Research On Measurement Of Airfoil Skin Friction Under Transonic Conditions

The measurement of skin friction plays an important role in improving the performance of the transport, and it is also an important basic research subject, which can be used to describe the fluid characteristics of the fluid surface in the flow field. In some emerging areas, such as flow control, the measurement of skin friction also has the same significance. Because the cruise speed of large aircraft is usually near transonic. This paper selects “research on measurement of airfoil skin friction under transonic conditions” as the research subject, and selects fluorescence oil film technique as the main measurement.Firstly, this paper begins from the oil film evolution under the action of the global skin friction and the deeply study of the mathematical model of the fluorescence oil film skin friction. And then,this paper puts forward an improved algorithm based on the assumption of time continuity and the iterative evolution algorithm of oil film based on the assumption of time continuity, from the two aspects of reducing the time and improving the accuracy of the calculation results. The simulation results of two kinds of improved algorithms show that the former can greatly reduce the computation time while ensuring the accuracy of measurement, and the latter can greatly improve the accuracy of the measurement while reducing the computation time.Secondly, in the transonic wind tunnel experiment, there are many unfavorable factors when acquire oil film evolution image on surface of the airfoil to be measured, especially vibration and distortion of the airfoil surface. In order to overcome these unfavorable factors, projection measurement technique for curved surface is introduced. For the problem of that the brightness of filming image is very low and marked points is not easy to extract, this paper begins from the principle of camera pinhole model, and proposes a new algorithm for corner detection, which is suitable for a particular case. According to the specific circumstances of the mapping model,we can deduce the relationship among the horizontal mark lines and the relationship among the longitudinal mark lines, which are parallel to each other on the surface of the airfoil. And this paper uses the method of redundant information fusion to reduce the error of the mark point extraction. Next, based on the calculation method of the internal and external parameters of the camera, the optimization algorithm based on the direct linear transformation algorithm is introduced in this paper. After that,this paper makes a comparison among projection measurement technique for curved surface, DLT algorithm of considering distortion and traditional Tasi two-step algorithm. And then this papercalculates their respective pixel errors and quantitatively evaluated the accuracy of the calibration results. The experimental results show that this method in this paper is superior to the other two algorithms in the measurement precision.Finally, the measuring technique of fluorescence oil film has been used in the practical application. The paper has measured the global skin friction of the NLF0415 airfoil and verified the result of drag reduction by riblets. Then, the paper has compared the experimental results between oil film technique and particle imaging velocimetry(PIV).And the results of the two experiments are consistent, which proves the feasibility of the measurement technique of the global skin friction of the fluorescent oil film.