Study On Bubble Sweepdown Characteristics Of Research Vessel With Sonar Dome

In natural state,there is a bubble layer near the sea level.When ship is sailing,these tiny bubbles will be entrained to the bottom.For scientific research vessels,bubbles near the sonar position will absorb or reflect sound waves.This phenomenon is called bubble sweep down.At present,installing a shroud is mostly used in engineering to deflect bubble flow and reduce bubble sweep down effect.However,the installation of shroud will increase resistance.In this condition,it is necessary to comprehensively study resistance performance and bubble sweep down effect on a scientific ship.In this paper,the numerical simulation of resistance and flow field of the bare hull model of a scientific research vessel is carried out.Based on the accurate simulation results,a shroud is added to the bottom of the vessel to discuss the resistance increase effect.Then,based on the streamline method,the bubble flow pattern is analyzed.At the same time,the analysis of the flow field results in Chapter 3 is carried out to explore the cause of bubble sweep down.Finally,a microbubble model was established based on the DEM method,and the differences between the bubble traces and streamlines were discussed.This paper mainly conducts a large number of numerical calculations on the resistance and flow field characteristics of scientific research ship under bare hull condition and with shroud appendage condition.First,summarize domestic and international research status of bubble sweep down,put forward the existing research problems,and then elicit the concerns of this research.Then,the main research content of this paper is determined:The resistance characteristics and flow field results under still water of a bare hull scientific ship model was analyzed.Based on above results,we install a shroud attachment on the bottom of model and research the resistance-incerase effect and bubble sweep down performance of the scientific research ship.At the same time,the effect of bubble deflection is also compared and analyzed.Finally,a microbubble model was established by DEM method.The correctness of DEM was discussed by simulating evolution process of gas nuclei in hydrofoil tip vortex.Then,a comprehensive analysis of bubble down sweep of the scientific research vessel was established to analyze the bubble trajectory of the scientific research vessel.The results show that the speed will affect resistance performance of ship model with appendages,and there would be resistance increase at high speeds.At the same time,the migration trajectory has a consistent trend and basically similar sweep angle at different speeds.After the shroud is installed,the flow trace in XY direction will have a large deflection,and the flow trace in XZ direction will have a smaller deflection.Bubble sweep down is mainly affected by vertical velocity around the bow and hydrodynamic pressure on the surface.The high-speed vertical velocity makes the bubble sweep down faster,which results in the advance of the sweep point.A tail vortex with a vortex strength around 120/s is formed on both sides of the shroud,which results in streamline shifting to both sides.In the end,the bubble trajectory and the streamline basically have same trend.In analyzing the bubble sweep down effect,the microbubbles can be regarded as streamline.