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Drag Reduction of Bodies of Revolution by Flow Control Using Bionic Non-Smooth Surface

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Tutor: RenLuQuan
School: Jilin University
Course: Agricultural Mechanization Engineering
Keywords: bionics,non-smooth surface,drag reduction,flow control,wind tunnel test,numerica
CLC: TB126
Type: PhD thesis
Year:  2007
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Abstract:
Research on drag reduction methodologies has received global attention during the recent several decades. Among all the kinds of the drag reduction methodologies, research on the passive flow controlling method with safe, environmental, and convenient bionic non-smooth smooth surface became the investigative hotspot. In this dissertation, the emphasis of our research is the drag reduction the bodies of revolution using the bionic non-smooth surface. Through theoretical analysis, experimental investigation and numerical simulation methods, the flow control behaviors of the bionic non-smooth were obtained. As the development of the research, the engineering application of bionic drag reduction method was explored.Based on the essential bionic research method, three simple non-smooth shapes, which were convex tubercle, dimples, and riblet, were obtained by abstracting non-smooth surface information of the plant and the animal surfaces. The sizes of non-smooth surfaces were estimated using the aerodynamics, optimized theory and the international drag reduction achievements with riblet surface.Experiments on the bionic non-smooth were performed using the experimental optimization method. The tests were performed in an intermittent semi-return type wind tunnel with built-in strain gage balance. The maximum sizes of the bodies of revolution were determined according to the sizes of wind tunnel test section. The bionic non-smooth bodies of revolution were processed by numerical control machining centre, which can make the models have high precision and surface smoothness. The wind tunnel tests indicate that, convex hulls, dimples and riblets can achieve drag reduction at different attack angles and different mach numbers, and the total drag can be reduced by 5.01% at Ma=1.2. If the Ma number is 2.51, the dimpled surfaces arranged on the forward and the whole body of bullet headed bodies of revolution can not reduce their total drag evidently, while the dimples arranged on the rearward of bullet headed bodies of revolution can reduce the viscous forebody drag and the base drag observably, and the total drag can be reduced by 2.98%.The regression equation of the parameters of the dimples arranged on the rearward of bodies of revolution such as diameter, depth, and the space between adjacent dimples in axial direction was obtained by Central Composite Design. With the optimized parameters of the surface configuration on rearward of the body of revolution, the total drag can be reduced by 3.81%. In this dissertation, the experiments with the single fact with Mach number were also carried on, and the drag reduction effect of the dimples arranged on the rearward of bodies of revolution was confirmed ulteriorly.The Computational Fluid Dynamics analyses using the RANS equations with SST k-¦Øturbulence model were performed to investigate the effect of the dimpled surface on the drag reduction of bodies of revolution. In order to achieve a high computational accuracy, prism grids were adopt in the boundary layer near the wall, and the distance of the first layer of grids to the wall be calculated detailedly. Moreover, the mesh parameters of the non-smooth models are constant with that of the smooth one except the non-smooth areas of the bionic models. Numerical simulations indicate that the bionic dimpled surface and the riblet surface can reduce the skin friction drag and the pressure drag evidently. The dimples reduce the total drag of the roundnosed body of revolution by 6.24% at 0.4Ma, and reduce that of the sharpnosed body of revolution by 2.12% at 2.51Ma. The riblets can reduce the total drag of the roundnosed body of revolution by 8.59% at 0.8Ma. The drag reduction of skin friction drag reduced by bionic non-smooth surface is accomplished through reducing the wall shear stress and the Reynolds stress, and the pressure drag reduction is achieved by reducing the base drag. The essential reason of the drag reduction is that the bionic non-smooth surfaces control and amendment the boundary layer effectively. The behavior of the riblet surface controlling the flow near the wall is that the riblet surfaces cut the low-speed flow near the wall effectively, restrain the low-speed flow concentrating in span direction, and then weaken the instability of the low speed steak produced by turbulent flow bursting. As a result, the turbulent intensity and energy loss are reduced. The controlling behavior of the dimpled surfaces is dissimilar to that of the riblet surfaces, but the result is the same. Firstly, the low speed rotating vortexes in the dimples make the gas-solid contact to gas-gas contact, which like vortex cushions. Therefore, the effect produced by the dimples can be entitled the vortex cushion effect. Secondly, the low speed rotating vortexes forming in the bottom dimpled surface can produce negative skin friction against to the other area, as is the driving effect. The final result of the vortex cushion effect and the driving effect is that the turbulence intensity near the wall is smaller, and the kinetic energy loss also correspondingly be reduced. The dimples and the riblets can also increase the base pressure, and reduce the vacuum degree of the truncate of the model, then reduce the base drag which is the part of pressure drag.In order to apply the bionic non-smooth drag reduction method to engineering practice, the experimental and numerical researches on the bodies of revolution with complex outline were performed. The dimples arranged on the rearward of the complex outline bodies of revolution can reduce the base drag evidently, while the dimples can not produce the anticipative non-smooth effect because of the disturb by the shock wave produced by the guiding belt on the complex outline bodies of revolution. Moreover, the large size dimples can also increase the shock wave drag. Therefore, the viscous forebody drag is increased, which dues to total drag can not be reduced evidently. However, the ringed surfaces arranged on the complex outline bodies of revolution can reduce the total drag more evidently than the dimpled surfaces. The optimum ringed surface, obtained by the uniform experimental design, can reduce the total drag by2.4%. The low speed rotating flows forming in the rings can give birth to the air cushion effect and the driving effect. As a result, the viscous drag is reduced. The ringed surface can also weaken the pumping action on the dead water region behind the model caused by the external flow, which induces the pressure drag reduction. Being similar to the dimpled surface, although the rings can increase the shock wave drag, the negative contribution of the shock wave drag produced by ringed surface to the total drag is smaller than that of the dimpled surface. Therefore, the total drag of the complex outline bodies of revolution can still be reduced.
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