| The sweeping downward phenomenon of bubbles is a critical issue for detecting ships,which will significantly reduce the performance of ship detection sensors,especially acoustic instruments.When the ship is navigating in the ocean,the microbubbles in the ocean surface sweep down to the ship bottom with the water flowing to the stern and then pass through the bottom area of the acoustic detector array.Therefore,the echo signal received by the sensor will be lost or damaged,which greatly reduces the efficiency,speed,depth,and bandwidth of the acoustic sensor.Consequently,the echo signal received by the sensor will be lost or damaged,which greatly reduces the efficiency,speed,depth,and bandwidth of the acoustic sensor.In this paper,the phenomenon,law,and mechanism of bubble downsweep are comprehensively analyzed through experimental tests and numerical calculations.Moreover,the bubble suppression principle of defoaming appendages is studied,and a reasonable suppression scheme for the interference effect of bubble downsweep is proposed.The main work of this paper is as follows.Firstly,the flow characteristics such as flow field structure and velocity vector in the key region of bubble downward sweeping flow were measured and analyzed by silk thread method and particle image velocimetry(PIV)technology.Based on visualization of flow field by silk thread method,the limitation of measurement dimension of 2D-3C SPIV(two-dimension three-component stereoscopic PIV)measurement system is broken through.The computed tomography(CT)sweeping measurement method is innovatively used to reconstruct the three-dimensional flow field of the measurement results of velocity distribution,streamline,and velocity vector around the bow area of the scientific research ship,the flow mechanism of bubble downward sweeping and the evolution characteristics of flow field affecting bubble flow are obtained.The bubble downward sweeping process is explored from macro flow phenomenon to microflow behavior.Then the numerical simulation was carried out to conduct a numerical study on the phenomenon of bubble downsweep.The flow field around the bow of the scientific research ship is investigated,and the numerical simulation result of bubble point is compared with the experiment data,which verifies the accuracy of the numerical model.By focusing on the analysis of flow field characteristics such as waveform,bow velocity field,bow pressure field,etc.,the mechanism of flow around the bow area and the cause of the bubble sweep-down phenomenon are expounded from the perspective of the flow field.In order to further study the effect of bubble sweep-downward on the performance of vessels,the advanced optical measurement method Planar Laser Induced Fluorescence(PLIF)was used to carry out experimental research on microbubble sweep-downward characteristics.The results were summarized into the microbubble distribution cloud images under different ventilation heights,and then the change trajectory of the microbubble cloud center coordinate position with the captain was obtained.The flow trace and motion characteristics of the bubble are studied in depth,and the bubble is quantitatively analyzed.By extracting the thickness,width and area of the microbubble cloud cluster for analysis,the downward scanning law of microbubbles at model scale is obtained.Since it is difficult to realize the state of the suspended bubble layer near the water surface in actual sea conditions,this section was studied by numerical methods.The Euler-Lagrangian coupling method,which is often used in the field of chemical particles,submarine pipeline oil and gas leakage,and ship ice area research,is employed to model the sweep-down motion of the ship hull.The Euler method is selected to model the gas-liquid two-phase flow field around the hull,the Lagrangian method is used to capture the bubble motion,and the Lagrangian bubble motion model is established in the Euler solution domain for a solution.By ignoring the breaking and coalescence characteristics of the bubbles themselves,the process of microbubbles sweeping to the bottom of the ship's bow under the action of the flow field is simulated,and the spatial motion characteristics of the microbubbles are studied.Two settings of bubble layer and bubble point are used to study the actual sailing state of the ship under natural conditions.The accuracy of the numerical calculation is verified through experiments.Furthermore,the influence of the speed and the original position on the movement characteristics of the microbubbles is analyzed and obtained.Finally,to study the suppression mechanism of bubble downsweep characteristics and give a reasonable bubble suppression scheme,various defoaming appendages were installed below the multi-beam installation position at the bottom of the scientific research ship model.First,the resistance performance of the research ship with appendages is analyzed,and the resistance results of various appendages are compared.Based on those results,the fairing with a better comprehensive effect is selected,and the flow field near the appendage is discussed utilizing numerical analysis.Finally,the flow from the bow to the bottom is analyzed,and comprehensively analyzes the anti-bubble performance and mechanism of the attached body of the research ship with a fairing. |
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