Bionic Study Of Drag Reduction Between Air And Non-Smooth Surface Of Blunt Body Of Revolution Model

The skin of living creature had non-smooth structure; it was found that the non-smooth surface had some relationship with the drag reduction. Combined the theory of non-smooth surface with the bionic engineering, the drag reduction of non-smooth surface was studied in the medium of air. And the different non-smooth structures were machined on the blunt turning model directly. The purpose was to study the effect of drag reduction due to the form, dimension, distribution and arrangement of non-smooth structures on the experimental model under different wind speeds, and proved the academic and experimental foundation to the newer technology of drag reduction. Based on the theory of bionic non-smooth surface and study of the anti-adhesion and anti-resistance about typical animals which live in the soil, air and water, combining with the interfacal model between air and solid system, the non-smooth surface design principle was exposed. Three typical non-smooth form such as riblet, convex dome and dimple concave form were preferenced as non-smooth form. The position of non-smooth structures were distributing on the blunt turning model could affect the characteristic of pressure force reduction was brought forward firstly. After that the dimension, arrangement and ration of depth and witdth of non-smooth structure could affect the characteristic of viscous force also considered. And a new idea about drag reduction about blunt turning model was put forward. According to the the principle of non-smooth surface, an orthogonal experimental design was adopted. In this experiment, six factors were considered, and every factor included three levels. The six factors were the form, dimension, distribution, arrangement of non-smooth structures on the experimental model and wind speed. According to the orthogonal table, 27 different kinds of non-smooth turning models and one smooth surface model were experimented in the low speed wind tunnel. The result of experiment was shown that the non-smooth surface causes drag reduction and the wind speed were the predominant factors affecting drag reduction. Except for wind speeds, the distribution of the non-smooth structure on the experimental model was also the prominent factor for the drag reduction. When the non-smooth structures were placed on the tails of the experimental models, the effect of drag reduction was greater than that on the head and on the whole model. When the non-smooth structure was distributed on the tails of the experimental models, the average drag reduction was 7.57%. Using analysis of variance, the optimal combination and levels were obtained, which were a wind speed of 44 m/s, distribution of the non-smooth structure on the tail of the experimental model, the configuration of riblets, diameter/bottom width of 1 mm, height/depth of 0.5 mm, arrangement in a rectangle formation. At the optimal combination mentioned above, the 99% confidence interval for drag reduction was (11.13 %, 22.3%). CFD software was used to simulate flow fluid characteristics of both smooth and non-smooth models. The non-smooth models, optimized by orthogonal experimental design, had optimal dimension and ways of distribution. The simulation was validated the correctness of orthogonal experiment. It was founded that the pressure drag on non-smooth surface was reduced; the rate of drag reduction was about 20%. Through analysis the wake and the characteristic of stream, the lost energy of non-smooth surface model was fewer than that of smooth surface model. So the total drag of non-smooth surface model was lower. As for the drag reduction mechanism of non-smooth surface, according to the effect of non-smooth structure on the velocity magnitude, vorticity magnitude of turning model as well as the momentum thickness of boundary layer, the mechanism of pressure force and total force reduction about non-smooth surface was exposed firstly. Through the analysis of the changes of velocity magnitude of non-smooth surface, the mechanisme of drag reduction was put forward. When the air was across the transitional area between smooth surface and non-smooth surface, the wind speed became higher. However, because of viscosity of air, small vorticity which had reverse direction was produced in the riblet. These small reverse vorticity could counteract the higher wind speed, and so the wind speed was depressed until the end of the turning model. In this condition, the phenomenon of air suddenly speed up on the cross section was prevented. And the bottom pressure of the model would not disturb sharply, so the pressure drag reduction was occurred. Through the kinetic equation, it can be seen that the relationship between the velocity and vorticty( (ω??)vρ), the changes of the vorticity volume ( vρ??)ω, and the dissipation of the vorticity ν? 2ωcould explained the mechanism of drag...