Aerodynamic Drag Reduction Research Of Non-smooth Auto-body Surface Based On The Design Of Experiments

The problem of energy consumption caused by the rapid growth of automobiles,and other environmental problems of toxic gas, particles, and greenhouse gasemissions that can’t be ignored should be solved. However, the method of reducingthe aerodynamic drag by non-smooth surfaces can not only improve fuel economyand the environment, but also provide technical guidance for breaking through thecurrent state of drag reduction bottleneck.This paper mainly includes four parts, the first part is the simulation oftwo-dimensional step model arranged with non-smooth elements; then simulatingMIRA automobile model arranged with non-smooth structures; and making the effectanalysis for each form of non-smooth elements and study the mechanism of dragreduction; the fourth is to make the orthogonal optimization on the basis of the secondpart.The research object of the first part is two-dimensional backward step，which isfrom the geometric features of tail steps of three-dimensional automobile model. Andapply the different forms of non-smooth elements to the different positions ofbackward steps. It’s found that the non-smooth surfaces can effectively improve thetail flow field, and decrease the energy consumption, and then reduce the dragcoefficient; At the same time, the primary and secondary orders affecting the dragreduction performances are given. What’s more, get the scheme from optimalcombination of levels according to the preliminary orthogonal experiment design.And it’s found through backward non-smooth elements not only affect the frictiondrag, but also reduce the pressure drag.The second part is to make the orthogonal experiment design for elements ofnotch-back MIRA model, and it’s found that the effects of positions, shapes forsurfaces with non-smooth elements, and their own feature sizes, including depth、 width and spacing between them on the whole automobile and components aren’ttotally consistent. The decrease of total drag can be that reduce the pressure drag andfriction drag of model, and also can be that largely reduce the pressure drag on thecondition that the increase of friction drag is not large. It can be gotten by analyzingthe effects of each factor and each level on drag reduction, the positions for elementsaffect the total drag reduction largest, and it can obtain the better effects of dragreduction when located in the baggage compartment. The effects of element shapescan’t be ignored. Among the schemes, the effects of groove located along the Xdirection airflow are obvious which makes up the weak of drag reduction for viscousdrag in this position, and then get better effects of reduction for total drag. The shapefeature of rotating can also get better effects, which guarantees the diversity applyingto engineering practices. However, the influence rules of drag reduction for spacingaffecting total drag and drags for each component are consistent, and the effect ofdrag reduction is better when the spacing is shorter and the density of non-smoothelements is dense; Instead, the sizes of elements affect less, which may be reasonedby that certain limits have been set to the element sizes considering the needs ofbeautiful and actual processing in this paper.The third part takes the models as the analysis objects, each model belongs toeach form of elements that has the biggest effect of drag reduction. And quantitativelycompare the effects of each element on components for each kind of drag, alsodiscussed the reasons of decreasing drag components. At the same time, state dragreduction mechanism for each form from the change of turbulent state、change ofvortex structure and vorticity、change of flow field near to the element, and also verifythe theories of “air bearing”、“protrusion height” which are put forward by scholars.The final part is based on the second part, and focused on the position, as theprimary factor, and amplified its effects on the flow field. Form the combinationscheme, and seek the better scheme for drag reduction among major factors. Throughthe simulation, it’s found that all combination schemes get good effects, and themaximum drag reduction effect can reach to7.41%, which is superior to the result ofbest scheme for single position:6.52%. And this provides a wider range of options for practical engineering application.This paper obtains the influence trends of all factors of non-smooth elementsacting on drag reduction rate by orthogonal tests, and states the mechanism ofelements with different forms that lead to the changes of drag in detail. The bettereffects of drag reduction are obtained by small range of orthogonal experiment design,and provide the theoretical basis for engineering application of non-smooth surfacesin automotive.