Skin-friction Measurements Using Elastic Film-based Sensor

The measurement of skin friction is of great significance to both the theoretical research and engineering application of fluid mechanics.Nowadays many kinds of skin-friction stress sensors have been widely used in many scientific and engineering fields.However,there is still a huge need of the skin-friction stress sensors especially one that can be capable of skin-friction measurements over extended regions of boundary surface in the water environment.The main work of this paper is to implement a very innovative skin-friction measurement method called S3F(shear and stress sensitive film)and apply it in the skin-friction measurement of open channel flow in order to verify its feasibility.In this paper,firstly the current common skin-friction stress measurement methods are introduced briefly.The basic principle of the S3 F method,the equipment needed and its advantages are introduced in detail.Then the paper explains how to implement and use the S3 F method in water environment from four aspects which are the optical measurement method,sensor fabrication and calibration,the finite element solution of shear stress and the application in open channel flow skin-friction measurement.In the optical measurement method part,the principle of the needed digital image correlation technology is introduced briefly.The method of making the tracking speckle pattern is described and the equipment needed for the optical measurement are introduced.In the sensor of S3 F method fabrication and calibration part,the physical and chemical properties of the polydimethylsiloxane(PDMS)which is chosen to be the raw material of sensor are introduced.The elastic modulus of PDMS are measured.A relatively wide range of elastic modulus(102 ~106 Pa)are obtained by changing the mixed ratio and curing temperature.The tensile tests are performed by using both the universal tensile testing machine and 2D DIC system.The comparison of the results obtained by the two measurement methods shows the superiority of the DIC method.In the skin friction calculation part,the governing equations of the deformed film are discussed briefly.The procedure of using the APDL of the finite element software ANSYS to model and input the measured displacement field as the boundary conditions is demonstrated in detail.In the application of skin-friction measurement of open channel flow part,the present common open channel flow bottom wall skin-friction measurement methods are introduced.The velocity distribution along the water depth is obtained with the ADV velocity probe,and the skin-friction stress is obtained indirectly by fitting the velocity distribution with the Coles formula.The results show that the results of the two methods are in good agreement,and the feasibility of the method is verified.