On the accuracy of turbulence measurements with inclined hot wires

Abstract

Since the accuracy of mean velocities and Reynolds stresses measured with inclined hot wires depends on the accurate knowledge of the hot-wire cooling law, each hot wire used in the boundary-layer experiment of Müller (1982, hereinafter referred to as I) had to be calibrated individually with respect to the magnitude and direction of the flow vector. In the present paper details of the calibration procedure and an example of calibrated data are reported. The directional hot-wire response was described by an effective cooling velocity, which was then used for the data reduction. The errors in the measured Reynolds stresses evaluated with an empirical cooling law as opposed to the actual one were estimated analytically from the governing equations and were confirmed by corresponding recalculations from the same set of measurements. Additionally, the validity of the conventional linearized method for evaluating the Reynolds stress tensor from the root-expanded equation for the cooling velocity was checked for increasing turbulence levels. In the test measurements all triple velocity correlations, which are usually neglected compared with second-order ones, were measured and taken into account in the data reduction.