Study On The Characteristics And Mechanism Of Air-injection Drag Reduction

In recent years,ship air-injection drag reduction technology has achieved more and more attraction owing to its obvious advantages,such as easy operation,environmental friendliness,low costs and considerable drag reduction potential.There are two main forms of air-injection drag reduction technology:microbubble drag reduction and air layer drag reduction.Though the idea of air-injection drag reduction is simple,there are still some problems in practical applications.For example,the difference of air-injection reduction effect in laboratory and ship practical application is relatively large.Thus,it is necessary to study the characteristics and mechanism of air injection drag reduction.The numerical models of air-injection drag reduction are established based on the open source tool of computational fluid dynamics OpenFOAM.Combined with the experimental results,the characteristics and mechanism of air-injection drag reduction are studied.The main work is as follows:Firstly,the numerical models for air-injection drag reduction are explained,including microbubble drag reduction and air layer drag reduction.For microbubble drag reduction,the air phase is considered as discrete phase and the liquid phase is considered as continuous phase.The numerical model of microbubble drag reduction is established using the Euler-Euler two-fluid model,considering the interphase forces,the effect of bubbles on turbulence,and the coalescence and breakup of bubbles.For air layer drag reduction,the numerical model of air layer drag reduction is established using the volume of fluid method considering surface tension and turbulence.Secondly,reduction of two drag components by microbubbles is studied numerically.The microbubble drag reduction of the 2-D flat plate is numerically investigated and the numerical model of microbubble drag reduction proposed in the paper is validated.In addition,the influence of bubble deformation and buoyancy on microbubble drag reduction is investigated and the flow field characteristics are analyzed.The mechanism of microbubble drag reduction is discussed.The microbubble drag reduction of the 2-D axisymmetric body is numerically studied.The reduction of two drag components by microbubbles is studied.The flow field characteristics are analyzed.The influence of tail shape on microbubble drag reduction is studied and the flow field characteristics are analyzed.Thirdly,the air-injection drag reduction of the 3-D axisymmetric body is numerically investigated using the numerical models of both microbubble drag reduction and air layer drag reduction.The phenomenon of the abrupt change of drag reduction rate is numerically studied and the mechanism of the drag reduction characteristics is studied.The data-coupling method based on the air flow ratio is proposed to evaluate the drag reduction in the abrupt change region.The flow field characteristics are analyzed.The mechanism of air layer drag reduction is discussed.The sensitivity analysis of influencing parameters of microbubble drag reduction is conducted and the influence degree of relative parameters on microbubble drag reduction is preliminarily discussed.Fourthly,the stability of air layer drag reduction is investigated numerically.Based on the numerical model of air layer drag reduction,the numerical model considering the motion is established by combining with the dynamic grid technology.The stability of air layer drag reduction of underwater axisymmetric body under periodical motion is studied.Considering two typical kinds of motion including heave motion and pitch motion,the influence of motion on frictional drag,drag reduction and air layer morphology is investigated.The influence of motion amplitude,motion period,flow speed and air flow rate on the stability of air layer drag reduction is investigated.Finally,a ship model is designed and a set of air-injection device is specially designed and developed.The air-injection drag reduction of the ship model with different draft,different speeds and different air flow rates is investigated experimentally,aiming to validate some conclusions obtained from the numerical study in the previous chapters.In addition,the effect of ship draft on air-injection drag reduction is analyzed.The ship air-injection drag reduction is investigated numerically,aiming to develop the forecasting technology of ship air-injection drag reduction and the related problems on air-injection drag reduction are further discussed.