| Boundary layer research is one of the most important research projects in aerodynamics (fluidmechanics), and skin friction is an important physical quantity for describing the state of turbulentboundary layer. But either for the computational fluid mechanics or experimental fluid mechanics,skin friction is the most difficult quantity to be measured. Turbulence is a worldwide problem of fluidmechanics that has not yet been fully understood. In the study of turbulent boundary layer, the skinfriction plays an important role in both assessing the result of turbulence modeling and validatingresults of numerical simulation. In this paper, the existing measurement techniques of skin friction aresystematically studied. Based on the classification of direct and indirect measurement, two morefactors, the sensor technology and spatial scales, are also introduced to describe the measurementtechniques of skin friction. Analysis shows that, among the skin friction measurement methods, theglobal measurement techniques of skin friction have obvious advantages in precision and spatialresolution, but there are many difficulties in realization and calculation. Comparing with the existingtechniques of global measurement, this paper introduces a new method that is based on fluorescenceoil film (FOF). The research is carried out from aspects of theoretical analysis, algorithm andexperiments.In theoretical research, started from the intensity of FOF, the paper explores the internal relationsand rules between the change of oil film fluorescence intensity and its movement process under theaction of the skin friction. Based on the research above, the FOF control equation is obtained.Combined With continuity equation, the mathematical model of the FOF global skin friction isestablished. In order to lay the theoretical basis for quantitative measurements of global skin friction,the simplified process of the model is analyzed in details. On this basis, the solutions characteristics ofthe model are analyzed in the steady flow for the one-dimensional and two-dimensional casesrespectively. The results show that fluorescence oil film solution for different instance under specificconditions (namely: s t0, s s0and s0) have similarity, and is quasi-similar ingeneral condition. It provides a basis for merging of solution sequence which improves the accuracyof the method. Based on the image processing technology, the thesis explores the quantitativemeasurement method of the evolution of fluorescent oil film, and establishes the mapping relationshipbetween global skin friction and the change of image grey scale. According to the penetrate boundaryproblem in the topology analysis of the measurement results, the Poincare-Hopf index theorem isintroduced, and the strategy for use is given in detail. Thereby the difficulties in the conversion pointjudgment are overcome. In the algorithm research, there are four issues, the method of solution, the optimization, themerging strategy and the projection calibration. For the first issue, the local algorithm based on theleast squares method and the global algorithm based on variation technology are introduced to solvethe aperture problems. Then the experimental study of simulating fluorescent oil film evolution isconducted to verify the feasibility and the effect of the specific parameters on these two algorithms.Subsequently, to solve the shortcomings of two methods above in anti-interference, computing speedand accuracy, an optimization algorithm is proposed. The optimization is based on the Robustnesstheory, which introduces the penalty function to enhance the robustness. In addition, the pyramiditerative evolution method is developed to take the advantages of the local and global algorithm. Inthe research of solution merging strategy, according to the solution characteristics of the steady flowoil film evolution equations, the research of merging method for solution sequence is carried outalong the time axis. The research finally makes the results independent from the initial state of oil filmto a large extent. For the projection calibration algorithm, an optimization is given according to directlinear transformation (DLT).In the experimental study, a channel flow is used, in which the global skin friction calibration iscarried out. In addition, the global skin friction measurement platform is developed, including theprojection, acquisition and data analysis of global skin friction. Using the above equipments, theresearch of measurement experiment is carried out to verify the theoretical analysis and the algorithmat typical surface and flow condition, for example, the impinging flow and the double delta wing.Also, the application to the wind turbine blade and flow control is explored. |
