting are used,one is equal front and rear venting,both are50%,and the other is unequal front and rear venting,with front side 70% rear side 30%.Compared with the previous period when only the bow was ventilated,it was found that the drag reduction effect was significantly improved by ventilating both the bow and the midship,and the drag reduction effect by injecting 50% of each microbubble into both the bow and the midship was superior to that when 70% was injected into the bow and30% was injected into the stern.To further improve the drag reduction effect,a new model combining the traditional v-shaped groove surface and the uniformly distributed spherical crown structure is proposed in this paper to obtain the effect of increasing the area of the contact surface and increasing the disturbance.The optimized double crown structure has a good effect on the improvement of the drag reduction rate,and the stabilized result is around 50%,which is better than the 40% stabilized drag reduction effect of the single crown structure.It is found that the volume fraction of air bubbles is significantly larger in shallow-draft than in deep drafts,and the air bubbles can cover the whole bottom of the ship completely when the air film is complete.At the draft depth of 0.010 m,the drag reduction rate was about 48%,but when the draft increased to 0.025 m,the drag reduction rate decreased to about 17%,so the drag reduction rate was higher at shallow drafts than at deep drafts.At the same time,the reduction rate is converted into carbon emission reduction,and the emission reduction values of all the ship’s reduction methods are very significant,especially the reduction amount of compound reduction method is outstanding,which indicates that the ship’s reduction has good energy-saving and environmental protection.