Investigation of a strain-gauge dynamometer for measuring boundary layer friction force on the wind tunnel wall

Abstract

A one-component strain-gauge dynamometer with a measuring range of 0-0.33 N was developed and manufactured at the Central Aerohydrodynamic Institute to measure directly the unsteady friction force of the boundary layer on the wall of a wind tunnel at very high Reynolds numbers. The subject of this study is to investigate the static and dynamic characteristics of the dynamometer and its systematic errors. The effects of the temperature, longitudinal, normal, and side forces and position of the longitudinal force application were investigated during the calibration. A block was designed and manufactured to calibrate the dynamometer using the dead weights in a wind tunnel. The calibration was carried out in laboratory conditions to determine friction force in the block. The developed dynamometer is a dynamic system with its own natural frequency of oscillations. The correction for the dynamometer's own dynamics is suggested on the basis of the method developed earlier. To calculate the correction to dynamometer dynamics the mass of the metric part of the dynamometer, its natural frequency in the absence of damping and the damping coeffi cient were determined. The correction is verifi ed by experiment with application of a step force. Effect of static and dynamic temperature components on dynamometer readings is investigated. Corrections are proposed to eliminate the systematic errors due to the effect of temperature on the basis of the previously developed method. It is shown that the side force does not affect the dynamometer readings, while the effect of the normal force is 1.1 % of the main sensitivity coeffi cient. The effect of the static temperature component on the sensitivity coeffi cient of the dynamometer is 0.017 %/°C. The total measurement uncertainty of the friction force dynamometer is estimated – the standard deviation of the measurement results does not exceed 0.076 % of the measurement range and the relative standard deviation of the main sensitivity coeffi cient is 0.03 %. The standard deviation of measurement of the nonstationary friction force will not exceed 0.86 % of the range of the dynamometer in the presence of oscillations of the sensitive element at its natural frequency.