Experimental Investigations of Boundary Layer Thickness Using Ultrasonic Transit Time Method

The widely used investigation method of fluid boundary layer is to record or simulate the current profile distributions near boundary, and then the loss of velocity can be estimate by integration. A directly measuring method of boundary layer displacement thickness by using ultrasonic transit time instrument is proposed. The problem of boundary layer measurement can be simplified to a easier measurement process of current velocity calibration, and the verification experiments are carried out by taking smooth plate as an example. An experimental platform of towing tank facility is established, the towing velocity is taken as the standard value of the outflow speed. The device for flat plate boundary layer measurement is regarded as an ultrasonic current meter. The inner side of the pair of plates equipped with ultrasonic probes can be considered as smooth surface, when the concave at the end of probes, installed by path axial angle, is filled with the material, which acoustic impedance is approximately equal to water. The measured value of ultrasonic current meter is equivalent to the difference between the outflow velocity and the loss caused by boundary layer. The accuracy of measurement result is ensured through high-precision geometric measurement, time delay calibration and sufficient zero-offset correction. In order to improve the time measurement resolution of the current meter, the range of flow velocity is set higher than 100mm/s. By changing the towing velocity and the characteristic position of ultrasonic probe installation, the Reynolds number range is 5e4 to 5e5. By analyzing the principle of ultrasonic current meter and towing tank facility, the uncertainty of displacement thickness measurement results can be properly evaluated. The measurement results of these experiments are close to the integration of flow field record by LDA.