A Wall Shear Stress Measurement Technique Using Double-layer Hot-film Sensor

Reducing the friction resistance is a very important work.The friction resistance is mainly determined by the distribution of the viscous wall shear stress.The viscous wall shear stress is also often used for dimensionless analysis of the parameters in the turbulent boundary layer.However it is difficult to measure the viscous wall shear stress.In this paper,a series of calibration-free wall shear stress measurement methods are developed based on the double-layer hot-film technology,and these methods are verified by experiments.First of all,this paper derives theoretically that the Joule heat generated by the doublelayer hot-film should be proportional to the one third power of the cosine of the incoming flow angle.Based on the directional sensitivity,a local two-component wall-shear-stress measurement technique is proposed using a pair of un-calibrated dual-layer hot-film sensors positioned perpendicular to each other.Three experiments are designed to verify the effectiveness of the method.The experimental results show that when the angle between the incoming flow and a single double-layer thermal film is within 60?,the Joule heat generated by the double-layer thermal film is proportional to the one third power of the cosine of the incoming flow angle.when the angle between the stress and the centerline of the sensor is within ±15?,the measurement error of the sensor to the wall shear stress is about 5%,and the measurement error of the incoming flow angle Can be controlled within ±5?.Subsequently,this paper also discusses the underwater measurement method of the doublelayer hot-film,and conducts the underwater measurement experiment of the double-layer hotfilm.The two experiments are the double-layer hot-film with coating and the hot-film without coating.The experimental results show that the two double-layer hot-films can realize the underwater calibration-free measurement of the wall shear stress.Since the coated double-layer hot-film will not be electrochemically corroded,the theoretical service life is longer.Finally,the dynamic measurement performance of the double-layer hot-film sensor is verified through experiments.The experimental results show that the double-layer hot-film can effectively respond to the dynamic wall shear stress within 300 Hz,but when the frequency exceeds 100 Hz,the dynamic response amplitude of the double-layer hot-film begins to decrease with the increase in frequency,and it begins to appear delay in response.When measuring the wall shear stress with high frequency changes,need to compensate the response amplitude and phase of the double-layer thermal film data.