| Development of fluid mechanics deeply influences aviation,bridges and industrial design today.The measurement of wall shear stress is important for fluid studies and is key to understanding flow and reducing drag.This paper investigates and improves a novel force balance for measuring shear stress in turbulent boundary layer by floating element supported by a blade.The paper first identifies the sources of perturbations affecting the force balance and takes actions to mitigate the introduction of perturbations.An experimental system is used to investigate the influence of the force balance components force transducer,support platform,force arm and mass on the stability of the force balance measurement.The stability and repeatability of the force balance is measured using a variety of indicators.Develop a model for force balance to analysis how force balance behave with different components.The perturbations can be attenuated but not eliminated.Experimentally compare different force transducers and support platforms.It has been found that the use of loadcell and high stability self-made support platform enables the force balance to achieve good measurement stability.The experiment compared the performance of the force balance with different force arms and different masses and found that the force balance was more reproducible when the force arm was about 15 times and lower mass.Comparing the measurement errors at different wind velocity and conclude that the repeatability error of force balance i s lower than 4%.Motion modelling of the force balance explains the underlying causes of the above performance of the force balance and summarizes directions for improvement.The reduction experiments were performed using a slit blowing reduction exciter.The difference between the mean shear stress reduction measured by force balance and hot-wire is within 4%. |
